CALIFORNIA AGRICULTURAL EXTENSION SERVICE CIRCULAR 128 September, 1944 PRODUCTION OF HEAD LETTUCE IN CALIFORNIA J. E. KNOTT and A. A. TAVERNETTI Cooperative Extension work in Agriculture and Home Economics, College of Agriculture, University of California, and United States Department of Agriculture cooperating. Distributed in furtherance of the Acts of Congress of May 8, and June 30, 1914. B. H. Crocheron, Director, California Agricultural Extension Service. THE COLLEGE OF AGRICULTURE UNIVERSITY OF CALIFORNIA BERKELEY, CALIFORNIA CONTENTS PAGE Production areas in the United States 3 Production areas in California 5 Climatic and soil requirements 6 Maintaining soil fertility 8 Preparation of land before planting 14 Seed production 16 Varieties and strains used in California 17 Varieties susceptible to brown blight 18 Varieties resistant to blown blight 18 Time of planting 21 Sowing the seed 21 Irrigation at planting time 22 Thinning and hoeing 23 Irrigating the growing crop 26 Cultivation 27 Yield 27 Harvesting 28 Packing for state use 30 Packing for shipment 30 Federal standards for head lettuce 32 California standards for head lettuce 34 Utilization of cull lettuce 35 Insect pests of lettuce and their control 36 Injury by birds 44 Diseases of lettuce and their control 44 Shooting to seed prematurely 51 Acknowledgments 51 PRODUCTION OF HEAD LETTUCE IN CALIFORNIA' J. E. KNOTTY and A. A. TAVEENETTP The area devoted to lettuce in California has decreased gradually from the high of 117,650 acres reached in 1932, but the output measured in crates per acre or carlot shipments has increased. Improved varieties and cultural prac- tices, and market price or demand have all contributed to maintaining the high level of production. Table 1 shows the California and total United States acreages only for the census years 1899 to 1939. During this period the Cali- fornia percentage of the total increased from 1.8 in 1899 to 65.9 in 1939. The TABLE 1 Lettuce Acreage in California and the United States by Census Years Census year California Other states Total United States Per cent California is of total 1899 46 595 6,121 31,650 60,565 106,050 80,286 2,586 4,894 15,423 38,384 53,207 48,370 41,431 2,632 5,489 21,544 70,034 113,771 154,420 121,717 1.8 1909 . ... 10 8 1919 28.4 1924 45.2 1929 53.2 1934 68.7 1939 65.9 Sources of data: 1899-1939: United States Department of Commerce Bureau of the Census. Censuses of 1900, 1910. 1920, 1930, 1940, and the Agricultural Census of 1925. 1934: United States Department of Agriculture. Agricultural statistics 1936. p. 143. Washington, D.C. 1936 total rail movement of lettuce originating in California in 1939 was 37,943 carlots, or about 72 per cent of the total for the United States. PRODUCTION AREAS IN THE UNITED STATES There has been a definite shift throughout the country from the butter-head type of lettuce such as White Boston and Big Boston to the crisp head typified by New York and the Imperials. Varieties have been developed that are par- ticularly adapted to certain sections. This has stimulated the planting of crisp-head lettuce. Big Boston and White Boston are now of relatively minor importance. Most of the lettuce shipments from the West are from Arizona, California, Colorado, Idaho, Oregon, and Washington. In the East, New York, Florida, Virginia, and the Carolinas are important in production. Figure 1 gives the carlot shipments of lettuce from important producing states for the five-year average 1938-1942. Though the shipping season of each locality varies slightly from year to year with seasonal conditions, the fiffure illustrates the dovetailing of the movement from the different districts. ^ This circular supersedes Extension Circular 105, Sead-Lettuce Production in California, by A. A. Tavernetti and John B. Schneider. 2 Professor of Truck Crops and Plant Physiologist in the Experiment Station. * Specialist in Agricultural Extension. [3] California Agricultural Extension Service [Cm. 128 ^ {0 -3 Ca//Torn/a [I/nper/a/ /a//e^] C<:7//forn/£7 fOff?er) Ar/zona Idafyo IVa^/?//?^/or) Ore^o/7 Ne^ York Co/oracfa -I I I 1- Ja/?. Feb. Mar Apr. Ma^ J^/ne Ja/i/ A^g- Sepf. Oct- Noy. Dec. Fig. 1. — Monthly carlot shipments of lettuce and romaine from important lettuce- producing states; 1938-1942 averages. (From: United States Department of Agriculture, Bureau of Agricultural Economics. Carlot shipments of fruits and vegetables by commodi- ties, states, and months. Annual issues.) Production of Head Lettuce PRODUCTION AREAS IN CALIFORNIA The two most important lettuce-producing areas for shipment are the Im- perial Valley and the Salinas-Watsonville-HoUister region, comprising the lower Salinas, Pajaro, and San Juan valleys. Considerable acreages are grown in Santa Barbara and San Luis Obispo counties on the coast and the Delano area along the Tulare-Kern county line in the upper San Joaquin Valley. Table 2 shows the relative importance of the counties in 1942. Plantings TABLE 2 Lettuce Acreage in California by Counties, 1942 Counties Season of harvest Spring Summer Fall Total Alameda Contra Costa Fresno Imperial Kern Los Angeles Marin Merced Monterey Orange Riverside Sacramento San Benito San Bernardino . . San Diego San Joaquin San Luis Obispo. San Mateo Santa Barbara . . . Santa Clara Santa Cruz Stanislaus Tulare Ventura Yolo Totals. acres 100 50 200 20,800^ 1,000 500 50 50 20,490 100 250 50 1,820 50 100 200 900 500 2,000 200 1.230 50 2,250 400 50 53,390 acres 100 150 11,050 100 300 500 2,000 150 850 acres 100 100 50 400 400 50 16,900 50 100 30 1,120 50 50 70 500 300 2,500 300 50 500 100 20 15.200 25,400 acres 300 150 250 20,800 1,400 1,050 50 100 48,440 150 350 80 3,040 100 150 270 1,700 1,300 6,500 650 3,740 100 2,750 500 70 93,990 * The cutting season in Imperial is in late winter and very early spring. California Cooperative Crop Reporting Service, Sacramento. California acreage of specified commercial vegetable crops by counties, 1937-1942. April, 1943. (Mimeo.) near San Francisco Bay and those near and in Los Angeles County are pri- marily for the local market. Homaine or Cos lettuce is important in the market- garden areas. Shipping Seasons for California Lettuce. — Lettuce moves from California in quantity all the year around. For the five-year average of 1938-1942 the lightest shipments were in March and December, with an average total of 840 and 889 cars respectively. In the Imperial Valley lettuce is shipped mainly from December to March ; in the Salinas Valley from April to November. The Delano area, included in the Central District, has two marketing seasons — March and April, and November and December. California Agricultural Extension Service [Cm. 128 Distribution of California Lettuce. — California lettuce is widely distrib- uted to the United States and Canada throughout the year. Though most of it is shipped in solid carlots, some is moved in mixed cars. A considerable quantity; in addition, reaches nearby markets by truck. A rather steady move- ment totaling about 600 carlots a month goes by truck to the San Francisco and Los Angeles markets alone. CLIMATIC AND SOIL REQUIREMENTS High-quality lettuce can be produced only where the temperature of the soil and air is moderately cool and uniform while the crop is maturing. This TABLE 3 Relation of Average Monthly Temperatures to Average Monthly Shipments of Lettuce Month of shipment January . . . February. . March April May June July August . . . . September October. . . November. December . Salinas Average temperature, in degrees Fahrenheit* Average shipments, in carloadst 2 2,483 3,350 1,416 2,566 2,708 2,656 2.294 2,674 236 B raw ley Average temperature, in degrees Fahrenheit^ 52.8 55.7 63 3 69.6 77 85 91 90.7 84,5 73 61.3 53.6 Average shipments, in carloads (Brawley and ElCentro)t 3,006 2,451 875 1 451 * Temperatures for Salinas based on 69-year averages, t Shipments based on the years 1938 to 1942 inclusive, i Temperatures for the Brawley district based on 33-year averages. Sources of data: Marketing Imperial Valley lettuce, marketing Salinas-Watsonville lettuce, annual summaries 1938-1942, Federal-State Market News Service. U.S. Weather Bur. Climatol. Data, California section 45 (13), 1941. requirement must be remembered especially by those who are attempting to mature lettuce in the summer. Fluctuating high and low temperatures do not promote good growth. High temperatures favor the development of seed stems, a bitter taste, loose heads, and diseases. Early in its development lettuce will tolerate considerable frost; but, if severely frosted when mature or nearly mature, it is more subject to slime and therefore does not ship well. Usually, frosted lettuce needs close trimming and is therefore not attractive. The matur- ing plants grow very little during continued frosty weather. Though the heads may become solid, they remain small, their leaf tips are injured, and the trade considers them undesirable. At low temperatures, increase in day length may result in more plant growth; but long days and low temperatures seldom occur together in our lettuce-growing areas. At high temperatures, day length has little effect on head formation or seedstalk development. Table 3 shows the interrelation of lettuce production and temperature for Production of Head Lettuce 7 two districts in California. A monthly mean temperature between about 52° and 62° F provides the best growth conditions for maturing the crop. Below this range, growth is slow, a fact accounting for the absence of winter ship- ments from Salinas. Above a monthly mean of about 62° the heads are poor in quality. In the Imperial Valley, plant frame growth occurs under a de- creasing temperature, heading after the range given above is reached. The favorable climate of the Salinas Valley for lettuce production is illus- trated further in figure 2. In summer the maximum soil temperature is usually higher than the maximum air temperature. Soil conditions would therefore Fig. 2. — Monthly average maximum and minimum air temperatures with comparable soil temperatures at a 6-inch depth, June, 1941, through May, 1944. As noted in the graphs, there was a 2 -month interruption in the soil-temperature records. (From daily records furnished by the Growers Ice and Development Company, Salinas.) be conducive to rapid growth ; yet the air temperatures during the same period are low enough to favor solid heading. The average soil and air temperatures were unusually high during the fall of 1943. This resulted in abnormal be- havior of the varieties planted for maturity at that time. The average soil temperature in January (about 50° F in 1942, 43° in 1943, and 47° in 1944) explains the slow growth of lettuce in winter. The rapid rise in soil and air temperature during the spring is accompanied by a rapid increase in soil nitrates, through the stimulation of bacterial ac- tivity. The graph shows clearly that the temperature conditions differ con- siderably from year to year. This explains why varieties behave differently from one year to the next and why fertilizer and cultural practices do not always give consistent results. Lettuce must not lack water during its growth period. The root system is relatively shallow. If the high ridges are allowed to dry out, the plants may not show severe wilting, since some of the roots will still be in moist soil. Growth may be less vigorous, however, than if the whole root system were absorbing water and if nutrients were being obtained from the upper fertilized area. Excessive water in the soil is, of course, undesirable. 8 California Agricultural Extension Service [Cir. 128 Although grown successfully on a wide variety of soil types ranging from clay loams to sandy loams and muck, lettuce attains its highest quality on fer- tile loam soils rich in organic matter. Where the fall crop is subject to high temperatures during its early development or matures in warm weather, the heavier types of soil should be used. They hold more moisture and may be cooler. The lighter, well-drained soils will serve for the crop that is planted and grown during the cool season. Because heavy-textured soils cannot be worked soon after a heavy rain, a definite planting schedule cannot easily be followed where rainfall is heavy during preparation and planting. All lettuce land should be well drained and properly leveled for irrigation. ^ MAINTAINING SOIL FERTILITY The soil problems involved in lettuce growing are chiefly those of keeping a good soil highly productive. Where good lettuce land is almost unlimited, as in the Imperial Valley and north-central California, this problem is not so difficult. The central-coast district or Salinas-Watsonville area has ideal conditions for summer lettuce ; but the area is limited. Here the only possible procedure is to plant in the same soils year after year. Crop Rotation. — Though as many as five or six crops of good lettuce have been grown consecutively, usually no more than two are profitable without some rotation or without an intensive fertilization practice. In the Imperial Valley, alfalfa is commonly used in the rotation. Lettuce is sometimes planted directly on alfalfa sod, but more often cantaloupe or some similar cultivated crop is grown first. As a rule it is difficult to make a good lettuce seedbed and to control weeds immediately after alfalfa. Lettuce growers of the Imperial Valley follow no one system of rotation. Some pro- duce an early crop of lettuce and a late crop of cantaloupes the same year, or plant lettuce for a late fall crop after a late crop of carrots. When flax is followed by lettuce, liberal fertilization is given. After being in cultivated crops for three or four years the land is usually resown with alfalfa. A deep- rooted legume followed by a series of shallow-rooted nonlegume crops makes a good rotation. In the San Fernando Valley and some other districts, spring lettuce is grown after fall crops of cauliflower, tomatoes, celery, or lettuce. Fall lettuce is often preceded by spring potatoes. In the Salinas-Watsonville-HoUister district more than one crop is usually produced on the same soil in a year (fig. 3). This district is fortunate because many crops can be profitably rotated with lettuce. The grower can work out a rotation with lettuce planted for harvesting in spring, summer, or fall. To counteract the combined effect of irrigation and cultural and harvesting opera- tions on the compaction of the soil during the growth of lettuce, it is well to rotate this crop with one that receives less working of the soil while wet. One can grow the following crops for spring harvest and still allow sufficient time to mature a crop of lettuce during the summer or fall : lettuce, garden peas, carrots, spinach, sugar beets, onions, garlic, vetch seed, and certain other seed crops. Possibilities for fall harvest after lettuce include lettuce, green peas, tomatoes, potatoes, carrots, cauliflower, spinach, celery, broccoli, and cabbage. Production of Head Lettuce 9 Though there are many rotation systems, the individual must determine the exact procedure to meet his needs. A few suggestions may guide him. One plan is to alternate two crops of lettuce with one rotation crop, an arrange- ment whereby lettuce can be grown on two thirds of the available acreage. Another plan now practiced successfully by a few growers is to carry out this rotation, but also to keep one fourth of the land in alfalfa for three to four years. Lettuce production in conjunction with dairying and alfalfa growing is an ideal combination. The refuse of all crops, such as straw, stalks, or unharvested plants, adds organic matter and should therefore be returned to the soil. If large amounts Fig-. 3. — Fall crop of lettuce nearing maturity in the coastal region. (From Ext. Cir. 105.) of such material are turned under, nitrogenous fertilizers are usually needed to offset the temporarily decreased fertility during decomposition. With a large percentage of the California lettuce grown on leased land, crop rotation as a practice is the problem not only of the grower and shipper but also of the landowner. As rentals are based mainly on the ability of the soil to produce marketable lettuce, landowners who wish continuously to enjoy a satisfactory income obviously must use practices that will keep their soil highly productive. A lease should not dictate just what crops are to be pro- duced ; but it might well stipulate the number of crops of a single commodity to be grown in succession on the same soil, the disposition of crop by-products having fertilizer value, and other similar conditions of soil management. Need for Organic Matter. — Organic matter in the soil is desirable for lettuce production. To be useful for plant growth, this material must be undergoing decomposition. The amount required varies with the soil type. As a rule sandy, porous soils are initially low in organic matter. In such soils the organic matter decomposes rapidly and depletion is much faster than in heavy soils. Organic 10 California Agricultural Extension Service [Cir. 128 matter lightens the heavier soils, thus improving aeration and water pene- tration. It is supplied as manure or as soil-improving crops. Manures. — Barnyard or other animal manure can be used on lettuce land. If it is composted with gypsum, the volatilization of the ammonia in the moist manure is reduced. The reaction is reversible on drying, so that losses may occur then. Fresh or unrotted manure should be plowed or disked under in time to decompose partially before the crop is planted. In the Imperial Valley the experimental use of barnyard manure, 10 tons to the acre, increased the total yield 54 per cent, caused the lettuce to mature 2 to 4 weeks earlier, and resulted in larger heads of better quality than the heads in adjoining un- manured plots. Manure on land in the Salinas Valley and other districts has also produced a striking response. Excessive applications, however, especially for summer lettuce, often result in loose heads. Soil-improving Crops. — ^Where the grower cannot obtain enough manure to supply the needed organic matter and plant food, he may use soil-improving crops (alone or with commercial fertilizers). To raise a green-manure crop is better than to leave the fields idle a part of the year. Such crops keep down the weeds, increase the organic content, and help conserve the soluble mineral elements of the soil, especially where the winter rainfall is heavy. They pre- vent erosion, help aerate the subsoil, and aid the favorable bacterial flora of the soil. The green-manure crop (preferably a legume) should be adapted to local climatic conditions and should produce a good tonnage of vegetable matter. It must be suitable for the planting time and the growing season. Usually the ground is thoroughly disked before the green-manure crop is plowed under. The crop should not be allowed to mature ; otherwise it will be hard to turn under, the seed may give rise to a volunteer crop, and decay vrill proceed much more slowly than if the green-manure crop were succulent. If the soil is dry, irrigation after plowing will hasten decomposition. Unless decomposition is fairly complete before planting of the next crop, injurious results may fol- low, through depletion of the nitrate supply in the soil by the bacteria that cause the decomposition. The addition of nitrogen will speed up the process. Green-manure crops are almost indispensable in the Salinas-Watsonville- Hollister area if lettuce is to be grown on the same soil over a period of years. More than two crops of lettuce should not be grown without at least one of green manure intervening, particularly if animal manure is not available. For green manure purple vetch and Melilotus indica or mixtures of these two are used almost exclusively. Since Melilotus indica will not germinate satisfactorily in warm weather, its use is confined to winter. Mixtures of it with vetch for winter planting are common. At other seasons, purple vetch alone is used. An August seeding of vetch for plowing under in November or December fits well into planting operations. Purple vetch, when seeded alone, is usually at the rate of 40 to 60 or more pounds per acre. First the seed is either drilled into the soil or broadcast, then ridges and furrows are made that cover the seed. The soil is irrigated to aid germination. Many growers prepare the soil and beds and drill the vetch in close-spaced rows on the beds and in the furrows. Production of Head Lettuce 11 Melilotus indica is usually broadcast on the ground in the early fall before the rains. Because the seed will not stand covering, no further work is done after seeding. From 15 to 20 pounds of seed is used per acre. When Melilotus indica and purple vetch are to be mixed, the vetch is drilled into the soil first, and then the melilotus is broadcast on top of the ground. When mixtures are used, about 10 pounds of melilotus to 30 or 40 pounds of vetch is satisfactory. The addition of the melilotus tends to hold the purple vetch up off the ground, preventing the considerable rot that sometimes occurs if the vetch is beaten down by heavy winter rains. Canadian Field Peas, sown at the rate of 70 pounds per acre, or Horse beans, at the rate of 75 pounds per acre, can be planted in late August or early September as a soil-improving crop. When the green-manure crop has made adequate growth, the fields are disked thoroughly and then plowed or chiseled. Side-dressing of lettuce with commercial fertilizers after turning under a legume green-manure crop can often be eliminated ; but one such dressing is usually made, the quantity of fertilizer applied being held to a minimum. Nonlegume green-manure crops such as sunflowers and mustard are some- times used. In general, however, these are less satisfactory than the legumes because their low nitrogen content requires subsequent heavy applications of nitrogen fertilizers. In the Imperial Valley, alfalfa is used rather often in the rotation. Green- manure crops are also employed to supply the necessary organic matter. The one now used in Imperial Valley almost to the exclusion of all others is the common legume Seshania macrocarpa. This is usually broadcast, 40 to 60 pounds of seed per acre, between May 15 and July 15, on land prepared flat for flood irrigation. It can be plowed under in 60 to 90 days. The advantages of sesbania are its suitability to Imperial Valley conditions, its quick growth, its heat tolerance, its cheap production, its adaptability to flood irrigation, and its rapid decay. It outgrows most weeds. Average costs of planting it in Imperial Valley are estimated at $10 per acre up to the time the crop is plowed under. The plowing is done without previous disking — usually with a notched disk plow. The growing of truck crops in beds and the "subbing" of the moisture to the plants tend to accumulate salts in the ridges. The flood irrigation required in growing the sesbania counteracts this tendency by leaching the salts to lower levels in the soil. Sesbania planted early in the season, plowed under in July or August, and then irrigated, is practically all decayed by the time lettuce is planted in October. Sesbania has certain disadvantages. It is susceptible to nematodes and re- quires high temperatures and abundant water. It breeds alfalfa caterpillars and cucumber beetles. Lettuce, however, being grown during the cool seasons, suffers little from nematode attacks. The insects, particularly the alfalfa cater- pillars, can be prevented from spreading to adjoining fields if the sesbania is plowed under at a medium stage of maturity, before it attains full growth. The ease and cheapness of growing sesbania may overbalance its lower produc- tion of dry organic matter. Commercial Fertilizers. — Besides the use of organic matter, commercial fertilizers may be needed to keep the soil highly productive. A complete fer- tilizer contains nitrogen, phosphorus, and potassium. 12 California Agricultural Extension Service [Cm. 128 The results of experiments by 0. A. Loreiiz in the Salinas Valley appear in table 4. These show the really small quantities of nitrogen, phosphorus, and potassium removed by a crop of lettuce. Nitrogen increased the yield in both spring crops, and phosphorus that in the phosphorus-deficient soil. Since potassium did not affect yield or quality, the amounts of it absorbed probably represent some luxury consumption — that is, more than the plant needs for growth. Two thirds of the amounts given in the table were taken up during the last month when heading took place. This fact emphasizes the need of having sufficient nutrients available in the root zone when heading begins. TABLE 4 Pounds Per Acre of Nitrogen, Phosphorus, and Potassium Removed by a Crop OF Lettuce from Fertilized and Unfertilized Soil* Soil not lacking in phosphorus Soil deficient in phosphorus Nutrients Sumnner cropt Spring crop Spring crop No fertilizer Complete fertilizer J No fertilizer Complete fertilizer} pounds 52 7 107 pounds 50 9 86 pounds 58 10 90 pounds 6 78 pounds 64 Phosphorus Potassium 9 98 * Calculated on the basis of a 90 per cent stand at 12-inch spacing in the row, 23,522 plants per acre, t No difference in yield or chemical composition between the fertilized and unfertilized plots in this crop. t Fertilizer: 120j)oundsbf^nitrogen (N), 120 pounds of phosphoric acid (PzOs), and 120 pounds of potash (K2O) apphed per acre. On the phosphorus-deficient soil the better growth and increased phos- phorus absorption resulted in 50 per cent more phosphorus being taken up by the plants. Yet the amount absorbed by the plant was small. Whenever the yield was improved the intake of the three nutrients was also increased. Neither the kind nor the amount of fertilizer can be specified for the entire state or even for any large district. Each grower should make several tests on his own farm to determine which fertilizers to use and in what amount. Lack of nitrogen is usually what limits the growth of the lettuce plant. Nitro- gen can be applied in the inorganic form as sodium nitrate, anhydrous am- monia, ammonium sulfate, ammonium nitrate, or calcium nitrate ; or in the organic form in such materials as Uramon, tankage, fish meal, cottonseed meal, and dried blood. If a complete fertilizer is to be used, it should be drilled in before or at planting (fig. 4). At planting time some growers drill the fertilizer 1 or 2 inches below the seeds, or in bands 2 to 3 inches to the side of the seed row and slightly below the seed, by using special attachments on the seeders. This appears to be a good practice. The phosphorus and potassium will not move much in California soils. They must therefore be put deep in the bed, where the soil will remain moist, so that roots can develop in the region of the fer- tilizer. Lettuce has responded to phosphorus fertilization on foothill soils Production of Head Lettuce 13 (Chualar and Greenfield series) in the Salinas Valley. Other soil series are known to be near the border line in respect to available phosphorus. Responses, therefore, are likely to be obtained in other parts of the state. Inorganic nitro- gen carriers may be applied immediately after the plants have been thinned and until about a month before harvest. Their application just after thinning and before the next irrigation is usually to be recommended. In the rainy season they can be broadcast or drilled shallowly on the surface of the beds ; the rains will carry the nitrates to the root zone. When the water is supplied by irrigation, the side dressing should be in the side of the furrow at about \. \ T Lf 9 M^' 1 %^~i ^H| ....^Jawm * affeff7H^ % uMmu ^^ i# m ^faM |Bg p »|VSlnHPn ft^ S-f 1 Fig. 4. — One type of equipment used to apply fertilizer and list in one operation. After the fertilizer is dropped, the lister covers it with soil. (From. Ext. Cir. 105.) the v>^ater level so that the dissolved nitrates will be carried into the bed as "subbing" occurs. If the dressing is placed lower they may be leached down- ward out of the root zone. Soluble fertilizer may be applied in the irrigation water. This method and the mechanical devices used in connection with it are covered by patents. Nitrate is carried into the soil as far as the water in which it is dissolved penetrates; this point, in some soils, may be past the zone of greatest root concentration. The nitrogen in the ammonia form is fixed in the soil of the furrow relatively near the surface — ^more so in heavy than in light soils. This is later changed to the nitrate form and carried to greater depths by subse- quent irrigation. In the Imperial Valley, fertilization with phosphate usually results in earlier maturity, larger heads, and higher yields. Applications are usually either 200 pounds per acre of treble superphosphate analyzing 43 per cent phos- phorus pentoxide (P2O5), or 500 pounds of ordinary superphosphate analyz- ing about 18 per cent phosphorus pentoxide. The treble superphosphate is preferred. It is placed 3 inches deep at planting time in bands on the furrow 14 California Agricultural Extension Service [Cir. 128 sides of the bed 1% inches outside the seeded row. Some growers follow the old practice of broadcasting the superphosphate before listing, or dropping it in a band in the center of the bed as it is listed. The superphosphate may be supplemented, after the lettuce is thinned, with a side dressing of nitrogen, such as nitrate of soda or a form of ammonia. With nitrate of soda the usual application is 200 pounds per acre ; sometimes there is a second application of 200 pounds 3 or 4 weeks before harvest. In all the lettuce-growing areas, side dressings of nitrogen are particularlj^ useful when cool weather retards crop development. During such periods, nitrate formation by bacterial action in the soil will be at a low level. Nitrates build up in spring and summer, reaching a peak in summer or early fall. One should usually not apply large amounts of nitrogen after the heads have be- Fig. 5. — A land pi©w used for grading fields for irrigation. gun to form. The use of inorganic nitrogenous fertilizers should be timed to produce a good growth before the heading period. To produce satisfactory- sized heads, a large frame should be developed. Too much available nitrogen at heading time may result in large, soft heads. If the temperatures are too low for growth, even nitrogen will not always bring the desired results. PREPARATION OF LAND BEFORE PLANTING Leveling. — In growing lettuce under irrigation the land must be smooth (fig. 5) and the beds of uniform height so that water can be applied without flooding certain areas and leaving others high and dry. Any leveling necessary should be done well before the planting season. If much soil must be moved, a test irrigation will settle it and will reveal the depressions and elevations. The necessary retouching can be done after the soil has become sufficiently dry. After leveling, manure or a commercial fertilizer is often added where the topsoil has been removed, so that a satisfactory crop will be produced. Preparatory Tillage. — In general the soil should be plowed and then disked and harrowed to a fine, mellow condition before the seedbeds are made. (fig. 6 ) . To make good beds or do good seeding on a cloddy soil is difficult. When seed- ing is done during very hot weather, however, aeration is facilitated and better stands are obtained if the soil is left a little crumbly. Production of Head Lettuce 15 When alfalfa land in the Imperial Valley is broken up for lettuce, it is usu- ally plowed several months in advance and left rough until about a month before planting. It is next floated, replowed, again floated, and then given a quick irrigation. Sufficient time should be allowed after the first plowing to kill the Bermuda grass and alfalfa ; and sufficient time after the second plow- ing to float, border, flood, dry out the soil, disk, float, and make seedbeds before the desired planting date. A common sequence of operations for land that has just grown a cultivated crop is plowing about 8 inches deep ; double-disking Fig. 6. — Preparing the soil in the Salinas Valley for the fall crop of lettuce after harvest of the spring crop. The land is first disked and then either plowed or deep-cultivated, as shown above, to loosen the soil before the beds are made. (From Ext. Cir. 60.) and floating; and then replowing, disking, cross-disking, floating, and bedding. In some districts more operations may be necessary, the method of handling depending a great deal on the soil type. Chisel cultivators, adobe harrows, and ring rollers are sometimes used in heavy soils. Planting Practices. — Three general planting practices are employed in growing lettuce in California. The crop may be seeded on level ground, on raised beds in dry soil, or on raised beds in moist soil. Planting without beds is used only in a limited way, in certain coastal valleys where no moisture except rain is available. Under those conditions seeding is done in rows 20 to 24 inches apart after the first fall rains, and harvest is in the early spring. Almost all the lettuce in California is planted on raised beds, which facilitate irrigation and drainage. The general custom is to grow two rows on a bed. In a few areas the plantings are made in single rows on slightly raised ridges thrown up on 24- to 30-inch centers. Making the Beds. — The width of bed, the distance between beds, and the 16 California Agricultural Extension Service [CiR. 128 depth of furrow vary in diif erent regions according to climatic conditions and the soil type. The beds are 18 to 20 inches or more in v^idth, and the furrows 18 to 20 inches or more (figs. 7 and 8). Seldom are the beds less than 3 feet from center to center, and usually they are about 40 inches. After smoothing they should be 6 to 8 inches high for the winter crop in order to provide drain- age, aeration, and increased sun and air exposure to give a warmer soil. This height allows for some settling. The beds for the summer and fall crop should be made only about 4 inches high (fig. 9), to reduce drying of the soil. The land must be well graded; otherwise there will be danger of flooding with K- -- 20 INCHES -^ 20 INCHES ->| ROW ROW WMiVK IRRIGATION y^^^W^WmW^MW^ J^r-^Vy'--: 'iNv._ FURROW ^^^^y-rX^:-;)};:0}OGE\ OR^'B Ep:^^\';■•:;;•:^f.':■;.^^v Fig. 7. — General type of bed and furrow used in the growing of lettuce in California. Dimensions vary according to season and local conditions. r^r^ .^.^^ Fig. 8. — Making lettuce beds for a fall crop in the Salinas Yailey with a triple lister. Double listers are also used for planting on a smaller scale. The beds are irrigated before seeding. (From Ext. Cir. 60.) these shallow beds, and the nutrients will then be leached too far below the roots of the small plants. More water is required to wet a high bed than a low one. The rough beds are made with a lister pulled by a tractor (fig. 8) . The rough beds may be shaped with a harrow or sled just before seeding or with the planter sled at seeding time. For very hard and cloddy soil, special equipment must often be devised to pulverize the beds so that they can be seeded. SEED PRODUCTION The production of lettuce seed is a highly specialized industry and not to be attempted by anyone not familiar with seed production. Figure 10 illus- trates an important step in breeding lettuce. Careful roguing of off-type plants is necessary in the development of the stock seed that is to be used in the production of a commercial seed crop. The plants selected for seed production Production of Head Lettuce 17 are stripped of heads to allow the seedstalk to develop and the mature seed is later harvested by hand or by machine. Numerous reliable companies in Cali- fornia specialize in lettuce seed. Growers should purchase seed only from a dependable source. Fig. 9. — Shallow beds make more efficient use of water than do high beds, but the land must be well graded to prevent flooding. Tf '^^te^^A^" 'X^^^^^ "2yfilA ,^^3 ^pOTI LQKw mm IPBJI^ ^B ^^^^'4;^^|^1HHB|I^K ^s^mBB ■■■ Fig. 10. — ^View of a lettuce-breeding plot. The plants to be self ed or crossed are enclosed in cheesecloth to prevent chance pollination. (Courtesy of the United States Department of Agriculture.) VARIETIES AND STRAINS USED IN CALIFORNIA The varieties of lettuce used for commercial shipment in California are the New York and Imperial, known to the trade as the "Iceberg" type. The true Iceberg variety, however, though resembling New York in habit of growth, has reddish leaves and is not grown extensively. 18 California Agricultural Extension Service [Cir. 128 The commercial lettuce varieties can be grouped into two general classes, based on disease resistance — namely, those susceptible to brown blight and those resistant to brown blight. All the resistant strains developed by Dr. I. C. Jagger of the United States Department of Agriculture were designated by him as Imperial. The stocks offered by various seed sources differ somewhat in their uniformity. Some are outstanding enough to be called ''strains." VARIETIES SUSCEPTIBLE TO BROWN BLIGHT New York Regular. — For many years the variety generally known as New York Regular or New York Special was the only one planted. It has been im- proved from year to year, and under favorable conditions produces a high percentage of uniform, good-quality heads. It and New York 12 and 515 are grown to some extent in certain market-garden areas where the soil is not infested with brown blight. New York 12. — This selection out of New York Regular was introduced by the Pieters-Wheeler Seed Company, Gilroy, California. An early, low-heading variety, somewhat lighter in color than New York Regular, it is very uniform in appearance and in time of heading. The head is exposed, with comparatively few wrapper leaves. New York 515. — This variety resulted from a cross between New York Regular and the true Iceberg and was developed by the Pieters-Wheeler Seed Company. It is partially resistant to tipburn. In general it resembles New York 12, being early and uniform, but is somewhat darker, with better and fuller wrapper leaves. There are improved strains of New York 515, such as New York 199. VARIETIES RESISTANT TO BROWN BLIGHT Imperial 152. — This variety is used for early-fall harvest on the coast from late July to mid-August plantings, and for early September plantings in the Delano area. The early-winter harvest in the Imperial Valley comes from September plantings. The plant is large, rather spreading, light grayish green, and set close to the ground. The leaf is thick, smooth, and rather soft. At first the heads are somewhat conical, with moderately twisted protecting leaves. At maturity they tend to be small, bald, flat, and very firm. The head leaves are long and overlap well. The base and interior of the head are well blanched. The variety is sure-heading, not inclined to bolt. Imperial 615. — This variety, a large winter type with spreading ground leaves, tends to coarseness. When young the heads are well protected by twisted leaves and are low and somewhat conical. At maturity they are often semibald and slightly flattened, with leaves well folded. The color is a dark grayish green. The stem is short; the base of the head smooth and well blanched. The variety is well adapted for producing early-spring lettuce along the coast from December and January plantings. It is used for the main crop in the Imperial Valley in plantings made from late September to the first of November. At Santa Maria, Imperial 615 is planted from mid-July to mid- March ; in Delano during November. It is not adapted to warm weather, for it bolts readily ; but in some of the cooler areas it will make a fair summer crop. Imperial D. — This resistant variety grows slowly. The plant is large and Production of Head Lettuce 19 coarse, rather spreading in habit and forming large heads. The color is dark, glistening green. In warm weather the plant suckers badly. In cold weather it forms good heads, firm because of the thick, crisp leaves. The interior is well blanched, and the flavor excellent. Imperial D is used somewhat as a late-fall lettuce for coast districts, being planted in late July and August. As it tip- burns badly, it is decidedly not a summer type. Imperial 847. — This is the principal summer variety. To make size it needs fairly warm weather. The heads are of medium size and well folded but tend, Fig. 11. — Imperial 847, the principal variety used for summer maturity in California. (Courtesy of Associated Seed Growers, Inc.) despite the many wrapper leaves, to be partly exposed when mature (fig. 11). The foliage is a lighter green than Imperial 615, being comparable in color with Imperial 152. Imperial 847 is planted from late March to early June in Salinas, from mid-March to mid-July in the Santa Maria area. Imperial 44. — The heads of Imperial 44 are well formed, with large, me- dium-green wrapper leaves, and are usually somewhat smaller than Imperial 152. Cold weather reduces the size greatly. The variety is slower bolting than most of the other Imperials. Planted during June and July at Watsonville and in certain other parts of the Salinas area, it produces good heads from August to mid-October. Heads maturing earlier in the summer may be affected with tipbum and slime. Great Lakes and Imperial 456. — These two varieties resulted from a cross between Brittle Ice and Imperial 152. Great Lakes comes from a line crossed to Imperial 615 and is therefore more robust than Imperial 456. Seed produc- tion is a difficult problem : both these varieties have a very short seedstalk, 20 California Agricultural Extension Service [Cm. 128 form hard heads, have tight wrapper leaves, and are slow to bolt even in hot weather. They are more resistant to tipburn and marginal burning of the outer leaves than are others. The foliage, a glossy, light green, is very distinct from that of the other Imperials. The leaf margins are much serrated or finely cut. The varieties are not yet well fixed for type. The protruding ribs detract from the smooth- ness of the heads. This condition and the tendency for the midribs of the head leaves to crack when the heads become too hard may interfere with packing for shipment. Great Lakes appears, however, to be well adapted for nearby Fig. 12. — Great Lakes, a heat-tolerant, slow-bolting variety for nearby markets. The background lines are spaced 12 inches apart. markets in inland areas of California where other varieties have not headed well in warm weather. In tests at Davis it has proved consistently better than any other variety tested for either spring or fall maturity (fig. 12). Imperial 456, under western conditions, usually forms too small a head to be as valuable as Great Lakes. Growers' E-4. — This large-framed variety, with dark-green leaves slightly curled on the margin, was introduced by Dr. L. E. Weaver of the Growers Ice and Development Company of Salinas. At maturity the heads are large, solid, and almost spherical in shape, with a high seedstalk. The variety appears fairly resistant to tipburn and is offered for late December to mid-March plantings in the Salinas Valley for April to July harvest. Growers' 253. — This large-growing variety has many light-green wrapper leaves. The heads are slightly flattened, with a somewhat spiraled cover, and are apparently less injured by frost than those of other varieties. Growers' 253 is planted in the Salinas Valley during the first three weeks of August for a late fall harvest. It requires a cool fall for solid heading. Production of Head Lettuce TIME OF PLANTING 21 Lettuce requires 60 to 150 days to mature, according to soil, climatic condi- tions, and the variety used. The longest time is required for plantings made in early winter ; the shortest time for those made for late summer or early fall maturity. The planting date influences to an important degree the choice of variety. The seasonal adaptations are discussed under the variety descriptions. In the Imperial Valley, seeding is done in September and October for winter harvest. The remainder of the year, the land is used for other crops. Fig. 13. — A three-bed sled that pulverizes and smooths the top of the listed beds, planting two rows of seed on each bed. Plantings in the Los Angeles area are continuous, so that lettuce moves to market the year round. The volume harvested from July to September is not great. In the Salinas- Watsonville-Hollister area, planting goes on throughout the year except between late August and mid-November. November plantings of Imperial 615 are best made without irrigation, to come up with the fall rains. The planting seasons in the Santa Barbara and San Luis Obispo districts resemble those at Salinas. Spring and fall crops are possible in the interior valleys. In the Sacramento Valley, August, January, and February plantings can be made, whereas in the upper San Joaquin late August through October is a suitable period. SOWING THE SEED Various tools have been devised for sowing. Two to six rows may be sown in a single operation with implements like that in figure 13. During the rainy season it can be planted i/2 inch deep. When it is to be 22 California Agricultural Extension Service [CiR. 128 irrigated up, the seed should be sown 1 inch deep, or slightly deeper. It will not germinate at high temperatures unless the soil is well aerated. In warm weather this aeration is accomplished by leaving the seedbed slightly coarser than during the cool season. According to Borthwick and Robbins,^ if lettuce seed is to be planted in a soil that has a temperature of 86° F or more during many hours of the day, good germination can be obtained by placing the seed in thin layers between moist burlap and then storing on ice 4 to 6 days. Good aeration must be provided during this storage period. When the seed is then dried and planted in moist soil, a high percentage of germination is obtained. Thick seeding is objectionable. The labor cost of thinning a thick stand of plants is expensive, besides the waste of high-priced seed. From 1 to 1^/2 pounds to the acre will suffice if all the seed is viable and if the soil temperature Fig. 14. — Irrigating lettuce beds before seeding, in the Imperial Valley. Water enters the furrows from the head ditch through small conduits. Usually only one is used for each furrow. The piece of shook beside the opening serves to regulate the flow. In most lettuce-producing districts another irrigation is given imme- diately after planting. (From Cir. 295.) and moisture are suitable. Even under favorable conditions, however, a little more seed may give a better stand. The rows should be not less than 10 inches apart. Some growers space them 14 to 17 inches to permit the development of large heads. When the rows are too close together on top of the bed, cultivation is more difficult, various dis- eases are apt to appear, and the mature heads are usually smaller. On the other hand, the plants usually grow better if not too near the edge of the bed. If a large acreage is being grown, successive sowings should be made at intervals of 10 to 14 days throughout the lettuce-planting season. In some projects planting is continuous throughout the planting period except when interrupted by adverse weather. IRRIGATION AT PLANTING TIME The irrigation system used at planting time varies with the locality, climate, and soil. In some regions the land is flood-irrigated, the beds are made, the * Borthwick, H. A., and W. W. Bobbins. Lettuce seed and its germination. Hilgardia 3(11) :275-305. 1928. Production of Head Lettuce 23 seed is planted, and no further irrigation is given until the seedlings are through the ground. This system can be practiced where the soil dries out slowly. Since beds of moist soil settle more evenly after irrigation than beds of loose, dry soil, preliminary flooding is advantageous. Where evaporation is very high it is customary, besides giving the prelimi- nary flooding, to irrigate immediately after planting or as soon as germination is desired. Rough beds are sometimes made in land that has not first been flooded. The beds are wet by running water in the furrows. This irrigation moistens and settles the beds, which, when sufficiently dry, are smoothed and planted. The fall crop of the interior valleys is usually planted in dry soil and then not irrigated until the grower desires to start the germination. The following plan is sometimes used for the early plantings in the Imperial Valley : After a preliminary flooding, the beds are made and then irrigated (fig. 14). Their tops, as soon as dry enough, are harrowed with a special tool equipped with runners that slide in the furrows and prevent the latter from being filled with clods. Seeding begins a few hours later, when the soil is dry enough for the planter to operate. The land is irrigated again, as soon after seeding as possible. In furrow irrigation the water should "sub" until it finally moistens the entire bed, but the flow may be shut off when the area of moist soil reaches the drilled rows. Judging from measurements of water, exorbitant amounts are often used to wet the soil in contact with the seed. In warm, dry weather, irri- gation immediately after planting is usually preferred. The water is some- times kept running in small streams down the furrows until the plants are up. If the water is cold enough to cool the soil, it will improve germination in hot weather. In cool weather such heavy irrigation is unnecessary. In heavy clay soils, deep furrows are used with a small head of water. If the water comes over the top of the bed, the soil will bake and crack when it dries, and the seedlings will not come through. Regardless of location, however, the surface soil should be kept moist until after the seedlings are through the ground. In the hot interior valleys more care is needed than along the coast, where moist air and cool temperatures prevent rapid evaporation. THINNING AND HOEING Lettuce should be thinned before the plants begin to crowd. In warm weather this time may be 3 weeks after planting ; in cool weather as long as 8 weeks. The plants are blocked out 12 to 14 inches apart in the row with special short-handled hoes and at the same time are thinned by hand to one in a place (fig. 15). Those not true to type should be removed. The plants should have plenty of room in the row. If they are too close the size of the head is reduced, with a delay in maturity. The season and the variety will determine the proper spacing. In cool weather 14 inches or more between plants will help give a larger head of the varieties that are apt to be small. In warm weather a 12-inch spacing will tend to give heads of the 5-dozen size instead of the 4-dozen size in large-growing varieties. Blocking of the lettuce by machine with subsequent hand-thinning of the remaining clumps of plants to a single stand is being more extensively practiced (fig. 16). 24 California Agricultural Extension Service [Cm. 128 Fig. 15. — Thinning lettuoe. A short-handled hoe is used to chop out most of the plants. All but one plant at a place are then removed with the free hand. '§1 i^'A £j Fig. 16. — A blocking machine used for spacing lettuce. The rotating curved blades cut off the row of plants except at the spaces separating the blades, where one or more plants slip through. The rows already blocked are seen on the bed to the left. (Courtesy of Spreckels Co.) Production of Head Lettuce 25 Thinning is important. Often when it is done under contract, part of the money is withheld until the job is finished satisfactorily. Beds and furrows are usually cultivated before thinning (fig. 17). Cultivation can be followed by a rolling (fig. 18) , which compacts the center of the bed to facilitate subbing and also lays the plants on their side, making hand-blocking and thinning Fig. 17. — Two views of the arrangement of shields, blades, and mulch er for close shallow cultivation of young lettuce. Fig. 18. — The beds are sometimes rolled just before thinning, to facilitate the operation. easier and more rapid. If a roller is to be used, the plants should be about ll^ inches high. A roller to cover three beds weighs 1,000 to 1,500 pounds. Growers may stipulate that all weeds left by the cultivator shall be removed by the thinners, that the plants shall stand at a specified distance in the row, and that no doubles shall remain. Some, however, remove the weeds and doubles in a separate operation. When there is danger from insects, thinning should be delayed, if possible, till the pests have been destroyed. The soil is replaced 26 California Agricultural Extension Service [CiR. 128 against the plant after thinning, as a side-dressing is given. This practice opens the furrows so that the subsequent irrigation can firm the soil about the plants, carry the nitrogen of the side-dressing to the plants, and help them recover from the disturbance incident to thinning. IRRIGATESTG THE GROWING CROP The details of irrigation vary considerably in different localities and at different seasons. Small plants need much less water than large ones. Less water is required when the weather is cool and humid than when it is hot, dry, Fig. 19. — Lettuce being irrigated about a month after thinning. and windy. The frequency of irrigation also depends on the water-holding capacity of the soil : as a rule, more irrigations are needed on light, sandy soils than on silt or clay-loam soils, retentive of moisture. In the opinion of many growers, water applied when the heads are maturing is apt to make them soft or loose, and heavy rains just before cutting make them open and spongy. Sometimes, however, one must apply water to prevent the soil from cracking, which would allow root aphis to attack the roots. Experience will teach the grower just when to irrigate. As a rule the plants need water when the moisture in the surface foot of soil is nearly depleted. Judging from recent irrigation experiments under the climatic conditions prevailing in the Salinas Valley, one irrigation about 30 days after thinning will produce as much lettuce as more frequent applications (fig. 19). A crop of lettuce will need not over 4 acre-inches of water in growing to maturity in the Salinas Valley. Actually, however, because of waste in applying the water, much greater quantities are employed in irrigating. Many growers use wooden or metal conduits or flumes to carry a small stream from the end ditch to the furrows between the beds. A conduit may be Production of Head Lettuce 27 made of four laths or slightly wider strips, such as cantaloupe shook, 20 to 24 inches long, nailed together. It should be long enough to fit well in the ditch bank. Pieces of shook or lath can be placed in front of the conduits to regulate the flow. Galvanized or iron pipes cut to the same length as the wooden con- duits are often used. Iron lasts longer. The usual diameter is 1% "to 1% inches. Metal siphons save much spade work and facilitate flow control. CULTIVATION If weeds are abundant one may have to cultivate before thinning. Often cultivation is needed to open the furrow and to replace the soil around the young plants after thinning or, later, to destroy weeds that rob the soil of moisture and soil nutrients (fig. 20). Beyond this point it is seldom necessary Fig. 20. — Cultivating partly growTi lettuce and clearing furrows for irrigation. or even beneficial, though certain soil conditions require special treatment. The ordinary beet or bean cultivator gauged for several beds is satisfactory. To insure cultivation close to the plant without injury, the number of beds cultivated in a single operation must correspond to the number seeded at one time. YIELD According to the Federal-State Market News Service and the California Crop and Livestock Reporting Service for the past five years, lettuce produc- tion has averaged slightly less than one-half carload for each acre harvested. Acre yields vary materially from year to year because of weather, insects, diseases, or poor market conditions. The average per acre for all sections of California from 1938 to 1942 was 153 crates on the basis of 5 dozen heads each. The crates contain about 75 pounds of lettuce. 28 California Agricultural Extension Service [CiR. 128 \ f^<^ir^sfe*^ Fig. 21. — Harvesting lettuce, showing the type of knife used and the method of cutting off the root just below the ground. (From Cir. 295.) Fig. 22. — A mechanical loader covering ten beds. The heads drop into the "baskets" on a truck. HARVESTINa Growers sometimes harvest too soon, especially if the price is high. Since lettuce should not be harvested until the heads are firm, frequent cutting is necessary. Harvesting should not be done when the field is muddy. Then, the plants being gorged with water, the leaves are crisp and brittle and break easily. If slightly wilted they are injured less in handling and packing and therefore will carry better. Lettuce should be cut just below the surface of the ground (fig. 21) . Most of the trimming of the outer leaves should be done at the packing shed. Production of Head Lettuce 29 Fig. 23. — Dry pack of lettuce in a field near Delano. Fig. 24. — The baskets are rolled on small wheels to the packing table. In this shed, after the lettuce is trimmed by the girls, the heads are carried on small revolving turntables to the men who pack. 30 California Agricultural Extension Service [CiR. 128 Harvesting is done by hand. A gang of men go down the rows, cutting the matured heads from the two rows adjacent to a furrow and tossing them onto trucks or trailers. The cutters may place the heads in windrows, from which another crew following toss the heads into large steel-framed crates or baskets on trucks or trailers. The lettuce is then hauled to the packing-shed. These bas- kets, having four small wheels on the bottom, can be rolled from the vehicle to the packing table. If the heads are packed in the field, one method is to have the men cut two rows each, trimming the heads and placing those from several beds together for packing. Fig. 25. — A modern packing-shed. The lettuce is dumped from trailers onto an endless conveyor, from which the heads are trimmed and placed on the packing table. (From Ext. Cir. 105.) In the larger lettuce-growing sections, mechanical loaders (fig. 22) are in general use. The conveyor, mounted on a truck body, moves at right angles to the rows just ahead of the harvesters. Each man cuts from the two rows nearest him, placing the heads on the moving belt, where they are elevated and drop into "baskets" on a truck or trailer. The apparatus covers eight to eleven beds. Lettuce heads that are developing a seedstalk, or have burst, or show tipburn or slime should be discarded in the field. In very warm weather or when freezes are expected, lettuce should be har- vested as soon as it is hard. In cool weather mature lettuce can remain in the field for some time without bursting. As a rule, however, it should not be held long if it is to be marketed in good condition, with a minimum of waste. PACKING FOR STATE USE The dry or field-pack method (fig. 23), is practiced for much of the lettuce used within the state. Under cool conditions it is satisfactory. The packers fill the lined crates as they work down the piles of cut heads. The covers are nailed on in the field. PACKING FOR SHIPMENT At the packing-shed the loose, diseased, and damaged leaves are trimmed off (fig. 24), and the small heads and loose heads culled out. When trimmed the heads are tossed to the packing table, where they are placed in the Standard California lettuce crate (fig. 25 ) . In each crate are put 24, 30, 36, 42, 48, 60, 75, Production of Head Lettuce 31 Fig. 26. — Cracked ice is placed between the layers of packed lettuce. From 25 to 30 pounds of ice is used in each crate. This cools the lettuce and tends to insure its reaching the eastern markets in good condition. (From Cir. 295.) Fig. 27. — Top-icing refrigerator cars. Portable ice-blower in action, showing the use of rubber hose. (From Ext. Cir. 105.) or 90 heads, according to their size. The growers and shippers, however, usually prefer heads that pack 4 or 5 dozen to the crate. The lettuce crate is lined with two strips of heavy waterproofed paper, which cover the bottom, sides, and ends, and fold over the top. This lining protects the heads from dirt and drying, keeps them cool and fresh, and guards them against mechanical injury. Three layers of heads are packed tight in the crate 32 California Agricultural Extension Service [Cir. 128 with stems up, generally with cracked ice between the layers (fig. 26) . Usually not over 30 pounds of ice is used in each crate. The crates should be placed in the refrigerator car as soon as they are packed. From 4,500 to 7,500 pounds of crushed ice, the amount depending upon the weather, is blown into the car, usually on top of the crates (fig. 27) instead of in the bunkers. The ice on top of the load preserves the ice in the crates and thus keeps the lettuce fresh for a long time. FEDERAL STANDARDS FOE, HEAD LETTUCE The U. S. Standard grades for lettuce formulated by the Bureau of Agri- cultural Economics, United States Department of Agriculture, as of March 15, 1934, reissued July 10, 1939, are as follows (numbers and letters in parentheses following grade terms indicate where such terms are defined under "Defini- tions of Terms") : The tolerances for the various grades are placed on a container basis. However, for a tolerance of 10 per cent individual packages in any lot may not contain more than one and one-half times the tolerance specified, and for one of less than 10 per cent individual pack- ages in any lot may not contain more than double the tolerance specified, provided that the entire lot based on sample inspection shall average within the tolerances specified. GRADES U. S. Fancy shall consist of heads of lettuce of similar varietal characteristics (1) which are fresh (2), firm (3), well formed (4) and well trimmed (5) ; which are not split, burst (6), or open, and which are free from decay, tipburn, russet, brown blight, doubles (7), and from damage (8) caused by seedstems (8a), broken midribs (8b), freezing (5 and 8c), dirt (8d), sunburn (5 and 8e), discoloration (5 and 8), disease, aphis (8f) or other insects, or mechanical or other means (8). In order to allow for variations incident to proper grading and handling, not more than 10 per cent, by count, of the heads in any container may be below the requirements of this grade, but not more than one-half of this tolerance, or 5 per cent, shall be allowed for decay affecting the compact portion of the head. Of this tolerance for decay, not more than two-fifths or 2 per cent, shall be allowed for slimy decay. U. S. No. 1 shall consist of heads of lettuce of similar varietal characteristics (1) which are fresh (2) ; which are not split or burst (6), and which are free from decay, tipburn, russet, brown blight, doubles (7), and from damage caused by opening (8g), seedstems (8a), broken midribs (8b), freezing (5 and 8c), dirt (8d), sunburn (5 and 8e), discolora- tion (5 and 8), disease, aphis (8f) or other insects, or mechanical or other means (8). Each head shall be fairly well trimmed (9) unless specified as closely trimmed (10). Not less than 75 per cent of the heads of Iceburg type lettuce shall be firm (3), and the re- mainder shall be fairly firm (11). Heads of Big Boston type lettuce shall be fairly firm (11) . In order to allow for variations incident to proper grading and handling, not more than 10 per cent, by count, of the heads in any container may be below the requirements of this grade, but not more than one-half of this tolerance, or 5 per cent, shall be allowed for decay affecting the compact portion of the head; provided that, of this tolerance for decay, not more than two-fifths or 2 per cent shall be allowed for slimy decay. This tolerance shall not permit in any lot of U. S. No. 1 Iceberg type lettuce fewer than 90 per cent of heads which are firm or fairly firm and free from defects, on the basis of a ratio of three firm heads to one fairly firm head. U. S. Commercial shall consist of heads of lettuce which meet all of the requirements of U. S. No. 1 grade except that they shall be free from serious damage by tipburn instead of free from tipburn. In order to allow for variations incident to proper grading and handling, not more than 10 per cent, by count, of the heads in any container may be below the requirements of this grade, but not more than one-half of this tolerance, or 5 per cent, shall he allowed for decay affecting the compact portion of the head; provided that, of this tolerance for decay, not more than two-fifths or 2 per cent, shall be allowed for slimy decay. This Production of Head Lettuce 33 tolerance shall not permit in any lot of U. S. Commercial Iceberg type lettuce fewer than 90 per cent of heads which are firm or fairly firm and free from defects, on the basis of a ratio of three firm heads to one fairly firm head. TJ. S. No. 2 shall consist of heads of lettuce of similar varietal characteristics (1) which are not split or burst (6), which are free from decay, from damage (8) caused by seed- stems (8a), and from serious damage (12) caused by wilting, tipburn, freezing, disease, insects, or mechanical or other means. In order to allow for variations incident to proper grading and handling, not more than 10 per cent, by count, of the heads in any container may be below the requirements of this grade. DEFINITIONS OF TEEMS As used in these grades: 1. "Similar varietal characteristics" means that the heads in any container have the same characteristic leaf growth. For example, lettuce of the Iceberg and Big Boston types shall not be mixed. 2. "Fresh" means that the head is crisp, although the wrapper leaves and the outer one or two head leaves may be slightly wilted. 3. "Firm," as applied to heads of Iceburg type lettuce, means that the head is compact but may yield slightly to moderate pressure; as applied to heads of Big Boston type let- tuce, means that the head is fairly compact. 4. "Well formed" means that the head is well shaped, and that midribs of the leaves are not abnormally prominent or protruding. 5. "Well trimmed" means that the butt is trimmed off close to the point of attachment of the outer leaves; that wrapper leaves are free from appreciable injury by any cause; that on heads of Iceberg type lettuce wrapper leaves do not exceed six in number, not more than one-half of which may be excessively large and coarse such as are characteris- tic of No. 6 strain; and, provided further, that the outermost leaves of the head show some shade of green color on a part of the leaves. "Wrapper leaves" means all leaves which do not fairly closely enfold the compact portion of the head. Heads shall not be considered well trimmed when the wrapper leaves are badly blistered or show yellow discoloration or more than slight brown margins. Heads with torn wrapper leaves shall not be considered well trimmed when such leaves appreciably injure the appearance of the head. 6. "Burst" means that the head is broken open. 7. "Doubles" means two heads on the same stem. 8. "Damage" means any injury which materially affects the appearance, edible or shipping quality of the lettuce except defects affecting wrapper leaves as restricted under definitions of "well trimmed," "fairly well trimmed," and "closely trimmed." The following shall be considered as damage: (a) Seedstems which are apparent upon external examination of the head. (b) Broken midribs, when more than two of the outer head leaves have the midribs broken in two due to abnormal growth conditions. (c) Freezing, when the head leaves show a brown discoloration over more than half of the crown, or when more than three of the outer head leaves show appreciable injury by freezing. (d) Dirt, when the head is smeared with mud, or when wrapper leaves are badly smeared with mud, or when the basal portion of the head is caked with mud or dry dirt. (e) Sunburn, when the head leaves show a brown discoloration over more than half of the crown of the head. (f ) Aphis, when the head proper is infested, or when the wrapper leaves are badly infested. (g) Opening, in hard or firm heads which have one-fourth or more of the head dis- tinctly separated from the remainder, or any degree of opening in fairly firm heads. 9. "Fairly well trimmed" means that the butt is trimmed off close to the point of at- tachment of the outer leaves; that wrapper leaves are free from serious injury by any cause; that, on heads of Iceberg type lettuce, wrapper leaves do not exceed ten in number, not more than six of which may be excessively large and coarse such as are characteristic of No. 6 strain ; and, provided further, that the outermost leaves of the head show some 34 California Agricultural Extension Service [Cir. 128 shade of green color on a part of the leaves. "Wrapper leaves" means all leaves which do not fairly closely enfold the compact portion of the head. Heads shall not be considered fairly well trimmed when the wrapper leaves show yellow or brown discoloration or brown margins to an extent that the appearance of the head is seriously injured. Any blistering except that causing yellow or brown discoloration which seriously affects the appearance of the wrapper leaves or any tearing of wrapper leaves shall not be con- sidered as serious injury. 10. "Closely trimmed" means that the head meets all requirements of "fairly well trimmed" except that the wrapper leaves shall be not more than 3 in number, none of which may be excessively large and coarse. 11. "Fairly firm" means that although the head is not firm, it is not soft or spongy. 12. "Serious damage" means any injury which causes the loss of a material portion of the edible part of the head. The loss of crispness due to freezing shall not be considered serious damage. Heads affected with tipburn shall be considered as seriously damaged when any single spot is larger than one and one-half inches in length and/or three- fourths of an inch in width. 13. "Fairly uniform in size" means that not more than 10 per cent, by count, of the heads in any one container may be one standard size smaller than the standard size head for the count packed. Example of Standard Size Head — The standard size head for a 4 dozen pack is that size which will pack tightly 4x4 heads of uniform size in a layer in the crate, assuming that the head has the average number of wrapper leaves found on all the heads in the crate. STANDARD PACK Heads of lettuce shall be fairly uniform in size (13), and tightly packed in uniform layers according to the approved and recognized methods, provided that a "bridge" may be used with sizes smaller than 5 dozen count. In order to allow for variations incident to proper packing, not more than a total of 15 per cent of the containers in any lot may not meet the requirements of the Standard Pack, but no part of this tolerance shall be allowed for packs which are excessively loose in the layers. CALIFORNIA STANDARDS FOR HEAD LETTUCE Head lettuce offered for sale in California must comply with sections 814 and 828 in the Agricultural Code of California (1941 revision) as follows: Head lettuce shall not be leafy without head formation and shall be free from slime, decay or rot affecting leaves within the head, internal insect injury, and free from seed stems which have so developed that they are apparent upon external examination; and free from serious damage caused by bursting, tipburn or freezing. Damage caused by bursting is not serious unless the head is burst open or is materially misshapen from this cause. Damage caused by freezing or tipburn is not serious unless it affects any portion of the head inside of the six outer head leaves. Not more than 10 per cent, by count, of the heads of lettuce in any one container or bulk lot may be below these requirements, but not to exceed one-half of this tolerance, shall be allowed for any one cause. Head lettuce, when packed or placed in layers in any container, shall not vary in size in any one container more than 10 per cent of heads which would tightly pack a size, larger or smaller, than the size marked, and they shall be tightly packed so that it is not possible without damaging or injuring the lettuce, to place additional heads in any of the layers of heads in the container. In addition to other packing requirements in this chapter head lettuce, when packed or placed in layers, shall contain either two dozen, two and one-half dozen, three dozen, three and one-half dozen, four dozen, five dozen, 75 or 90 heads of lettuce per standard container numbers 45A and 45B with a slight bulge of crates when lidded. Each crate of three and one-half dozen count shall have each layer arranged with four rows of three, four, and four and three heads. In the case of sizes 75 and smaller per crate, a bridge of from three to six heads shall be permitted and in the five-dozen size a bridge of four heads shall be permitted. Production of Head Lettuce 35 All containers of lettuce shall bear upon them in plain sight and in plain letters on one outside end, the name of the person who first packed or authorized the packing of the lettuce, or the name under which such packer is engaged in business, together with a sufficiently explicit address to permit ready location of such packer, and in figures not less than one-half inch in height the exact number of heads contained therein; pro- vided that in the case of 10 per cent of the crates in any lots the contents may vary not more than three heads from the count as marked. All containers of head lettuce shall be standard containers numbers 45A, 45B, or 45C. Standard containers numbers 45A, 45B and 45C, when lidded, shall have a lid not over Dimensions No. and name Depth inside in inches Width inside in inches Length inside in inches 133^ 13 9 or 9^ 173^ 17M 13 21^ 45B Standard lettuce crate . . 215^ 450 Half lettuce crate 215^ 25 inches in length. The inside length shown herein above for the standard containers numbers 45,^ 45A, 45B, 45C and 46, shall be a minimum length, with maximum outside length of these containers of 24^ inches ; and the inside lengths of these containers shall be measured between the end slats, except that if flat end posts wider than 1^/2 inches are used, the inside length shall be measured between the posts. UTILIZATION OF CULL LETTUCE In packing, large quantities of leaves and cull heads accumulate. The leaves analyze 94.7 per cent water, 1.27 per cent protein, 0.3 per cent fat, 2.9 per cent carbohydrates, 0.7 per cent fiber, and 0.9 per cent ash. Although this material contains such a high percentage of moisture — 94.7 per cent^ — nevertheless its feed value for beef and dairy cattle is considerably higher than the chemical analysis indicates. In the Salinas Valley about 20,000 tons of cull lettuce are fed to cattle annually. The beef thus produced amounts to perhaps 400,000 pounds a year. The cull lettuce is loaded into dump trucks from the lettuce-packing sheds. It is then hauled to fields and dumped in large piles where cattle have free access to it. Although this is not the most economical way of securing its maxi- mum feed value, such disposal is quick and cheap from the standpoint of the lettuce grower. Some of the feeders dump the culls in open racks or mangers, so that very little waste occurs. A mature beef animal fed on straight lettuce will eat about 100 pounds per day and will gain 1 to 1% pounds per day. Both beef and dairy cattle gain and produce more if dry grain hay or straw is added as a supplement. Success in feeding the cull lettuce depends on securing a continuous supply over a period of several months. Feeding cattle on lettuce that is dumped in large piles will attract numerous flies. This fly problem is usually combatted by disking under the decomposed lettuce and manure from time to time and feeding on new, clean ground. All types of cattle do best when they are only gradually brought on the full feed of this product. Some deaths have occurred when thin, hungry cattle have had free access to fresh, green, cull lettuce before becoming accustomed to it. ^ Containers 45 and 46 do not pertain to lettuce. 36 California Agricultural Extension Service [Cir. 128 ,«.7 INSECT PESTS OF LETTUCE AND THEIR CONTROL Cutworms. — Common species of this serious pest are the greasy cutworm, Agrotis ypsilon, and the variegated cutworm, Peridroma margaritosa. The cutworm usually cuts off the young lettuce plants at ground level. The best control is the use of a poisoned bait, prepared as follows : Paris green (or arsenic trioxide) 1 pound Bran 25 pounds Molasses 2 quarts Sufficient water to make a dry mash The replacement of 20 per cent of the bran with citrus meal often increases the attractiveness of the bait. A little amyl acetate can be used to rej^lace or supplement the citrus meal. Sodium fluosilicate at the rate of % pound may be substituted for paris green in the above formula; or calcium arsenate can be used at the rate of 1% pounds. In preparing the bait, thoroughly mix the poison and bran or citrus meal. Dissolve the molasses in a little water, and add to the dry mixture while stir- ring. Continue to add water until a crumbly mash consistency is obtained. The water and poison must be uniformly distributed in the bait mixture. In using poison bait after the plants begin to head, he extremely careful not to let the mash fall into the heads. The freshly prepared bait is spread on the top of the beds in the early evening, for the cutworms feed at night. A lettuce planter or fertilizer machine can be used to distribute the poison. Usually 20 to 30 pounds of the bait is necessary per acre; a second or third application may sometimes be needed. Dry, commercial baits are available on the market ; these usually require 15 to 40 pounds to the acre for a single application. Calcium arsenate or cryolite may be used as either dusts or sprays, but only when the plants are young, because of the danger of poisonous residue. These materials are for the control of certain species such as the yellow-striped army worm. Corn Ear worm. — In the coastal areas the caterpillars of the corn ear worm (Heliothis armigera) often cause considerable damage to lettuce. The adults fly at night, depositing the eggs singly on many plants, especially corn, cotton, tomatoes, and tobacco. Under favorable conditions the larvae mature in 14 to 20 days and enter the soil to pupate. There are three or more broods a year. The result is a gradual build-up of the population in the fall. Lettuce is not a preferred host plant, but is selected particularly in the fall when adults emerge from beans, corn, or tomatoes. For this reason, avoid planting lettuce adjacent to these preferred crops. Often the moth does not complete its life cycle on lettuce, but at harvest time the mature caterpillars are found in the hearts of the heads. The larvae have been first observed the latter part of June and * This section was contributed by W. H. Lange, Junior Entomologist in the Experiment Station. "^ The California Agricultural Code reads : "When using materials which are known to be lethal to bees, livestock or other property, exercise every precaution to protect all property from damage and, as a means thereto, use only such methods of application and under such climatic conditions at the time of application as shall confine the material applied sub- stantially to the premises of intended application so as not to cause appreciable loss of bees, livestock, or other property on adjacent premises." Production of Head Lettuce 37 have reached a peak during September. The use of insecticides has not proved successful for this insect when heads are formed and the larvae are inside. Dusting and spraying the small plants as described under the alfalfa semi- looper may give some control. Lettuce Boot Aphid. — The lettuce root aphid {Prociphilus hetae) clings in cottonlike masses on the main taproot and small, secondary roots (fig. 28). Severe injury may occur from May to September. Often the heads wilt, nor- 28. -The lettuce root aphid, showing white cottonlike masses on the roots of lettuce. mal head development stops, and, in severe infestations, the plants are killed (fig. 29). The infestation is spread by the winged forms, which seek out new plants to infest. They are more abundant in heavy soils that tend to crack and crumble than in lighter, more sandy soils that remain compact. The explana- tion is that the winged individuals crawl into cracks in the ground to give birth to living individuals. Other host plants include sugar beets, docks, and pig- weeds. On docks, colonies have been found every month of the year. In areas where injury occurs, avoid planting lettuce adjacent to sugar beets, and keep the edges of fields and ditches free from weeds, especially docks. If the soil on top of the bed cracks badly, the dispersal forms may gain entrance to the taproots. Keeping the plants growing vigorously with adequate fertilizing and proper watering will often help to avert serious injury. Soil fumigants 38 California Agricultural Extension Service [Cir. 128 have not proved satisfactory when applied after wilting was noticed. Dichlor- ethyl ether, applied as discussed under wireworms, aids in averting serious injury if used before infestations become severe. Natural enemies include syrphid fly larvae {Metasyrphus wiedemanni is a common species) , the larvae and adults of many common ladybird beetles, and the larvae of several flies (Chloropisca spp.) . A fungus disease is also effective in killing entire colonies. Other ApJiids. — Several aphids occur aboveground on the leaves, especially in spring. (Myzus hieracii and Macrosiphum euphorhiae are common species.) Usually control is not necessary ; but they can be controlled by spraying with nicotine dust. Free nicotine (80 per cent), used to make a 2.5 per cent dust, has proved satisfactory in the Salinas Valley, where dusting temperatures Fig. 29. — Injury to plants by the lettuce root apliid, showing extreme wilting of the plants. usually run below 70° F. It can be incorporated with cryolite and a nonlime carrier to make a combination dust for loopers and aphids. Syrphid fly larvae and the larvae and adults of ladybird beetles are common predators. Wireworms. — Wireworms or the larvae of click beetles injure lettuce by cutting off or burrowing into the taproot. They are shiny, yellowish brown, segmented, and % to 2 inches long. The common species in the Salinas Valley and coastal California are three closely related forms in the genus Limonius {L. canus, L. calif ornicus, and L. infuscatus) . Injury occurs in the spring and summer when they come to the surface and feed, but may also occur in the fall on young lettuce. During late fall, winter, and early spring there is no injury, for the larvae remain inactive deep in the soil. From one to several years may be spent in the larval stage. The adults emerge in the spring and fall and spread the infestation. Control is best accomplished by attracting the larvae to a bait crop, such as cull beans (small white or lima beans are better to use than dark beans), and then destroying them with a poisonous gas. A good time to treat is after a spring crop, when the areas of infestation are well delineated. The ground should be clean-cultivated, well pulverized, and moist enough for the germina- Production of Head Lettuce 39 tion of beans. The trap crop is drilled into the soil about 4 inches deep. In 2 to 10 days the wireworms will congregate in sufficient numbers to justify use of the fumigant. Granular calcium cyanide should be drilled into the bait row about 1/2 inch below the beans at the rate of 100 pounds to the acre, or 6 pounds to 1,000 lineal feet of row. A side-dressing fertilizer machine or a bean drill is convenient for applying the material. Immediately after this procedure, use a roller to compact the soil and to aid in holding in the fumes. The moisture in the soil reacts with the calcium cyanide to form hydrocyanic acid gas, which destroys the wireworms. In the experiments conducted to date, it has been found difficult to apply calcium cyanide directly to the injured plants : dif- fusion of the gas is not sufficient to reach all of the wireworms present on the roots. In the Salinas Valley, the growing of alfalfa has been found to increase the populations of wireworms, especially when spring irrigations are practiced ; and lettuce following alfalfa may suffer serious injury. Crop rotation and summer fallowing are helpful in reducing damage from wireworms. Often one can plant in late fall or early spring and thus escape serious trouble even in fields where wireworm populations are high. Certain crops such as broccoli, cabbage, celery, and peas can often be grown on wireworm-infested soil in the Salinas Valley without having important losses. A method of minimizing harm to an individual crop is to use a solution of dichlorethyl ether. The emulsified material is used at the rate of % gallon to 100 gallons of water. When wireworm injury first occurs, just before or just after thinning, apply the solution directly to the plants ; or, in a normal plant- ing, turn two streams of liquid on the bed. Remove the spray nozzles and whirl plates, and have just enough pressure for adequate discharge of the solution. The amount will vary from 300 to 600 gallons to the acre, depending upon the soil type. The solution should not be applied after the plants are more than 2 inches high, or control will be difficult. The best time of application is toward evening, although morning may be advisable in areas where winds are a prob- lem. The solution kills wireworms on contact and acts as^ repellent for several weeks. This delay allows the plants to become firmly established before new attacks occur. Alfalfa Semilooper. — Along the coast, the caterpillars of the alfalfa semi- looper {Autographa calif ornica) often cause considerable defoliation to young lettuce plants. The adult moths lay the round, greenish- white eggs directly on the upper surfaces of the leaves. In 5 to 10 days these hatch into green larvae, which crawl in a semilooping fashion. The caterpillars cast their skins four times, attaining a length of about 1 inch. Larval development requires 12 to 42 days, depending upon the season. Pupation occurs aboveground, usually on the plants, in a white, silken cocoon. The harm done to lettuce is most noticeable just after thinning, when the caterpillars congregate on the remaining plants and damage the terminal shoots. This injury prevents normal growth and head development. The cater- pillars usually mature and leave the lettuce before the plants begin to head up, so that damage is less than might be expected. Mature plants that have been attacked are susceptible, however, to slime rot because of the injured tissue. This pest can be satisfactorily controlled with a 33 per cent cryolite dust 40 California Agricultural Extension Service [^i^- i^s applied at the rate of 15 to 30 pounds per acre (fig. 30). In the dry, interior valleys a 50 per cent calcium arsenate dust can safely be used at the same dosage. Spraying with lead arsenate, 3 pounds to 100 gallons of water, is also effective. The dusts or sprays should be applied just after thinning, though in heavy infestations the plants may require treatment before thinning time. These ma- terials must not he applied after the lettuce is more than one-third developed. Caterpillars. — Occasionally other larvae besides the semilooper and cut- worms injure lettuce planted near uncultivated fields or poorly tended or abandoned crops. The caterpillars hatch in these neglected areas and when mature, or nearly so, migrate to seek more food or a place to pupate. At such §^^0C '^-m ^ "'-^^^m Sy-^^i" "* -'%ii*'«^t**<2»'„ . ''^mi^^^mj?^^ ^ .Tjmm Fig. 30. — Power duster for controlling leaf -eating insects. As much as 60 acres can be covered in one day. (From Ext. Cir. 105.) times, usually late in the summer or early fall, they attack limited areas of lettuce. If abundant and migrating in a definite direction, they can be trapped in a trench dug with the steep side toward the lettuce. Often in the fall the caterpillars of the acraea moth {Estigmene acraea) appear, eating everything in their path. The woolly-bears or salt-marsh cater- pillars may injure lettuce, especially the young plants. In the immature stages the caterpillars are hairy and gray. Later they turn darker, with yellow broken lines and brownish red hairs. At maturity they are about 2 inches long. They pupate in the fall and spring; and later on in the spring the moths appear. These have a wing expanse of 2 to 2% inches. The females are white except for the abdomen and the hind wings, which are orange. Wings and abdomen are spotted with black. The acraea moth can be controlled with a 40 per cent cryolite dust, or with arsenical dusts and sprays. These materials must not be applied after heads are formed. Barriers and ditches are often used ad- jacent to sugar beets and to other fields from which the caterpillars migrate to lettuce. Grasshoppers and Crickets. — ^Young fall-planted lettuce is occasionally damaged by grasshoppers or by crickets eating the outer leaves. Such damage Production of Head Lettuce 41 may be expected where lettuce fields adjoin alfalfa, pasture, or grain land. These fields should, if possible, be thoroughly disked or harrowed in winter. Alfalfa land that is being broken up for lettuce should be disked and harrowed before plowing, to destroy grasshopper eggs and various stages of insect pests found in the topsoil. As a regular practice, grass and weeds should be burned and the ground worked up along the roads and ditch banks near lettuce fields, for the grasshoppers lay their eggs in the undisturbed surface soil of these areas. As soon as the grasshoppers or crickets appear, irrigate heavily to moisten the ground and to close any cracks. The most satisfactory control is the poison bait recommended for cutworms. Citrus meal or freshly ground citrus fruit should be used in the bait. This bait should be applied as directed for cutworms, preferably in the morning for grasshoppers, or in the evening •.for crickets. The precautions used with this poison are the same as those ex- plained in connection with the control of cutworms. The Coast Lined June Beetle. — In the fall of 1936 the coast lined June beetle {Polyphylla crinita) was found, for the first time, to be infesting lettuce fields on the Pacific Coast. The larvae feed 5 to 6 inches below the soil surface. The taproot is cut oif , and the lower portion of the root is completely eaten, so that the plants wilt and die. The stands are thereby materially reduced. The adult beetle is 1 to 1% inches long, very robust, and brownish gray, with a series of parallel longitudinal white lines on the dorsum. The larvae or "grubs" are large, white with a brown head, and 1% to 2 inches long. They usually occur in sandy soil. The adults appear in the spring, and injury often follows the "grubbing out" of native shrubs when lettuce is grown for the first time. Indications are that they have a three-year cycle. There are no practical means of control in lettuce fields. The False Chinch Bug. — This small insect {Nysius ericae) occurs in the Salinas Valley periodically. To be of any economic importance the pests must appear in countless numbers. They usually injure young lettuce plants before or after thinning, to such an extent that fields often require replanting. Most of the contact insecticides are of little avail. Practically 100 per cent control of the larvae has been secured with a spray containing % per cent pyrethrum and % per cent light summer oil, emulsified in water. This treat- ment has little effect on the adults. Nematodes. — On sandy and light soils the garden nematode may cause con- siderable loss. The pest is easily recognized by the nodule growth that it causes on the roots. When attacked early in growth the lettuce fails to reach maturity, remaining dwarfed and unthrifty. Damage usually occurs first in small areas, which continue to increase in size from year to year. Other than crop rotation, no control is economical. Garden Centipede. — Garden centipedes are white, extremely small creatures scarcely % inch long, which live in the soil. They have numerous legs — unlike insects, which have only three pairs. In the late spring and summer, centipedes feed on the roots of the small lettuce plants, causing brown discolorations and a stunting and disfigurement of the root system. This injury stunts the plant and prevents normal head formation (fig. 31). Treating the soil with carbon disulfide, 2 ounces to every 18 inches of soil area, has proved effective but costly. Recently dichlorethyl ether, used in the same manner as discussed 42 California Agricultural Extension Service [Cm. 128 under wireworms, has been found to reduce the population so that a crop can be raised on badly infested fields. The material should be applied just before or after thinning, when the plants are 1 to 2 inches high, at the rate of 400 to 600 gallons per acre. Irrigating in 24 to 36 hours often helps to diffuse the material through the beds and prevent injury to the plants. Fig. 31. — Upper row: three lettuce plants injured by garden centi- pedes, showing stunting of the plants and injury to the roots. Lower row: three plants from the same area after treatment with carbon disulfide, showing normal growth. Pea Leaf Miner. — Occasionally maggots of the pea leaf miner {Liriomyza flaveola) attack the leaves and petioles of lettuce. Infestations are more severe in the fall, and on small plants. The females feed on the leaves, thrusting the ovipositor into the tissues and feeding on the resulting droplet of cell sap. The resultant punctures are often very numerous on the upper surfaces of the leaves. Eggs are deposited inside the tissues; and the maggots mine the leaves. Production of Head Lettuce 43 dropping to the ground when mature to pupate in the soil. A complete cycle may take only 30 days during the summer and fall. No control is known, but treatment is usually unnecessary because the plants are able to outgrow the injury. Vegetable Weevil. — The vegetable weevil (Listroderes obliquus) injures lettuce, especially along the edges of fields where infestations occur on mallow and pigweeds. The first adults appear in March and feed on the tops of various weeds and vegetables until June, when they enter an estivation period. In September they emerge from estivation and begin their e^^g laying, which usually continues until March and April. The larvae feed on vegetables from October to the following April. On lettuce the most damage occurs during January and February, when the small larvae feed on the center leaves of lettuce and the older larvae feed on the outer leaves and crowns of the plants. Often adults and larvae cut the plants off entirely — an injury similar to that caused by cutworms. When mature the larvae enter the soil, usually near the surface, and form pupae in earthen cells. Control consists in applying a 40 per cent cryolite dust at the rate of 25 to 30 pounds to the acre. Use the dust before — not after — head formation, to avoid leaving a poisonous residue. Diabrotica, or Cucumber Beetle. — This common beetle sometimes damages small lettuce sufficiently to require control measures. The beetles feed on the leaves. Control consists in applying a 40 per cent cryolite dust as in treatment for the vegetable weevil. Never use the dust late in the growth of the crop after the heads have begun to form. Beneficial Insects. — Many insects that occur on lettuce feed on aphids and other injurious insects. Because of their beneficial role, they are mentioned here in order that they may not be confused with the insects that cause actual damage. Syrphid Flies. — Adult syrphid flies are gaudy, striped flies that occur about the lettuce plants when the females are depositing eggs. The larvae commonly are found among aphids, on which they feed voraciously. The larvae are some- what flattened, enlarged at one end and pointed at the other, legless, and usually active. Among aphid colonies the eggs are often observed to be elon- gated and white and (under magnification) to have a finely reticulated surface. Ladybird Beetles. — The adults of these beetles are generally recognized, but the larvae may be mistaken for pests. The common larvae on lettuce are elongate, flat, six legged, and blackish, with orange spots. The yellowish eggs are laid in masses, on end, and are usually found on the leaves near aphid colonies. Both adults and larvae prey on aphids. Green and Brown Lacewings. — The adult green lacewing {Chrysopa cali- fornica is a common species) is a beautiful green insect % to % inch long, with lacelike wings. The larvae are flat, with elongated bodies tapering toward each end. Their greatly enlarged, sicklelike jaws are used to capture, puncture, and suck the juices from aphids and other small insects. The brown lace- wing (Hemerobius sp.) is similar, with brown wings, and of smaller size. Damsel Bug. — This grayish bug (Nahis ferus) is common on lettuce, where it feeds on aphids, leaf hoppers, and other small insects. It is i/4 to %6 inch long, and the nymphs are similar except that the wings are absent. 44 California Agricultural Extension Service [CiR. 128 INJURY BY BIRDS In some sections of the state English sparrows and horned larks often de- stroy many acres of young lettuce. Probably the best control is to place a large number of scarecrows throughout the field in the form of bright pieces of cloth or paper fastened to laths or sticks. The use of a shotgun for a day or two will help to scare the birds away. Scattering chicken feathers has proved suc- cessful in some districts. DISEASES OF LETTUCE AND THEIR CONTROL^ Lettuce is no exception to the rule that with intensive propagation of one crop, one or several diseases gradually become epidemic. In California four Fig. 32. — Comparison of brown-bliglit-resistant and nonresistant lettuce. The two center rows are 100 per cent infected. The outer rows of resistant strains show normal growth. (From Ext. Cir. 60.) diseases have become serious within the past twenty years. Brown blight (cause unknown) has been present in Imperial Valley since 1912. Downy mil- dew (Bremia lactucae) occurs in Imperial Valley, but is usually more prev- alent in the coastal sections. Tipburn (actual cause unknown) occurs annually in most of the important lettuce districts and is thought to be induced by cer- tain unfavorable climatic conditions. Slime (caused by Botrytis spp. and certain bacteria) usually attacks plants previously injured by tipburn. Be- sides these four diseases, minor ones occur annually but do not usually cause heavy losses. Spotted wilt has become serious in two localities and is present in others. Drop (Sclerotinia spp.), mosaic, and yellows have been found in Cali- fornia. Fortunately for lettuce growers, brown blight is now under control through the utilization of resistant strains ; and some progress has been made with tipburn. Strains that resist tipburn are not liable to become slimy ; hence slime control largely depends upon success in breeding tipburn-resistant strains. Brown Blight. — Lettuce brown blight is known to occur only in California ^ This section was prepared bv C. E, Scott, Extension Specialist in Plant Pathology; and by members of the Division of Plant Pathology. Production of Head Lettuce 45 and Arizona. The disease may manifest itself at any time after the seedlings have developed five leaves, usually after thinning. Plants attacked when small assume a somewhat mottled, sickly, yellow color, become rosetted, and never head. If attacked after heading, the entire plant may become yellowish ; and dead, brown irregular streaks and blotches may develop in the leaves. Although brown blight threatened to become so severe in Imperial Valley that lettuce could no longer be profitably grown, the disease is now under control through the efforts of the late Dr. I. C. Jagger of the United States Fig. 33. — Spotted wilt on head lettuce. The brown dead spots and curvature of the leaf are characteristic effects. The smaller leaf shows marginal wilting. (From Ext. Cir. 105.) Department of Agriculture. Dr. Jagger, who began his investigations in Cali- fornia in 1922, distributed numerous strains resistant to brown blight that succeed on infested soil where others are a total loss (fig. 32) . The adaptability of these strains is discussed elsewhere in this circular. Spotted Wilt. — The spotted-wilt virus produces in head lettuce a very de- structive disease characterized by brown dead spots, marginal w^ilting, and curvature of the leaves (fig. 33). One side of a plant is usually affected more than the other. The brown dead spots and streaks are especially conspicuous in the basal portion of the midvein. The leaves of intermediate age are first to show symptoms, but soon the dead spots occur on all the younger leaves, in- cluding those in the heart. Young plants flatten out and die (fig. 34) a week or two after symptoms first appear. Plants infected late may show no external symptoms ; but the internal leaves are affected, and the freshly cut surface of the stem usually shows some brown discoloration at one side. In advanced cases a slimy soft rot of the internal leaves may be present. This disease is extremely serious in a few localities in the fall crop and oc- curs elsewhere to a limited extent. It is at its worst near San Pablo and in the Salinas-Watsonville district. It is known as the San Pablo disease. 46 California Agricultural Extension Service [Cm. 128 The virus that causes spotted v^ilt is carried by thrips. It occurs also in other crops such as tomatoes, peppers, broad beans, chicory, spinach, and celery, and in numerous ornamentals such as nasturtium, aster, cineraria, calla lily, tuberous begonia, gloxinia, calceolaria, delphinium, dahlia, petunia, zinnia, and sweet pea. It also occurs in nettle, Jimson vreed, mallow, and chickweed. Infected plants of any species are potential sources of infection for head let- Fig. 34. — A young lettuce plant affected with spotted wilt. (From. Ext. Cir. 105.) tuce. The disease seems to be worse in lettuce fields adjacent to old lettuce planting's, alfalfa, and ornamentals. The disease is usually not prevalent in the early spring crop, probably be- cause the winter rains greatly reduce the thrips population. No control is known. Spraying to kill thrips has not been effective. Numerous varieties of lettuce, including Dr. Jagger's brown-blight-resistant strains, have been tested ; but all proved susceptible, as was also wild lettuce. Brown blight and spotted wilt are similar in that dead spots in the leaves and stunting are symptoms. Plants infected with spotted wilt while young show a curling of the petioles, a curvature of the leaves, and a wilting of the most severely affected leaves, followed rather promptly by collapse of the en- tire plant. In contrast, young plants infected with brown blight show only the dead spots in the leaves and the yellow and stunted condition. These dis- Production of Head Lettuce 47 eases are particularly difficult to distin^ish when the plants are infected late in life, the dead spots in the leaves of the mature heads being the only con- spicuous symptom of each disease. Downy Mildew. — In the coastal sections downy mildew {Bremia lactucae) is usually present every season ; in Imperial Valley, only occasionally when there is an unusual amount of rain and dew. Certain favoring environmental conditions such as relatively low air temperature and periods of cloudy or r^'**^ %-^ Fig. 35. — Downy mildew on lettuce leaf. (From Ext. Cir. 105.) rainy weather or heavy dews determine the likelihood of there being injury from this disease. Evidence of its presence is found on either the leaves or seedstalk, and it may occur in any stage of plant growth. On the outer leaves, yellowish or light-green areas develop on the upper surface, usually delimited by the larger veins ; the corresponding lower surface is usually covered by a dense, white, fluffy growth of the fungus (fig. 35), which produces the spores capable of infecting other leaves. Infected spots may enlarge or several grow together until much of the leaf may be killed. Under California conditions infected plants seldom die prematurely, but continue to grow more or less salable heads. Irregular white blotches may appear on infected seedstalks. When infected heads are shipped under refrigeration to distant markets, decay may follow mildew injury. The fungus produces resting spores, which may persist in the soil for a considerable period. Tiphurn. — Tipburn is a nonparasitic disease generally prevalent when 48 California Agricultural Extension Service [CiR. 128 warm, bright days follow periods of foggy or cloudy weather. It is somewhat more severe in poorly drained areas. It is characterized by dark-brown dis- coloration near the margin of the leaves (fig. 36). The development of these small dead spots seems to prevent the passage of water to edges of the leaf ; and that portion outside the spots becomes wilted and yellow and soon dies, # flHI^ "- MC/ ,w! P% .^ Bjj^^^^^'* '** 1W^^ r^ -^^P^^B ^^^^^^^^^^1^^' ^^^^j^S^H| } ' \^0^' . ^^^^PM^^HH^H^^^^^^^p' \ > / ? Fig. 36. — Tipburn on head lettuce. This appears as dark areas on the margins of the head leaves. (From Ext. Cir. 105.) leaving a dead, brown strip around the edge of the leaf. New York 515, Growers' E-4, and Great Lakes are somewhat resistant, but not immune, to tipburn. Slime. — Lettuce heads affected with tipburn often develop a slimy soft rot in the field or during transit and storage (fig. 37) . This condition is caused by a number of different bacteria. Heads subjected to frost injury or excessive irrigation or recently infected with spotted wilt often develop slime. Slime develops most rapidly during warm, damp periods, particularly on warm nights ; it is checked by cold, dry weather. In the field, control usually resolves itself into prevention of tipburn. Strains resistant to tipburn are like- Production of Head Lettuce 49 wise less subject to slime. Where slime is a serious factor, plantings should be regulated so that the crop will mature in cool weather. The use of high, well- drained beds aids in preventing slime. Harvested heads showing any indica- tion of slime should not be packed for long-distance shipment, for slime is apt to spread during transit. Fig. 37. — Slime on head lettuce. The affected portion is brownish black. (From Ext. Oir. 105.) Anthracnose. — This disease is characterized by numerous small, dead, brown spots on the older leaves, especially at the base on the midrib, and by a shot-hole effect when the centers of the spots fall out. The infected older leaves may dry up, fall off, and blow away, often leaving the plant with a long bare stem and a small white head. The disease is caused by a fungus {Marssonina panattoniana) , which is spread by rain. Anthracnose is prevalent only in crops exposed to long rainy periods. Drop. — In this disease, first the older leaves wilt and fall flat on the ground; 50 California Agricultural Extension Service [Cm. 128 then follow wilting and death of the entire plant. This condition results from a soft watery rot of the stem at the ground line caused by the fungus Sclero- tinid sclerotiorum (or the closely related ;8^. minor). When a wilted plant is overturned, a cottony growth of white mold is usually visible on the rotted stem, and occasionally black sclerotia are present. The disease is favored by Fig. 38. — Head lettuce affected with big vein. (From Ext. Cir. 105.) wet soil and low temperatures. This fungus may also cause a watery soft rot of the heads in transit. Yellows. — Affected plants are stunted and yellow, and remain spread out instead of heading, the young leaves being yellow, elongated, cupped outward, and variously twisted. The diseased plants are scattered through the field in varying but usually low precentages. This disease is caused by the California- aster-yellows virus, which is transmitted by certain species of leafhoppers, especially the aster leaf hopper. The same virus affects celery, carrots, parsley, parsnips, and numerous ornamentals and weeds, in which it is variously char- acterized by dwarfing, yellowing, twisting of petioles, proliferation of shoots, and greening in the flowers. It occurs in aster, chrysanthemum, zinnia, mari- gold, strawflower, godetia, buttercup, and California poppy. June Yellows. — This disease occurs in the early summer crop. The outer leaves are light yellow or white between the veins and rather thick and leathery. Unlike the leaves in virus yellows, the younger leaves in June yel- lows are green; and often whole fields are more or less uniformly affected. This Production of Head Lettuce 51 disease appears to result from unfavorable soil or climatic conditions prevail- ing at some point in the life of the crop. Big Vein, — In this disease the veins are thickened and translucent, and the leaves are tough and crinkled, the green areas between the conspicuous white veins bulging upward (fig. 38) . Affected plants are scattered through the field, and the disease is more abundant in crops maturing during the colder part of the year. Big-vein plants continue to grow and head, but are reduced in size and quality. The cause is apparently in the soil, since experimental steriliza- tion of the soil prevents development of the disease. Gray Mold Rot. — This may occur as a side rot in which one side of the head is covered with a velvety gray mold (Botrytis cinerea) , which has invaded and rotted some of the leaves. Often this fungus invades the stem through an old weakened leaf. Gray mold rot may also follow tipburn or spotted wilt. SHOOTING TO SEED PREMATURELY Some loss is always caused by the plants' going to seed prematurely. This trouble usually results from unfavorable soil, moisture, and climate. It is difficult to determine, in all cases, the contributing factor or factors. The best lettuce seed available should be purchased ; but, above all, growers should not attempt to grow lettuce under high -temperature conditions, on steep slopes, or on light sandy or gravelly soils not retentive of moisture. Some varieties bolt to seed much more readily than do others. ACKNOWLEDGMENTS For help in preparing this circular the authors are indebted among others to R. Albaugh, E. S. Fry, M. W. Gardner, L. G. Goar, P. A. Kantor, 0. A. Lorenz, W. H. Nixon, J. B. Scherrer, G. W. Scott, L. E. Weaver, and T. W. Whitaker. 22m-9, '44(1025)