i^ 1% Division of Agricultural $ c i e rt c e $ UNIVERSITY Of CALIFORNIA Sweet Potato Production and Handling in California CALIFORNIA AGRICULTURAL Experiment Station Extension $ e r v ic e CIRCULAR 431 +Jo\V many of these baskets will YOU get th?s year? In 1952 about 10,000 acres of California soil produced an average yield of 1 15 fifty-pound baskets per acre. Each basket shown here equals 25 real baskets of sweet potatoes. Good cultural practices . • § can increase the state average yield to 200 or more baskets per acre. -Here's how you cah improve your yield Follow a seed-improvement program Transplant in moist ridges and maintain good stands Irrigate adequately and fertilize early Control diseases in field and storage Use disease-free seed, cure and store properly, sort and treat seed after storage, use new clean plant-bed sites, rotate fields. Store seed (and crop) carefully Sort out diseased and cull roots, move crop directly to storage, cure at 85° F and high humidity, store at 55° F and avoid chilling. WHERE TO FIND IT The Varieties Grown 7 Propagation of This Year's Crop 9 Transplanting and Managing the Crop 16 Pests and Diseases of Field and Storehouse 19 Harvesting and Marketing the Crop 26 Storage 30 Seed Improvement Program 35 FRONT COVER PHOTO: Picking up sweet potatoes after digging, and placing them directly into storage boxes. In most instances this method will be preferred to piling the potatoes for snapping. Hill selection of sweet potatoes— a desirable step toward getting disease-free, high yielding seed that is true to type. For a further discussion of this and the other steps involved in increasing your yield, see pages 35 et seq. THE AUTHORS: P. A. Minges is Extension Vegetable Crops Specialist, Davis; L. L. Morris is Associate Professor of Vegetable Crops and Associate Olericulturist in the Experi- ment Station, Davis. June, 1953 SWEET POTATO PRODUCTION and HANDLING IN CALIFORNIA P. A. MINGES L. L. MORRIS California grows about 2 per cent of the sweet potatoes produced in the United States — approximately 11,000 acres an- nually. In 1952, the California crop was valued at about five million dollars. Most of the crop is produced on the sandy soils of the San Joaquin Valley and south- ern California, where yields are about equal to the United States average. Production costs of this warm-season root crop are relatively high even though the crop can be grown on rather inexpen- sive land. They involve overwinter stor- age of "seed" potatoes, maintenance of plant beds, and considerable labor for transplanting and harvesting. In fact, costs are generally higher than for other vegetable crops that can be successfully grown on sandy soil, such as water- melons, cantaloupes, or sweet corn. Ob- taining a supply of good seed is an im- portant obstacle facing anyone who considers growing sweet potatoes. This is a crop where experience is very advantageous. On the other hand, the opportunities for improving practices to increase yields and quality are probably as great as for any other vegetable crop grown in California. In general, the acre- age per farm is small, the crop often being handled as a family enterprise. In the San Joaquin Valley, sweet-potato production is frequently associated with dairying: the sandy soil, the size of the farm, the desirability of crop rotation, compatible labor requirements, the use of unmarketable potatoes as feed, the uti- lization of the manure all favor such an arrangement. Much of the crop is marketed soon after harvest during the summer and fall, mostly on the Pacific Coast. A small per- centage is processed as whole potatoes, as baby food, and in soup. The crop may have possibilities as a commercial source of starch and as livestock feed in a dehy- drated form. After the first of the year, potatoes from the southern states begin moving into California markets. Sweet potatoes are well adapted to farm storage, and more of the California crop could be marketed during the winter and early spring if adequate storage fa- cilities were available. Since overwinter "seed" storage is also necessary to the sweet-potato farmer, this circular treats fully the problems of storage and plans for handling them. Climate and soil requirements Sweet potatoes thrive during hot days with maximum sunshine and warm nights. Young plants are easily injured by light frosts, and their growth is re- tarded by cool soil temperatures or cold nights. In the fall the vines are killed by moderate frosts, and the roots may be in- jured by chilling during heavy frosts or prolonged periods of cold, wet weather. [5 A warm growing season of 4 to 5 months is necessary for good production. Thus only the warmer areas of the state are suitable for growing sweet potatoes. Select well-drained, sandy soils or light, sandy loams for this crop. The type of soil influences the yield and ap- pearance of sweet potatoes. Roots are of best color and smoothness when grown on very light sands, but yields are usually better on the light, sandy loams. On heavy soils the potatoes are rough, often veiny, misshapen, and of poor color, all of which greatly reduces their market value and acceptance. Sweet potatoes are rela- tively sensitive to salt or alkali condi- tions. Areas of production Commercial. Merced County, with approximately 4,000 acres centered in the Atwater district, is the leading sweet- potato county. Usually from 1,500 to 2,000 acres are planted in the Manteca district of San Joaquin County. Other sweet-potato districts are Bloomington, in San Bernardino County; Turlock, in Stanislaus County; Costa Mesa and West- minster, in Orange County; Coachella Valley, in Riverside County; and Shafter, in Kern County. Scattered acreages are planted in Fresno, San Diego, Tulare, and Los Angeles counties. Occasionally plant- ings are made in the Sacramento Valley. Home garden. Sweet potatoes are most popular as a garden vegetable in the San Joaquin Valley and in southern Cali- fornia. They can be grown in other areas where soil and climatic conditions are suitable. "Sweets" and "yams" The edible storage roots develop in a bunch around the main stem, making up what is called a "hill." Development con- tinues until limited by cool weather, frost, lack of water, or removal of the tops. Thus the crop does not have a growing period of definite length and may be harvested when the roots reach a market- able size. Sweet potatoes are classified into moist and dry types. The moist are known popu- larly as "yams" and the dry as "sweet potatoes" or "sweets," although both be- long to the same species in the morning- glory family. Cinnamon Vine, a true yam belonging to another family, is used for food in tropical countries but in the United States is grown only as an orna- mental. All sweet-potato plants have creeping vines, some as long as 12 feet. Certain varieties with very short vines are termed "bunch type." Sweet potatoes seldom flower in California. The term "seed" re- fers to roots saved for propagating pur- poses. Mutations. Sweet potatoes have a marked tendency to mutate or "sport." An example of a mutation for skin color is shown on page 8. This tendency, while providing a means of originating new varieties, accounts for much of the variation in commercial varieties. Con- stant selection is necessary to keep varie- ties uniform in skin color, flesh color, and general type. Mutations, though seen most frequently in the roots, also occur in the vine. [6] VARIETIES . . . These are the ones most commonly planted by California sweet potato producers Varieties of the moist or "yam" type are the most popular on the Pacific Coast, although there is always some demand for the dry type. Porto Rico and its strains The regular Porto Rico has a long, vigorous vine with coarse, reddish-purple stems. Leaves are shouldered, large, and green except for a purple stain at the base of the blade and on the veins. The potatoes have smooth, pale, rose-colored skin and vary in shape from fusiform (long with a large middle and tapering to both ends) to globular, as shown on page 8. Because potatoes tend to set some distance from the stem, digging is often difficult. The flesh of the potatoes is orange-yellow to salmon and when cooked is moist and very sweet. If properly cured and stored it keeps well. Because of mutations and the lack of selection, seed stocks used in California are extremely variable. Numerous strains of this variety have also developed through an occasional selection program or by importation of seed from other states. Including these strains Porto Rico is the main variety grown in California. At present there is confusion in the nomenclature of the Porto Rico strains. A strain very similar to the regular Porto Rico, except for a more uniform and deeper red skin color, is generally known as Improved Red or Improved Red Porto Rico, though some growers call it just Porto Rico. Occasionally it is erroneously High yielding hills of the three main varieties grown in California. Left, Porto Rico; center, Yellow Jersey; right, Hawaiian. The Porto Rico includes several red-skinned strains. Potatoes of the Hawaiian variety are often longer and more slender than those shown here. [7] s Variations in root shape of Porto Rico. The intermediate shapes are most desirable for the market. called "Key West." A second strain char- acterized by a reddish-purple skin has been called "Key West" and "Velvet." A mutation of similar description has been termed Porto Morado by J. C. Mil- ler, of the Louisiana Agricultural Experi- ment Station. A third strain has a dark reddish-purple skin similar to the second strain, but in addition has considerable White ski n mutations on potatoes of the Porto Rico variety. purple pigment just beneath the skin and purple streaks in the flesh. This strain is usually known as Velvet but has been termed Double Red and also "Key West." The name "Key West" is poor for any of the above strains because of confusion with the name Key West Yam, which is applied to an old variety formerly grown in the southern states. It is similar to the regular Porto Rico except that its flesh is cream to yellow in color. The Bunch Porto Rico is similar to the regular Porto Rico, but the vines are shorter and it is considered lower in yielding capacity. Jersey group The Yellow Jersey, sometimes called Little-Stem Jersey, is the principal va- riety of the dry type grown in California and is second to Porto Rico in impor- tance. The vines are smaller, with slender, green, hairy stems and small, heart- shaped, green leaves. Roots are light golden-yellow, often russeted, quite regu- larly uniform in shape, and smooth or sometimes veined. The potatoes bunch well in the hill. Flesh is yellow and when baked is very dry, firm, and fairly sweet. This variety is only a fair "keeper" under California conditions. The Maryland Golden, a variety of the Big-Stem Jersey type, is sometimes grown for the early fall market because it [8 reaches market size fairly early. It has a golden skin and orange flesh and is more moist when cooked than Yellow Jersey. It often cracks badly if harvest is delayed until late fall. Hawaiian This variety was introduced into Mer- ced County about 1920 by Joseph Lial, a sweet-potato grower in that district. Mr. Lial has stated that he brought it from one of the Pacific Islands shorly after the Spanish-American war and maintained it in his garden near Freeport, California, until he moved to the Atwater area. The term "Kanaka" is also applied to this variety. The vine is short and bunched, with deeply lobed leaves and has a reddish tinge at the base of the leaves and on the stems. The potatoes are usually russeted, dull yellow to cream in color, and vary in shape from fusiform to short chunky. The flesh is light yellow and at harvest cooks very dry, but after several months' storage it cooks to a moist texture of rather good quality. It is an excellent keeper and is grown to some extent in Merced County to store for the late winter market. Newer varieties The Ranger is a new high-yielding variety, introduced into California for canning purposes. The skin of the pota- toes is light rose, but the flesh is a deeper orange than Porto Rico or its strains. It is very moist and of excellent quality when cooked. The variety has been se- lected for large-sized roots, which are desirable for canning but not for market use. Later planting, earlier harvesting, or closer spacing might help to obtain better market sizes. The light skin color has also adversely affected its market acceptance. The Heartogold (formerly L-138) is a recent introduction by the Louisiana Experiment Station. The skin is nearly white, and the roots are rather poorly shaped; but the flesh is a deep orange that is very moist, tender, and of excel- lent quality when cooked. In limited tests in California it has consistently out- yielded Porto Rico and Yellow Jersey. Despite its high quality, California mar- kets probably will be slow to accept this variety because of its light skin color. Several selections developed by the U.S.D.A. and tested in California during recent years have exhibited high yielding capacities, and certain ones appear desir- able for either market or processing. One or more may be named and introduced. Objectives of current variety-improve- ment programs include resistance to stem rot (wilt), good internal flesh color, and fine quality, all combined with high yielding ability. PROPAGATION . . . getting ready for the current year's planting • . . four kinds of plant beds Sweet potatoes are propagated from "seed" roots which, when placed in warm plant beds, produce slips (also called sprouts, plants, or transplants). When the slips reach a suitable size they are transplanted into the field. This is the only method of propagation used in Cali- fornia. In some southern states, the early plant- ings are established by the above method, but for later fields vine cuttings taken from the early fields are used in place of the plant-bed slips. The use of vine cut- tings offers definite advantages for prop- agating the "seed" crop in California and may have possibilities for establishing late commercial fields in areas with a fairly long, warm growing season. For further description of this method see the discussion on pages 35 to 38. [9] Planting small pieces of roots directly in the field (comparable to the practice for Irish potato tubers) has been tested but does not appear practical. Seed stock The usual practice of growers is to use field-run seed consisting of very small and unmarketable potatoes. Instead, the seed roots should be carefully selected in the field at time of harvest. Good seed is free of all diseases and nematodes, true to type, uniform in color, and capable of good sprout production. Roots from % inch to 2 inches in diameter produce bet- ter plants than smaller ones. Tests have shown that cured and properly stored seed has a higher plant-producing capac- ity than seed that has been poorly handled or stored under adverse conditions. From 300 to 500 pounds of seed are needed to produce the plants for each acre planted. The actual requirement will de- pend on the plant-producing capacity of the seed, the conditions provided in the plant bed, the number of pullings to be given, the variety, and the plant spacing to be used in the field. Good quality Porto Rico or Yellow Jersey seed should pro- duce 1,500 to 2,500 desirable plants per 100 pounds of seed on the first pulling. Maryland Golden produces fewer sprouts than Porto Rico. However, different strains within the same variety may vary in their sprout-producing capacity. Few sources of good seed exist in Cali- fornia at present. Sometimes good seed can be imported from other states, but often imported seed is little better than local stock. It is expensive and may rot badly before it can be bedded. Also there is the danger of introducing new diseases or pests. Until reliable seed sources de- velop, growers will have to save their own seed and conduct individual improve- ment programs. New growers should get the best seed available and immediately begin their own improvement program. Four types of plant beds are used Since it is usually necessary to start plant production under adverse climatic conditions, special beds are needed to provide the moisture and heat for satis- factory sprouting and plant growth. Open-heated beds are provided with a source of bottom heat but have no frames or covers. Additional protection is often afforded by windbreaks and by having black paper on the soil before emergence of the plants. Because of the relatively mild weather in most sweet- potato districts, this commonest type of Electrically heated, open plant beds of sweet potatoes in the Atwater district of California. r io i bed is usually satisfactory for starting the plants. However, cloth covers should be available on frosty nights. Covered-heated beds may be desir- able for extra early plant production or where frost protection is needed for the plants. Frames are usually made of wood and covered by sash, with glass or glass substitutes. Open-unheated beds similar to those shown here, but without bottom heat, can be used for late beds or where weather conditions are warm and mild. Covered-unheated beds (or cold- frames) have also been used under mild conditions. Methods of heating With heated beds, heat from the sun is supplemented by bottom heat provided by electricity, hot water, hot air, or fer- menting organic matter such as animal manures or cotton hulls. The last method is the most common because of low initial cost but has the disadvantage that there is little control of the heat. Of the organic materials, fresh, strawy horse manure gives the best results. Since this is usually not available, cow manure, cotton hulls, straw, or similar organic materials are commonly used and may be Covered plant beds may be desirable in some cases. Note electric cable connections. mixed together. Avoid manure from places where cull sweet potatoes have been fed to livestock. Place the material that is to furnish heat in a pit below the plant bed, and thoroughly wet and tramp it down. Pro- vide at least 8 inches of compacted mate- rial and cover it with about 3 inches of sand upon which the roots are to be bedded. Unless you can accurately predict the heating characteristics of the material used, wait a few days before bedding the seed. A short period is needed for the material to start heating, and under cer- tain conditions an undesirably high bed temperature is then produced for a brief time. The delay in bedding is therefore desirable to obtain warm soil tempera- tures and, in some cases, to avoid the short period of undesirably hot tempera- tures. A more common problem is getting the material to heat adequately. The addition of a light top dressing of a nitrogen fer- tilizer directly to the material may hasten decomposition and improve heat produc- tion. The use of covers with manure- heated beds will usually improve the heat- ing efficiency. Accurate observation of temperatures is essential with beds of this type where no automatic control is possible. Electric heating cables, although ex- pensive, are probably the most satisfac- tory over a long period because of easy installation, accurate temperature control (see p. 15 for optimum sprouting tem- perature), and long life under proper care. An electrically heated bed can be expected to produce the maximum num- ber of plants per unit area. Loop the lead- coated cables back and forth at 9-inch intervals and then cover with 3 inches of sand. The temperature is regulated through a thermostat bulb placed at the level of the potatoes and in a spot repre- sentative of the entire bed. Cables should be used in 60-foot lengths (on 110 volts) or 120-foot lengths (on 220 volts) , giving [in '& s\\\\\\ V ^l.ll 1 j' ' to 0) o "> a > o E CD c a o r u V V/vV Y^vV S'5'' rN /a /> A a connected load of 400 and 800 watts, respectively. The 120-foot cable will heat about 90 square feet of hotbed space. One thermostat may accommodate several units of cable. The cost of heating will depend on the weather, type of bed, and management of the bed. A rough estimate of electrical consumption for covered beds under Cali- fornia conditions is % to 1% kilowatt hours per square yard of bed space per day, and for open beds 1 to 3 kilowatt hours. Most of the heating is required during the first part of the plant-bed sea- son; in general little heat will be needed after the plants are up. Hot water also has been successfully used to provide bottom heat in sweet- potato plant beds. Where a natural sup- ply of warm water is available, this method appears attractive. One-inch pipes may be placed 12 to 18 inches apart and covered with 4 to 5 inches of sand. Thermostatic control of the water circu- lation is desirable. Plant beds can be heated by buried flues, which conduct hot air and gases under the bed from an underground fur- nace. This system requires careful atten- tion to maintain desirable temperatures uniformly throughout the bed. To construct beds Plant beds are made 6 to 10 feet wide and of varying length. They should be excavated to a depth of 12 to 14 inches for those heated with organic materials, and to a depth of 6 inches for electrically heated beds. For open beds, little other construction is needed. Side walls made of one-inch boards will prevent the cav- ing-in of the soil and will help confine the heat to the plant-bed area. There should be enough space between parallel beds to allow for the ditches and dikes necessary to take care of surface drainage and runoff. Ditches alongside the beds are used by some growers to carry water for irrigation. For covered beds frames of either wood or concrete, such as are commonly used for other plant-growing structures, are necessary to support the covers. A half- gable-type frame placed with the slope to the south is recommended. The low wall should rise 10 to 12 inches above the plant-bed surface and the high wall 2 to 3 feet. The spacing of rafters or cross bars will depend on the width of the bed and the type of cover used. Glass or glass-substitute sash are the most effective covers because they trans- mit a maximum amount of the solar heat. Although seldom used in California, they would be desirable for covering extra early hotbeds or for coldframes not using supplemental heat. Cloth covers made of white, light-grade muslin sheeting are sometimes used, espe- cially if there is a danger of frost after plant emergence. These covers help to raise the air temperature in the bed on sunny days and can furnish protection against light frosts. However, they have the disadvantage of excluding a consid- erable amount of sunlight and solar heat. If the covers are removed on sunny days the soil will heat better, and there will be less tendency toward leggy, succulent plants. Muslin covers are much cheaper than sash and are more convenient to handle and store when not in use. You can find more information on plant beds for sweet potatoes in U.S.D.A. Farmers' Bulletin No. 2020, "Commer- cial Growing and Harvesting of Sweet Potatoes." Establishing beds Locate plant beds in well-drained areas free of disease and such pests as nema- todes or wireworms. Choose sandy soils, or at least use sand in the bed. The area should be fully exposed to the sun and if possible protected from cold winds. It is important to put beds on new ground each year. The common practice of using the same site year after year is very poor. Diseases and pests build up rapidly in old plant beds and in sweet- [13] potato fields. If it is necessary to use a site a second year, clean out the used beds immediately after the plant-growing season, and haul all old sweet-potato roots from the area. Soil fumigation of the plant-bed area may be used to reduce the population of nematodes and wire- worms, but cannot be relied upon to erad- icate nematodes completely nor to control the common sweet-potato diseases. When to plant. Most plant beds are established during March in the princi- pal sweet-potato districts. Under favor- able propagating conditions about 5 weeks are required from the time of bed- ding to produce suitable plants. With poor heat in the beds or with cloudy, cool weather, from 6 to 10 weeks may be necessary. Preparation of the beds should start 1 to 2 weeks before time for bed- ding. In the desert areas bedding time will depend on the proposed planting date. First prepare the seed. When the beds are ready, carefully sort the seed stock, on removal from storage, to elimi- nate all decayed roots and any showing signs of disease or nematodes. The roots should then be treated (see page 23) to kill disease spores carried on the po- tatoes and to reduce the danger of rot- ting in the plant bed. Bed the seed imme- diately after treatment. If seed is removed from storage and sorted a few days before bedding, seed treatment at the time of removal will help reduce decay. Then bed the seed. Arrange seed po- tatoes in the beds as closely as possible without touching. Approximately 40 square feet of bed area is required per 100 pounds of the usual small-sized seed. Large potatoes will require less space. Some growers find it useful to lay 2-inch- mesh poultry wire over the potatoes to re- duce disturbance of the seed roots at pulling time. Then cover them with sand to a depth of at least 2 x /2 inches. Spread the sand carefully to obtain uniform depth and to avoid compaction of the sand in spots. The beds should then be watered to insure good moisture. Finally, cover the surface. Black building paper placed directly on the sur- face of the bed has definite advantages for both open and glass-covered beds. It can aid in maintenance of desirable tem- peratures by reducing loss of supplemen- tal heat and by increasing the absorption m The usual procedure for placing seed in beds— the potatoes are placed by hand so they do not touch each other. [14] of solar heat. It also holds moisture in the soil. Put this cover over the bed imme- diately after bedding, and remove it when the plants start to emerge. If it is used on beds that are otherwise open, a slight slope of the bed surface will aid in shed- ding rain. Management of beds Fertilizing is seldom necessary in beds heated with manure or organic materials. In other types some nitrogen may prove beneficial if the plants turn yellowish or appear stunted. The nitrogen can be added by dissolving 1 pound of nitrate of soda or ammonium sulfate in 4 gal- lons of water and applying this amount to 500 square feet of plant bed by means of a sprinkling can. Wash the fertilizer solution off the foliage with fresh water to avoid burning the leaves. Weeds sel- dom present a serious problem in the plant bed, and those emerging ahead of the sweet potatoes can be destroyed by flaming. If you use cloth or glass covers, remove them during the day after the plants emerge to encourage normal, stocky growth. During the last week or two before pulling the plants, leave the covers off at night to toughen the young plants. Temperatures. Optimum soil temper- atures for sprout development are 75° to 85° F. At lower temperatures sprout growth is slow, practically stopping at 60° F or below. If the plant-bed tempera- ture goes above 90° F for even a short period, the potatoes may be injured and may then rot quickly. Overheated beds can be cooled by watering. You will need an accurate soil thermometer to check the beds and to set the thermostats for electrically heated beds. After most of the plants are up, soil temperatures be- come less important. Watering. Keep the soil moist until the plants are up and well established. Add only enough water to moisten the soil a little below the depth of the pota- toes. Overwatering will tend to keep the Most of the sprouts grow from the stem end. Transplant on right is a good one— it is stocky and has well formed leaves. beds cooler. Overhead sprinkling is rec- ommended. Toward the end of the plant- growing period, withhold water to harden the plants. Pulling plants A desirable plant for transplanting is well hardened, stocky — with 6 to 8 inches of top and 2% to 4 inches of stem below the ground level — and free of diseases and pests. The plants should have 4 to 5 leaves that have changed from light to dark green. •i 4 •" ■f ^S| |gg| . t # Range in size of transplants. The two center plants are most desirable. [15] Selective pulling. Since some plants reach the transplanting stage ahead of others, it is often necessary to go over the beds 2 or 3 times. Water the bed well, then remove the plants that are ready using a side pull rather than a straight upward jerk. Do not disturb the imma- ture plants and the seed roots more than necessary. If you pull several plants in a bunch, separate them so that men doing the transplanting will not be slowed down. Discard any cull plants at the time of pulling. Finally, water the bed again to settle the soil. Place plants upright in boxes or bas- kets, and cover them with wet burlap sacks for hauling them to the field. They should be planted as soon as possible to insure good stands. Holding or freshen- ing plants in sumps or basins is undesir- able because of the hazard of spreading stem rot and other diseases. Trimming or pruning the foliage or the roots is not recommended. TRANSPLANTING . . . into prepared fields . . . spacing plants properly . . . and MANAGING the crop The main steps in preparing fields for planting are (1) turning under crop resi- dues and (2) forming the ridges. In some cases land leveling may be needed to permit efficient irrigation of the crop. If there is any possibility that root-knot nematodes or wireworms are present in serious numbers, fumigate the soil 2 to 3 weeks before planting. To insure a good job of fumigating, the field surface should be smooth, the soil in good tilth, and any Ridges for transplants are commonly spaced 38 inches apart— 10 to 15 inches high after planting. trash well pulverized or decomposed. Work the soil no more than essential, as excessive tillage causes unnecessary loss of moisture. Sweet potatoes are grown on ridges 8 to 15 inches high and 36 to 42 inches from center to center. A common spacing is 38 inches. Flat planting is unsatisfac- tory because of difficulty in digging. Field tests in other states have shown that yields are better on ridges 8 to 15 inches high than on lower ridges. Ridges are made with a lister. When the field has been fumigated, or if there is little trash, one listing may be the only soil preparation needed. If a cover crop (see page 18) is to be worked in, the field should be disked, plowed, perhaps floated, and then listed. The fertilizer may be applied in the ridge while listing, but take care to place it deep enough to be below the roots of the new transplants. To allow time for settling, which will pro- vide a firm bed for planting, form the ridges a week or more in advance. If ridges are dry, it is advisable to pre- irrigate to insure good moisture in the ridge at time of planting. Transplanting Sweet-potato plants are set in the field by use of transplanting machines or by hand. From 3 to 5 acres can be planted in [16 a day with a 2-row machine. It is impor- tant to adjust the packers on the trans- planters carefully to insure good firming of the soil around the roots of the trans- plants. With the transplanting machines water is applied at the rate of about 1 pint per plant. If the ridges are moist at the time of planting, the water supplied with the transplanters will usually be adequate for some time. With hand planting or if ridges are dry at the time of transplant- ing with machines, apply an irrigation immediately following the planting. Since high yields depend upon good stands, make every effort to insure good survival of the plants in the field. Replanting a field by filling in the missing plants seldom proves satisfactory unless a high percentage of the plants fail to survive. In this case, it may be most practical to plant the entire field a second time. Starter solutions, made by adding fer- tilizer to the water used at transplant- ing, may give the plants a faster start but have not been proved to increase yields. Transplanting into the field begins in late April and ends by mid-June in most districts. In general, the earlier plantings will give better yields because of a longer Transplanting, watering, and firming the soil around the roots in one operation. Immediate irrigation may be desirable. growing season. In the Coachella Valley the main planting period is February 15 to April 15, though earlier and later plantings are sometimes made. Plant spacing. Usual spacing for the Porto Rico variety is 10 to 14 inches be- tween plants in the row, and for Jerseys from 12 to 18 inches. Close spacings tend to give higher yields per acre, reduce the number of jumbos, and in the case of Porto Rico encourage bunching of the roots in the hill. The wider spacings are A field of Porto Rico variety near Manteca. The strips of grain afford some wind protection during the early growing season. [17 used for plantings that will be harvested early because the potatoes will size up more quickly. From 10,000 to 15,000 plants are required per acre. Cover crops Because the sandy soils used for sweet potatoes are subject to blowing when bare, it is often advisable to seed a crop of rye or other grain in the fall to hold the soil during the winter. Under most conditions the cover crop should be worked into the soil about one month ahead of the expected planting date. In some areas where young plants are likely to be damaged by strong spring winds, narrow strips of grain can be left at in- tervals of 20 to 40 feet at the time the field is prepared for planting. These strips will afford protection for a month or so after planting and can be plowed in as the grain begins to mature. Fertilizers Although sweet potatoes are grown on relatively infertile sandy soils, the fer- tilizer requirements of this crop are not great. On some soils, yields have not been increased by fertilizer applications, prob- ably because of good previous soil man- agement or heavy use of fertilizers on previous crops. Where fertilizers have im- proved yields, nitrogen seems to be the principal element needed. Tests indicate that 40 to 60 pounds of nitrogen per acre is adequate in the San Joaquin Valley. This amount can be sup- plied in 200 to 300 pounds of ammonium sulfate. In southern California from 60 to 100 pounds of nitrogen may be re- quired. On a few of the very poor soils in Merced County, a complete fertilizer may increase yields above those obtained from nitrogen alone. If a complete fer- tilizer is to be tried, an 8-10-12 mix at the rate of 750 pounds per acre is sug- gested. Apply the fertilizer in the ridges at the time of listing, or as a side dressing at planting or shortly thereafter. If you use ammonia gas, apply it in the first or sec- ond irrigation. In applying fertilizer, take care to place it where it will be in moist soil throughout the growing season, yet where leaching by irrigation water will not be excessive. A band placed at approximately the same level as the water in the irrigation furrow and 2 to 6 inches from the transplant will usually meet these requirements. Cultivation Cultivation is usually needed to control weeds during the early part of the grow- ing season and maintain the ridges. To reduce the amount of disturbance to the vines it should always be made in the same direction on a given row. With proper attachments on the cultivator the early vines can be trained along the ridges to facilitate irrigation. Cultivation should cease midway through the growing sea- son. Usually one or more hand hoeings are needed, and at times some hand weed- ing is necessary after cultivation is stopped. Occasionally sweet-potato vines take root at the nodes, where in some cases small potatoes develop. It is questionable that such rooting is harmful, and fur- thermore it is kept at a minimum by the usual cultivation practices. Irrigation Sweet potatoes develop an extensive root system, penetrating on good, deep soils to a depth of 5 or more feet. At the time of planting, the soil including the ridge should be well filled with moisture. Additional water is then seldom required for a month or six weeks. However, sweet potatoes should not be allowed to suffer for moisture at any time except at the end of the growing season, when the soil is to be dried out for harvest. With good soil moisture, yields may double within a period of 2 to 3 weeks during September. Some growers withhold water to keep potatoes from becoming oversized, but this tends to reduce total yield. Whenever possible, it is preferable to hold down size by close spacing rather than by withhold- [18] ing water. Furthermore, a few oversized potatoes are not serious if a high per- centage of the other roots reach market- able size. Contrary to popular opinion, irrigation water properly applied does not in any way injure sweet potatoes or cause scurf. In some sections of Stanislaus and San Joaquin counties, because of rather ex- tensive subirrigation from a high water table, fairly good crops can be produced without surface irrigation. In most other sections, 4 to 8 irrigations, providing 18 to 24 acre-inches of water, give the best yield. Furrow irrigation is the common method, but overhead sprinklers could be used very satisfactorily, if needed to ob- tain good distribution of water on un- leveled rolling land. At each irrigation, apply enough water to penetrate to a depth of 5 or 6 feet. Check the depth of water penetration with a soil probe at various points in the field. Rotations In order to hold diseases and pests in check, sweet potatoes should not be grown on the same land more than once in 3 years. Sometimes two successive crops can be grown satisfactorily if the land is then taken out of sweet-potato production for 3 or 4 years. The im- portant sweet-potato diseases are not com- mon to other crops grown in California. Alfalfa is a good crop to use in rotation. PESTS and DISEASES . . . ways to avoid trouble during the production and storage of the crop Pests and insects Nematodes and wireworms are the most important pests in California. Sweet potatoes are relatively free of other insect trouble. Root-knot nematodes. These very small soil pests attack the root systems of many plants including sweet potatoes. Both yield and market quality of the Yel- low Jersey and Porto Rico varieties are seriously reduced when this pest is prev- alent in the field. Nematodes may pro- duce small galls or swellings on the fine feeder roots; but they enter the storage roots and feed in the tissues beneath the skin without causing the common galls found on carrots and other root crops. Symptoms include decayed areas under the skin, surface blemishes and pitting, deformed roots, poor color, and some- times severe surface cracking. The vines of infested plants are usually stunted and yellowish, the leaves may show brown, dead spots, and with severe infestations the plants may be killed. Porto Rico sweet potatoes from a field where root-knot nematodes reduced yield and im- paired quality. Nematode injury can range from severe cracking to superficial surface blemishes. [19] Whenever possible, it is best to plant sweet potatoes on soils free of this pest. However, nematodes are found in varying numbers in most of the soils devoted to sweet potatoes in California. They can also be carried to the field on the roots of plants; therefore it is important to obtain nematode-free plants for transplanting into the field. The sources of infestation for plants are infested seed stock and plant-bed soils. Make an effort to obtain plant-bed soil that is free of nematodes; if this is impossible, fumigate the soil before bedding. The use of vine cuttings planted in clean soil is the surest method of obtaining clean seed. If fields suspected of being infested with nematode are to be used for growing sweet potatoes, fumigate the soil with dichloropropene mixtures (such as Shell D-D) or with ethylene dibromide. Use the former at the rate of 200 pounds (20 gallons) per acre, while ethylene dibro- mide should be applied at the rate of 3 gallons of actual material per acre. Double the rates for treating soil to be used in plant beds. Apply the treatment at least 2 weeks before planting. The soil temperatures should be above 50° F and the moisture content of the soil slightly below field capacity. The machines used to apply the materials in the field should be accurately calibrated to insure that adequate amounts will be applied uniformly over the field. The chisels should be 12" apart for ethylene dibromide and not over 18" apart for D-D. About 8 inches is a good depth to apply the fumigants. Although the cost is relatively high (about $40.00 per acre), the treatment has proved profitable where nematodes are a problem. Wireworms. These smooth, round, hard, yellowish-brown worms cause damage (1) by feeding on the roots and stems of young plants after transplanting, (2) by feeding on the storage roots later in the season, thus greatly reducing their marketability, and (3) perhaps by spreading diseases such as black rot and Common Defects of Sweet Potatoes Left to right: Wireworm injury; veining; cracking; cut made by digger; skinning. [20] scurf. The insect is prevalent in many of the soils used for sweet potatoes. Soil fumigation with 2 gallons of actual ethy- lene dibromide per acre is quite effective in controlling wireworms if applied when the soil temperatures are 50° F or above. Some growers use lindane, which suc- cessfully controls wireworms. This is ap- plied to the soil in either dust or spray at the rate of about % pound of actual lindane per acre. However, the question of contamination and resulting off-flavors has not been adequately answered, and therefore lindane application is not rec- ommended at present. The State Bureau of Chemistry states that it should not be used as a soil treatment where sweet potatoes will be grown within 2 years. When the soil is infested with both wireworms and nematodes, ethylene di- bromide at the rate of 3 gallons per acre or a dichloropropene mixture at the rate of 30 gallons per acre may be used. Cutworms. These worms, which feed on the plants at night, cause damage by cutting off the young transplanted plants near the soil surface. Spreading moist poison baits over the ground in the eve- ning is a good control measure. If you suspect cutworms apply the bait the eve- ning of the day the plants are set out. Dusting 5 or 10 per cent DDT on the ground around the plants may give con- trol of some types of cutworms. Other insect pests. The morning- glory leaf miner has been observed on sweet potatoes in a few isolated areas of this state. The larvae of this insect mine in the leaves, and when present in large numbers may defoliate the plants. It is not yet known whether this pest will be- come important in the main sweet-potato areas. Flea beetles occasionally attack young plants but seldom do much dam- age. The sweet potato root weevil has been very injurious in various sections of the South but thus far has not been introduced into California. Rodents. In fields pocket gophers and other rodents are sometimes troublesome. Control measures are outlined in Cali- fornia Extension Service Circular 138. In storage (see page 34) mice and rats often cause damage by feeding and by spreading diseases, particularly soft rot. Make every effort to exclude these pests from storage. Watch for evidence of dam- age by rodents, and take immediate steps to poison or trap any that may have gained entrance. Field diseases Stem rot. This disease, also called wilt, is caused by a fungus of the Fusa- rium group and lives within the roots and stem of the plant. Stem rot is prevalent in all the sweet-potato districts of California, often resulting in serious reductions of stands and yields. Sweet-potato plants may become infected from the following sources: (1) infected seed roots, (2) in- fected soil in the plant bed, and (3) in- fected soil in the field. Aside from direct infection of plants produced on infected seed roots, the prin- cipal entrance of the fungus appears to be through wounds made in pulling and handling plants at transplanting time. For example, considerable spread may take place when the plants are dipped or soaked in basins or sumps while being held before setting in fields. Field infec- tion after transplanting appears to occur mainly when the stem or roots are injured by insects, tillage tools, or wind. Young plants infected in the plant bed often die soon after transplanting. Sur- viving plants first show a few bright yel- low leaves around the crown, then dwarf- ing of leaves and vines, and often (later) badly rotted stems. Although they may survive throughout the season, the pota- toes are usually small and yields poor. Plants infected in the field may sometimes show no external symptoms and may pro- duce good yields, yet the potatoes if saved for seed will carry the disease. These plants can be recognized by splitting the main stem, to show a dark discoloration of the water-conducting tissues. [21 Left: Stem rot. Note the rotten condition of the main stem and sprouting of the storage roots. Right: The damage caused by black rot, a serious field and storage disease. The stem-rot fungus lives in the pota- toes through the storage season without causing external symptoms. Spores of the disease picked up in the field during harvest may adhere to the roots during storage. Black rot. This fungus disease causes potatoes to rot both in the field and in storage. Also it can attack the under- ground stem of the growing plants, caus- ing dark, decayed spots and sometimes resulting in death of the plant. On the potatoes themselves, this rot is character- ized by dark, slightly sunken, corky areas on the surface. Spots are usually circular, with the decayed tissue sharply defined from the healthy tissue. As decay de- velops, the spots penetrate deeply into the flesh of the potato. When cut, the decayed tissue often takes on a greenish tinge. A bitter taste is associated with this rot. All potatoes showing visible spots of black rot are culls, and in California the disease can cause severe cullage in the field and after storage. Plants can become infected in the plant bed from diseased seed roots or from infected soil. The disease can be carried on seed potatoes in both active and spore form and can live in the fields or plant- bed soil for a year or longer. Scurf. This fungus disease lives on the outermost layers of cells on the potatoes, causing superficial round, brownish- black spots. Individual infections are relatively small, but as shown here they may be numerous enough on a root to cause almost complete discoloration of the surface. While eating quality is not harmed, the poor appearance of the dis- eased potatoes greatly reduces market ac- ceptability. Contrary to popular opinion, this trouble is not caused by moisture al- though moist conditions favor its develop- [22] ment. It develops on potatoes in the field, and in storage when spores are carried in from the field. The disease or its spores can live over for a season in hotbed sites or in the field as well as on sweet potatoes. Soil pox is a disease of the Actino- myces group that causes common scab on Irish potatoes, and though it is not as yet prevalent in California, growers should watch for it, since once established in the soil it can persist for a long time. The main damage is malformation and pitting of the potatoes. Texas root rot, a serious disease of sweet potatoes, cotton, alfalfa, and many other crops in the southwest, is present in some soils in the southeastern part of California. The fungus causes a firm, brown rot that completely destroys the root. Sweet potatoes should not be planted where this disease is known to exist. Common soil fungi such as Rhizoc- tonia and Pythium may occasionally attack the stems or rootlets of plants in plant beds, but so far as is known these fungi seldom prove serious in the field. Foot rot and another rot caused by Sclerotium bataticola have been reported in California, but the diseases have never become serious. Other somewhat minor diseases includ- ing mosaic and internal cork, found in the southern states, are not common in California, but these could be brought in on imported seed. To control field diseases The precautionary approach needs to be emphasized. It would be well to check the following steps: 1. Use disease-free seed. 2. Provide clean soil in the plant beds. 3. Rotate crops to prevent build-up of diseases in the field. 4. Treat seed to kill spores on the sur- face of the potatoes (see below). 5. Use a root dip for the transplants. With the possible exception of the last step, the disease-control program to be successful must include all of the steps listed above. Neither cull potatoes nor the manure from animals fed on culls should be dis- tributed on soil used for growing the plants or crop. The spread of most dis- eases is helped by wounding. Therefore, avoid unnecessary mechanical injury in harvesting and handling operations, in pulling plants, transplanting, and cultiva- tion. Also avoid setting plants in sumps or basins of water after pulling. Seed treatment. Various fungicidal dips may be used to treat seed just before bedding. Such treatment will kill spores carried on the surface of the seed roots, but cannot be expected to kill any disease that has penetrated into the skin or flesh. A barrel, several porous containers, and a drainboard are the equipment needed for treating the seed. Semesan Bel and Wettable Spergon are instant dips; immerse the seed in either only long enough to insure thorough wetting of all surfaces. Direc- tions for making the suspensions are pro- vided by the manufacturers. One pound # \ Scurf, although confined to the peel and not affecting eating quality, can make potatoes un- marketable. [23 Some of these fungicides are very poisonous to both humans and livestock. Extreme caution is necessary in handling and disposing of the materials and the treated seed stock. Sweet potatoes should not be treated before storage, especially with materials involving mer- cury. of Semesan Bel is enough to treat 1% tons of seed. Mercuric chloride (corrosive sub- limate) at a 1-1,000 strength (4 ounces in 30 gallons of water) or a 2V2 P er cent borax solution may also be used for seed treatment. With these materials potatoes should be dipped for 10 minutes, drained, and bedded. In Kansas, Dithane D-14 at 2 per cent has been recommended. With the highly corrosive mercuric chloride, wooden or other nonmetallic equipment should be used. Of the four treatments Wettable Sper- gon is the least effective against disease spores. Semesan Bel and mercuric chloride may cause some reduction in sprouting. Slip treatment. Dipping the roots of the slips or transplants may give some additional protection against diseases when the other control measures are practiced. Wettable Spergon at the rate of 2 ounces in 1 gallon of water may be used for this treatment. Phygon XL at 1 ounce per gallon has been recommended in Kansas. Semesan Bel and yellow cop- per oxide have also been used, but are somewhat toxic to the plants. Dip the roots just before transplanting them, as injury to the plants may occur if some time elapses between treatment and plant- ing. Storage diseases Black rot. This disease often causes very serious losses during storage. The rot can develop from spots which become infected before harvest but are too small to be detected at that time. In addition, wounds inflicted during the harvesting operation offer potential areas for infec- tion. Thus careful handling is important. If black rot is apparent in an appreciable amount at time of harvest, growers are advised not to attempt storage. The field control of black rot is the most practical way to control it as a stor- age disease. The role of curing in rela- tion to the later development of black rot is not clear. It is possible that the curing treatment may hasten its development, but it is not likely that the ultimate amount is increased. This rot is favored by temperatures of 60° to 80°F. If curing is practiced, then the potatoes should be warmed rapidly and cooled rapidly so that they will be in this temperature range for a minimum period. Recent investigations point to the value of a heat treatment immediately after har- vest to eradicate this disease from sweet- potato roots. The sweet potatoes are held at 110° F for 1 day and are then cured. Although commercial testing may be de- sirable, the adoption of this procedure cannot be recommended at present. V ^^fea^ Soft rot, a common storage problem that oc- casionally develops in the field. It is accom- panied by a sweet odor. [24] Soft rot. This disease caused by the fungus known as bread mold (Rhizopus nigricans) can cause serious losses dur- ing storage. It gains entrance through wounds and causes a soft, sweet-smelling decay, which often involves the entire potato. The presence of any soft rot ren- ders the potato a cull. Under some con- ditions the surface is covered with a fuzzy growth of "whiskers." The occur- rence of this disease usually reflects poor handling and storage practices. Bruised, chilled, or uncured potatoes are most sus- ceptible. Both table stock and seed stock are especially susceptible after removal from storage. Prompt bedding of the seed stock after sorting and removal from stor- age, plus seed treatment, will help control the rot in the plant beds. Plants growing from infected seed roots usually show a rot on the basal portion of the stems, making poor plants for transplanting. Surface rot. Surface-rot lesions are circular, regular in shape, hard, sunken, and grayish-brown. By the end of the storage season the lesions pretty well cover the surface, and the potatoes are shrunken, dry, and sometimes mummi- fied. The causal fungus belongs to the Fusarium group but is not the one that causes stem rot. Potatoes put into storage while the surfaces are still wet and not cured are most likely to develop this disease. Dry-end rot. This trouble, common on stored potatoes in California, is ap- parently due to drying of the tissues under the broken ends of the potatoes. Secondary organisms then enter the af- fected tissues, causing decay when the potatoes are placed in moist conditions. Proper curing is the best control for this disorder. Scurf, discussed above as a field dis- ease, continues to develop in storage and is best controlled by sorting at the time of harvest. Dry rot, caused by the fungus Diaporthe, usually enters the potato at the stem end, from which it spreads «' Evidence of surface rot, a storage disease that can follow harvest injuries. slowly through the root to give a shrunken, wrinkled, and finally mummi- fied potato. To control storage diseases 1. First control field diseases. 2. Harvest and handle potatoes care- fully to avoid bruising and and skinning. 3. Avoid chilling before and after har- vest. 4. Sort out all diseased potatoes at harvest. 5. Cure potatoes properly. 6. Control rodents in the storage. 7. Maintain sanitation and good stor- age conditions, as follows : Clean storages of all refuse immediately after the storage season in spring. In the fall, before the potatoes are brought in, fumigate the storage room with formal- dehyde or spray with copper sulfate (1 pound to 20 gallons of water) to kill spores of the storage diseases. Do not attempt fumigation without previous ex- perience, unless Farmers' Bulletin No. 1442 is available. This bulletin, "Storage of Sweet Potatoes," can be obtained from your Farm Advisor's office. 25] Growth disorders Veining and cracking are rather common growth disorders. Occasionally the percentage of affected potatoes is high enough to reduce the grade materially or cause heavy cullage. Factors causing these disorders are for the most part un- known; therefore few suggestions can be offered for their control. The Yellow Jersey variety is more sub- ject to veining than the Porto Rico. The disorder does not seem to be carried in the seed, as smooth seed roots taken from smooth hills may produce a veiny crop and vice versa. Some growers believe irrigation practices may have a bearing on the development of veining, but there is no experimental evidence to sup- port this idea. Growth cracks can apparently develop at any stage in growth of storage roots. Those occurring early in the growing sea- son are usually healed over by digging time. Some varieties may crack badly some years if allowed to grow late into the fall, while a few tend to crack or "pop" during digging operations. Gen- erally the faster-growing and therefore larger potatoes are most prone to crack- ing. Heavy applications of nitrogen may aggravate this disorder, according to evi- dence obtained in the southern states. The presence of root-knot nematode is definitely a factor in the cracking of Porto Rico potatoes. Possible ways to reduce the amount of cracking are control of nematodes, limiting the number of fast-growing potatoes by obtaining a uni- form stand of closely spaced plants, and maintaining favorable moisture condi- tions throughout the growing season. Sweet potatoes sometimes show exten- sive sprout development at the time of digging. These sprouts are unsightly and tend to lower the attractiveness of the potatoes. Extra labor is required to brush them off, and the wounding involved is undesirable. The sprouting seems worse when vine growth is materially slowed down or stopped before harvest while temperature conditions are still favorable for sprout initiation. For example, roots of plants severely damaged by stem rot show extensive field sprouting. An unex- plainable amount of sprouting is some- times general in a crop where most of the vines are healthy and appear normal, aside from slow growth due to the fall weather conditions. Yellow Jerseys ap- pear more inclined to sprouting of this nature than Porto Ricos. HARVESTING and MARKETING ... the prevention of injury to the potatoes . , grading and packing Sweet potatoes can be harvested when- ever the roots reach a salable size. Under good growing conditions, fair yields can be obtained in 90 to 100 days; and some growers, particularly those in Coachella REMEMBER TO SAVE GOOD SEED Three seed-improvement programs are discussed on pages 35-38. It will pay you to read these pages before harvest- ing your crop. Valley, start harvesting early to supply California markets during the summer and early fall. The major portion of the crop is harvested during October and early November, at the completion of a long growing season and before the onset of cold, wet weather. During this harvest period, the potatoes may move directly to market, or they may be placed in either temporary or overwinter storage. The usual harvesting period for the San Joaquin Valley extends from August 1 to November 15; and for southern Cali- [26] fornia, August 15 to December 1. In Coachella Valley, the main harvesting period is mid-July to late September, though a few late plantings may be held for harvest during December and Janu- ary. Harvesting operations 1. Cut the vines. 2. Dig or lift the hills out of the ground. 3. Snap the potatoes from the vines and put them in market or storage containers. Under most conditions it is necessary to remove the vines just before digging. Cutting the vines by hand, using short- or long-handled knives, is slow and difficult work, especially when the vines are heavy. Sharp rolling coulters or disks attached to tractors are sometimes used to cut the side vines, leaving only a few along the top of the ridge to be cut by hand. Re- cently various types of vine-cutting machines which do an acceptable job at a great saving in labor costs have been developed by growers. Especially adapted moldboard plows, lifters, and modified Irish-potato diggers have been used for digging the potatoes. Plows with the upper part of the mold- board cut off and with three or four rods attached in place of the removed portion of the moldboard have been most gen- erally used. The lifter is a wide, U-shaped blade with upward-curving rods attached to its trailing edge. The blade is run under the ridge to loosen the ground and cut oif the roots beneath the potatoes, allowing the rods to push the hills partially up out of the ground. With either of the above machines it is necessary to lift or "scratch" the hills out. Irish-potato diggers have been adapted by shortening the chain to about 3 feet and removing the shakers. Such potato diggers speed harvesting, and work fairly well when the soil is moderately dry, though they tend to cause more skinning than the other tools. When these diggers are operating properly, potatoes are laid out on top of the ground. Snapping and sorting sweet potatoes from a "heap pile." It is common practice to collect the hills of potatoes into piles and then to snap and grade at the piles. Piling is advantageous when two or more grades of market potatoes are being made, or if hill-selected seed is being saved. Another method is to snap directly from the row, placing the potatoes from 3 rows onto the center row, from which they are picked up and sorted. A third method con- sists in placing the potatoes directly in a container as they are snapped from the row. Harvest injuries can result in the en- trance of decay organisms, increased water loss, and an unsightly appearance. Some mechanical injury is of course un- avoidable, but be careful to keep injuries at a minimum. The skin of a potato is thin and easily damaged or removed during harvest. Skinning will be more serious with immature potatoes from actively growing vines than with mature potatoes from vines that have ceased to grow. The moisture condition of the soil is also important. It should not be dry enough to form clods, nor should it be wet. Skinning can often be reduced by [27] allowing the potatoes to dry on the soil before handling. Two or three hours, even, will not injure the roots on bright, sunny fall days. However, under extreme conditions heat injury is possible and must be borne in mind. In some produc- tion areas, workers are required to wear cotton gloves to minimize injury. To re- duce bruising, the potatoes should be handled as little and as carefully as pos- sible. The proper adjustment, speed, and operation of the digger is essential to minimize skinning, bruising, and cutting of the potatoes. With the modified Irish- potato digger a layer of soil should be carried to the rear of the chain to cushion the roots. Yields. Yields range between 50 and 300 bushels (501b.) of marketable pota- toes per acre. The state average is about two and three-fourths tons (110 baskets) per acre, with a good commercial yield reckoned at from 5 to 7 tons. The aver- age yield could be raised considerably by the development of a good seed-improve- ment program, better control of diseases, early planting, and the adoption of im- proved irrigation and other cultural practices. Chilling injury Sweet-potato roots may be injured by exposure to temperatures below 50° F either before or after harvesting. The time of exposure required to cause injury is shorter as the temperature lowers. Freshly dug or uncured potatoes are more sensitive to chilling than cured potatoes or those that have been held for some time. Potatoes injured by chilling are more subject to rotting, and in storage they may develop an unattractive color and surface depressions sometimes re- ferred to as "cold spotting." Further- more, the table quality of the potatoes may be reduced. Injury from chilling may occur in the field after the soils have become cold and wet and during or following periods of heavy frosts. No toxic substance appears to be involved with frosted vines, and it is not necessary to remove them until the potatoes are to be dug. The important thing is to dig and move the potatoes to a warm place as soon as possible. Har- vested potatoes should definitely not be left exposed in the field when tempera- tures are dropping below 50°F for several hours during the night. Common market containers: Used apple box; bushel basket; lug. The potatoes shown here are a strain of Porto Rico. [28] Although several days of exposure to chilling temperatures may be necessary to give injury symptoms, it is best to avoid all unnecessary exposure to tem- peratures below 50°F. Potatoes that have been subjected to chilling temperatures should by all means be cured at the start of the storage season. Marketing Most sweet potatoes on the market dur- ing the summer and early fall are freshly dug. At other seasons most of the potatoes have come from temporary or regular storage. Study the provisions of the State Agri- cultural Code in regard to labeling, packs, containers, and minimum requirements of quality. Containers. The packaging of sweet potatoes in California is regulated by the following provision in Sec. 819 of the Agricultural Code of California: "All sweet potatoes when in containers which are closed must be in standard containers numbers 27, 47, 48, 48A, and 50A. The container requirements of this section shall not apply to sweet potatoes in sacks or containers which are not closed." Of the containers listed, only numbers 27 (the standard lug box) and 50A (the standard bushel basket) are commonly used by California growers. The continuous-stave-type bushel bas- ket of 50-pound capacity is the most com- mon container. Although seldom used, paper liners and a corrugated pad under the basket lid are recommended to re- duce injury to the potatoes during han- dling and transit. The standard lug box containing about 30 pounds is rather commonly used for local markets, especially in southern Cali- fornia. Second-hand apple boxes and cit- rus crates are occasionally used without lids. The wire-bound crate with paper liner holding one bushel is now the main container used in the southern states. Washing and waxing. The washing and waxing of sweet potatoes have been practiced to improve their appearance. Food coloring has sometimes been added to the wax to obtain a red color. How- ever, the California Bureau of Fruit and Vegetable Standardization has taken the stand that this is a misrepresentation of the product. A decision of the Superior Court of California has substantiated this stand. Waxing at time of marketing may in some cases have sufficient sales value to warrant its use, but at present no recom- mendation can be made for the practice. Washing and waxing do not improve keeping quality and are not recom- mended for potatoes to be placed in storage. Grading and packing. The Califor- nia Agricultural Code establishes the minimum standard for sweet potatoes that can be offered for sale. Grades have also been set up by the U. S. Department of Agriculture but are not widely used in California. In addition, the trade rec- ognizes certain unofficial grades. The No. 1 grade includes sound, clean potatoes, relatively free of defects, that range in size from 1% to 3% inches in diameter and from 3 to 10 inches in length. Larger potatoes of equal quality are classified as "Jumbos." Potatoes of lower quality or smaller sizes are sometimes classified in a No. 2 or second grade. The potatoes within a container are jumble-packed. It is advantageous to ar- range the top layer systematically, espe- cially when the bushel basket is used, but a deceptive pack must be avoided. Handling. Most California potatoes are transported by truck, though some are shipped into northern markets by rail. Sweet potatoes should be moved rapidly through the market channels to reduce losses from shrinkage and decay. After removal from storage, potatoes are especially perishable because soft rot de- velops rapidly. Recommended tempera- tures during transit and handling are 50° to 60° F. Price trends. Prices are normally [29] lowest during the main harvest period of October and early November, when growers' prices may range from 2 to 6 cents per pound. After December 1, prices usually begin to rise and may con- tinue upward until the following fall. On- the-farm storage of sweet potatoes or extra early production may enable grow- ers to take advantage of these higher prices. STORAGE ... if properly done will lengthen the marketing period . . . also result in better "seed" Storage permits extension of the mar- keting season of table stock and is neces- sary for the seed selected for the crop following. If proper storage conditions are provided, potatoes for market can be stored satisfactorily for 5 to 6 months after harvest. Practically all storage of California sweet potatoes at present is on the farm. In general, the methods and structures now in use could be greatly improved. Storage involves fixed costs such as capital investment, fuel, and labor. In ad- dition, there are the variable costs, result- ing from loss of weight and the develop- ment of storage rots. Loss of weight, due largely to water loss, is most rapid imme- diately after harvest. During the first two or three weeks, potatoes can be expected to lose from 2 to 6 per cent of their orig- inal weight. After this, the rate of weight loss under commercial storage conditions is about 2 per cent per month. Severe skinning during harvesting, low humidity during storage, or excessive sprouting may cause higher rates of water loss. Loss from rots is one of the chief hazards of storage and under commercial conditions varies from nil to as much as 50 per cent or more. If storage is to be profitable, it REMEMBER . . . Harvest and handle potatoes carefully. Sort out diseased and cull roots. Do not store if Black Rot is prevalent. Move crop directly to storage. Provide an adequate storage house. Cure at 85° F and high humidity. Store at 55° F— avoid chilling. is necessary to keep all costs at a mini- mum. Temporary storage Sweet potatoes for market may be held for 4 to 6 weeks in various shelters or structures if temperatures are maintained above 50° F and the humidity is kept fairly high. It is important to protect the potatoes from cold drafts and from sun and rain. Curing is probably of less value for this short-time storage than for over- winter storage. Field piles. In the San Joaquin Valley field piles are often used for temporary storage. From 250 to 500 pounds of po- tatoes are carefully stacked in piles about 2V2 to 3 feet high. The piles are covered with wrapping paper, and then sweet po- tato vines or straw are placed over the paper. Contrary to common opinion, curing is not accomplished by holding potatoes in field piles, and their use is undesirable for potatoes going into regular storage. Chilling injury has been frequently re- ported when potatoes were held in field piles too late in the fall. Accordingly the use of field piles for storage is both lim- ited and risky. Regular storage Only sound, disease-free, marketable roots and good seed should be placed in storage. Discard unmarketable roots be- fore storing, and sort out badly skinned potatoes for immediate sale. Potatoes har- vested while the weather is still warm and before the fall rains begin are the most desirable for storage. It is advisable to [30] '<$&'- A field pile. Although often satisfactory for short time holding, use of these piles previous to overwinter storage is not recommended. move the potatoes directly to the storage house at time of harvest. Bulk or container storage? With bulk storage, the bins are usually 6 to 8 feet in width and depth and 5 to 6 feet high. The potatoes are placed by hand or poured very gently. We recommend you use containers, which have definite advantages over bin storage: (1) Less handling eliminates some in- jury. (2) Less work is needed to fill the storage. (3) Circulation of air around the po- tatoes is improved. (4) Repacking for market is faster. One disadvantage of container storage is that fewer potatoes can be stored in a given volume of space, but containers permit higher stacking than is possible in bin storage. Apple boxes, citrus boxes, field crates, or baskets are suitable storage containers. Baskets, not commonly used in Califor- nia, would require lids and care in stack- ing to prevent mechanical injury. With baskets an offset arrangement in stacking would be desirable, and decking may be necessary if the stacks are more than five baskets high. The containers should be filled level full as the potatoes are snapped and taken directly from field to storage. Stack the containers so as to provide for air circu- lation. Leave an air space of two to four inches between the stacks and all wall surfaces. Channels of similar width should extend through the stacks at inter- vals of every two boxes. Curing. The purpose of curing is to encourage development of new skin or "periderm" at wounded surfaces. Curing is not a drying process; in fact, drying should be kept to a minimum by main- taining the humidity as high as possible. A good layer of skin over the entire sur- face of a sweet potato prevents excessive moisture loss and acts as a barrier against the invasion of storage diseases. At a temperature of 85° F and with the rela- tive humidity above 85 per cent, the new skin layer forms in 8 to 10 days, or less. [31 At ordinary storage temperatures it forms very slowly if at all. Begin the curing process as soon as possible after the potatoes are harvested. Fill the house or compartment within a few days; then close it and raise the tem- perature to 85° F. It is recommended that the potatoes be cured in the room or compartment where they are to be stored and that they should not be handled in any way until sorted for market. In early fall, desirable curing tempera- tures can often be obtained without artificial heat. The production of heat by the potatoes helps to raise the temper- ature and maintain it during the curing period. Little or no artificial heat may be needed after the potatoes have reached the desired temperature. In fact, it is possible for the potatoes to become over- heated even though the source of arti- ficial heat has been shut off. Place accurate thermometers in vari- ous locations and levels since wide ex- tremes in temperature can exist within the house or compartment. Fans may be needed to maintain uniform tempera- tures. In order to hold the relative hu- midity above 85 per cent, it is necessary to keep the house closed and usually to sprinkle water on the floor each day. Ventilation from outside is not desirable unless necessary for temperature control. Make an accurate check on humidity each day, using a dependable hygrometer. The rate of water loss from the potatoes will be about 3 times as rapid at a given relative humidity under curing tempera- tures as compared to storage tempera- tures. Thus the loss of salable weight can be serious if the relative humidity is not maintained at a high level of 85 per cent or above. Maintain curing conditions for at least 8 days and not longer than about 14. A period of 10 days is recommended; al- though there is some experimental evi- dence that 4 to 6 days are enough to ac- complish curing, we cannot make such a recommendation at present. To establish the length of the curing period, take into consideration the expected rate of cooling the house. If slow cooling is anticipated, the curing period should be held to a minimum. Some sprouting will occur during curing, especially in the hottest parts of the house. If sprout initiation becomes general, terminate the cure re- gardless of the time elapsed. After completing the cure, lower the temperature as rapidly as practicable to 55-60° F. Failure to cool the potatoes in a short period may result in excessive sprouting. To cool the house, ventilate during the night and close it during the day when outside temperatures equal or exceed house temperatures. The genera- tion of heat by the potatoes also makes cooling of the storage more difficult. Limited trials indicate that some of the growth regulators, effective on Irish po- tatoes, will also inhibit sprout growth on sweet potatoes when applied at the start of the curing period. Definite hazards are involved, and we do not yet know enough about these hormones to recommend them. Any commercial trials attempted with these materials should be conducted with caution and on a limited scale. Conditions for storage. Hold the temperature during storage as constant as possible within the limits of 55 to 60° F. A temperature of 55° F and a rela- tive humidity of 80 to 85 per cent are recommended as the optimum. Tempera- tures somewhat below 50° F will not necessarily prove harmful if of short duration, but storage temperatures must average above that. If they are above 60° they are conducive to a shortened stor- age life, sprouting, and decay. Condensa- tion can normally be prevented by good insulation and proper circulation of air within the storage so that potato or wall surfaces do not get cold. Ventilation from the outside is necessary only for tempera- ture control and to prevent excessive con- densation. Avoid cold drafts on the po- tatoes. [32] Storage structures The most important requirement of a sweet-potato storage is the ability to maintain temperature and relative-hu- midity conditions, as outlined above. If you are building a new storage, the type of structure will depend upon size of enterprise, capital investment possible, site, and other probable uses of the build- ing. This will of course vary considerably from grower to grower, and one design or plan is not applicable to all conditions. Although most California storages are of the cellar or semi-cellar type, above- ground structures if properly constructed are entirely satisfactory. A plan giving construction details for a house 24 feet wide and of varying length is available from the Division of Agri- cultural Engineering, Davis, California. Various plans and construction details are also shown in U. S. D. A. Farmers' Bui. 1442— "Storage of Sweet Potatoes." Site. In selecting the location for the storage, consider accessibility, drainage, and the exposure of necessary openings. A slope in the natural grade of the site may make it feasible to place the storage partially below grade and to have the floor at truck-bed height. Size. The size of storage needed to store a given quantity of potatoes will depend upon the design of the house and the method of storage. Space for sorting and packing should be given considera- tion in some instances. One cubic foot of 1. INSULATE POINTS TO CONSIDER WHEN BUILDING OR REMODELING A SWEET POTATO STORAGE HOUSE Insulate roof . . . or provide a loft with Store in containers insulation Side wall insulation is also desirable Use at least Vi" insulating board or equivalent 2. VENTILATE When necessary to control temperature or condensation Use ridge vents Vent through ducts — avoid direct draft on potatoes Use Va" hardware cloth over all Ifc* openings for rodent control 7mm Moisture barrier, paint, or metal foil on inside walls is recommended 3. HEAT W$///$s/s Provide for: about a 10 day cure at 85° F and relative humidity of 85% average storage temperature of 55° F not to drop below 50° F at any time Blower and thermostat arrangement is desirable [33] storage space will hold about 25 pounds of potatoes if they are in containers, and about 35 pounds if in bulk. However, containers permit better use of the floor area and can be stacked to a greater height than is possible with bulk storage. It is advisable to divide large storages into compartments so that the potatoes from a few days' harvesting can be closed up and cured without delay. These com- partments might range from 500- to 1,500-bushel capacity. Temperature and humidity control The proper control of temperature and humidity is dependent upon insulation, ventilation, and heating. Most sweet-po- tato storage houses in the state are poorly insulated, and very few are equipped for heating. Mechanical refrigeration of stor- ages may prove desirable and practical to prolong the storage period into the late-spring and early-summer months. Such refrigeration would also be of value in removing the heat from the potatoes following the curing period. Insulation is needed to help maintain desirable temperatures within the storage during the winter, as well as to prevent walls and ceilings from becoming so cold that condensation of water vapor can occur. With most wall and ceiling con- struction, at least V2 i ncn of good insulat- ing material is needed. Although artificial heat is not always essential under California conditions, provision for heating is recommended. Heat is usually necessary during the cur- ing period and may be needed during storage to maintain temperatures above 50° F. Electricity, oil, or gas provide satisfactory sources of heat. When oil or gas heaters are used, the burners should be adequately vented to the exterior. The size of heater required will depend upon the size of the storage and the amount of insulation. Since sweet pota- toes generate a certain amount of heat of their own, a heating unit considerably smaller than required for the same space in an average home will suffice. A 30,000 B.T.U. heater will probably be adequate for a storage of 7,500 to 10,000 cubic feet capacity. Portable electric heaters prob- ably give the most flexible arrangement for meeting the requirements of curing and storage heating. Uniform distribu- tion of heat throughout the house is im- portant and may present certain prob- lems. The source of heat should be at or near the floor level. Thermostatic control is recommended. The desirability of cooling the pota- toes quickly after the cure is completed and of maintaining even temperatures throughout the storage make it necessary to provide for good air circulation around the potatoes. In filling the house, stack crates on slatted floors; keep the stacks at least 2 inches from all walls and one foot or more from the ceiling. In bulk storage, the bins should be not more than about 10 feet square and 6 feet in height. Separate the bins by slatted double walls providing 4 to 6 inches of air space. Fans may be needed to circulate the air within the storage, particularly during the cur- ing period. With poor air circulation, temperatures may vary as much as 15 to 20 degrees. Intake ventilators in the side walls and overhead exhaust ventilators are essential for temperature and humidity control. Space these ventilators about 8 to 10 feet along the side walls and ceilings. The in- take vents should open into the storage at or near the floor level, but they should be constructed to avoid drafts of cold air directly on the potatoes. Provide all vents with tight doors and cover with 14-inch wire mesh to keep out rodents. When the storage is being cooled, especially after curing, fans may be useful to help the exchange of air with the outside. [34] SEED IMPROVEMENT next year's crop . . . suggested , . . getting ready for Several methods are A sweet-potato seed-improvement pro- gram involves the selection and produc- tion of seed free of seed-borne diseases, free of nematodes, true to type for the variety or strain, and uniform in color. Take care also to develop seed lines of high-yielding plants, that set 5 to 7 po- tatoes bunched well in the hill. Develop your program through "hill selection" or a combination of hill selec- tion and the use of vine cuttings. Al- though some improvement can be seen after one year, a continuing program is essential to develop and maintain high- quality seed. In addition to the selection program, use new fields for growing the seed crop, or fields that have been out of sweet potatoes for several years. Hill selection In hill selection, seed is kept only from desirable hills selected at harvest. After digging, select the hills first for type and the desired root color, then check them for diseases and nematodes. The stem-rot disease may not show externally unless the stem is badly rotted, and so to check on this disease it is necessary to split the main stem. Discard all hills that show internal brown discoloration in the stem (stem rot) and any trace of black rot, scurf, or other disease on the roots. All potatoes from selected hills except the extra large ones may be saved for seed. Either "foundation seed stock" or "com- mercial seed" may be selected by this method. Vine cuttings Vine cuttings, being free of diseases and nematodes, aid in eliminating the carry-over of these troubles to the new seed crop. The cuttings are taken in early summer, when plants in the field have Common Practice . Not Recommended Selecting seed from a cull pile. The correct way is shown on page 4. [35] PROCEDURE NO. 1 (HILL SELECTION) Fall, First Year Hill Select Seed from Commercial Field Store Selected Seed in Clean Storage Spring, Second Year Establish Commercial Field from Selected Seed Fall, Second Year Hill Select Seed from Commercial Field Repeat as Above PROCEDURE NO. 2 (HILL SELECTION + SEED FIELD) Fall, First Year Hill Select Seed from Commercial Field Store Selected Seed in Clean Storage Spring, Second Year Establish Seed Field in Clean Site Fall, Second Year Spring, Third Year Hill Select Seed from Seed Field Store Selected Seed in Clean Storage Establish Seed Field and Repeat as Above Save all Clean, Small, and Medium- Sized Roots for Commercial Seed * Store Commercial Seed in Clean Storage * Establish Commercial Field 36 PROCEDURE NO. 3 (HILL SELECTION + VINE CUTTINGS + SEED FIELD) Fall, First Year Hill Select Seed from Commercial Field Store Selected Seed in Clean Storage Spring, Second Year Early Summer, Second Year Fall, Second Year Establish Early Field from Selected Seed to Produce Vine Cuttings Establish Seed Field in Clean Site by Use of Vine Cuttings Hill Select Seed from Seed Field Save All Clean Roots for Commercial Seed Store Selected Seed in Clean Storage Store Commercial Seed in Clean Storage Spring, Third Year Plant for Vine Cuttings Establish Commercial Field grown vines 18 to 24 inches long. About 6 to 8 inches of the tips of the vines are cut off and immediately planted on ridges like ordinary transplants. The cuttings are set 4 to 5 inches deep and spaced 12 to 15 inches apart. They root in 2 to 3 weeks in moist soil. If planted in late June or early July they will produce sev- eral potatoes per plant by mid-October. This method can be used for seed increase from foundation stock obtained by hill selection. Three seed-saving procedures In Procedure No. 1 (see charts above) seed is hill-selected from commercial fields in the fall at the time of harvest. All the seed required for next season's crop can be obtained the year the im- provement program is started. This in- volves considerable labor to select large quantities of seed. It is most likely to be practical for the grower with a small acreage. In Procedure No. 2 only a small amount of seed stock is hill-selected in the fall. This is used the next spring to plant a seed field that will be harvested entirely for commercial seed. Each fall a new supply of selected stock will be hill- selected from the seed field. This proce- dure requires less labor than Procedure No. 1, but two years will be needed to obtain commercial seed, and the improve- ment program will be slower. In the fall of the third season, follow the steps shown for the second fall, and so on. Procedure No. 3 involves the use of vine cuttings and requires two years to [37] *. Potatoes on the left are typical of those commonly used for seed. Those on the right are more desirable in size and shape. obtain commercial seed. Although the use of vine cuttings alone can correct the disease and pest problem, hill-selection is necessary to improve color, type, and yielding capacity of the seed stock. In the fall of the second year seed may be hill- selected from either the early field planted from selected stock or from the seed field established by vine cuttings. Where nema- todes are present the latter course is ad- vised. This procedure should prove prac- tical under California conditions. Dur- ing the third and following seasons, follow the outline for the second year. Care of the seed Potatoes for seed should be harvested during favorable weather periods before there is danger of chilling injury to the roots. In general, it will be best to have the seed harvested and stored by the middle of October. Medium-sized seed roots will keep better and will produce better sprouts than very small ones. Han- dle, cure, and store seed carefully to avoid unnecessary losses during the winter and to maintain high sprout-producing ca- pacity. Practice all precautions listed under control of diseases during the plant-bed, growing, and storage seasons. Seed-producing enterprise One of the three procedures (prefera- bly 3 or 2) could be used to produce seed for sale. At present there is a good de- mand for improved seed, which leads to the possibility of a new enterprise in California. A seed-certification program might then logically follow. [38 In order that the information in our publications may be more intelligible, it is sometimes necessary to use trade names of products and equipment rather than complicated descriptive or chemical identifications. In so doing, it is unavoidable in some cases that similar products which are on the market under other trade names may not be cited. No endorsement of named products is intended nor is criticism implied of similar products which are not mentioned. Cooperative Extension work in Agriculture and Home Economies, College of Agriculture, University of California, and United States Department of Agriculture co-operating. Distributed in furtherance of the Acts of Congress of May 8, and June 30, 1914. J. Earl Coke, Director, California Agricultural Extension Service. 10m-6,'53(A6134)A.A. [39 IN PRODUCING one fourth of this nation's vegetables There's a bumper crop of jobs in a wide variety of rewarding fields — all helping Califor- nia produce one fourth of the vegetable crop of the United States. growing • shipping • developing new varieties • producing seed sup- plies • chemical fertilizers • pest control • processing contacts. These jobs are hard to fill, for they need very special training. These jobs are thoroughly covered by the University of California vegetable crops curric- ulum at Davis. It provides a balanced and thor- ough course covering all aspects of vegetable production and handling in different areas. breeding • handling • storage and transit • crop varieties • insect studies • plant diseases • weed control • vegetable growth. These jobs require special skills that can be developed only through practical experience under able guidance, using the many and varied facilities that the College of Agriculture at Davis can offer. For further Information • . • write to or 140 acres of land for class work and experiment • greenhouses and spe- cial equipment • irrigation facilities • a faculty of national reputation. Department of Vegetable Crops UNIVERSITY OF CALIFORNIA Davis, California see your University of California Farm Advisor for college entrance requirements