s^ ^Ci^FORNIA AGRICULTURAL EXTENSION SERVICE JUNE, 1949 POTATOES THE C 6hJ^E G E O F a G R I C U ■•^ ee i^ 1 2s Si s 5 ^ ■4^ 1 cd ee ce ce < •J •J >A » CO z .2* tt ^ o 5 ■■i s. :a mJ s is •43 z z s. o >- .- 3 1 0) 1 03 1 iJ •o lU A A 5 A ^ Q a ^ ^ flj •1^ |i4 o o Q § 03 i o 1 a 1 H «2 03 P4 03 03 u> ? ? Xi ua U) 111 1 £ 2 s M O 0} o o < 1 ■+a •a ■a •O t3 > 3 J^ e8 cd 52 2 1 u. o H p2 ^ * s « « & s 0, o bo » 2| U) o O o ^ c8 cS H^ iJ ij iJ •^ H" (O bo s .g 111 T3 ^ ^ ■i^ 1 1 -S tt •rj C CO c8 o S < ^ Xi « ■O fl «=»< X CO bo bo bo bo bo [m u d rt •S c3 .s ^ 73 til > ^ t ^ rt -CJ s © cd $ 2 i 9 0) (-1 ^ a a ^ ft >? u> M a CO c CO rt CO -2 1 « ^ 0) P r« ' « •> rC n tS 2 S,2 1 le S,£ S.;S' •5 ! % t V CO eg ■^ o U} •J3 s § •§ i O •c > O (4 ^ El O < X en O < Z < 3 CO . w ^^i^ 40. ^-^^ ■^MjL^^ '**?«*► F ?* .--^ ^^m '^■t^^' ■^"^^^0^^ ' Ti.^>. Calrose possesses suflScient resistance to late blight to protect it against mild attacks of the disease. It is quite tolerant to high air temperature and requires a relatively long period of growth, which should enable it to supplement other va- rieties in extending the length of the har- vesting and marketing period in districts where such procedure is desirable. Russet Burbank. Synonyms: Netted Gem, Idaho Russet, California Russet, Golden Russet. The Russet Burbank, origin unknown, is a late-maturing variety grown to some extent in the Tulelake district, at Santa Maria, and at the higher elevations in the Sierras. The plants are medium to large, spreading. The flowers are white. Tubers are large and long, regular in shape, skin russeted, heavily netted. The eyes are numerous, well distributed and shallow. The flesh is white. There is a tendency for this variety to produce tubers with knobby second growth when grown on heavy soil or with an irregular moisture supply. Triumph. Synonyms : Bliss, Bliss Tri- umph, Red Bliss, Coconino Red, Ha- waiian Rose, Stray Beauty. Triumph originated in Connecticut and was introduced by B. K. Bliss and Sons in 1878. It is thought to be the result of a cross between Peerless and Early Rose. This variety is grown in Butte County, in the Colma district, and to a limited extent in Kern County. It is of early maturity. Plants are large to me- dium spreading; stems thick with nodes slightly swollen and somewhat reddish purple in color. The flowers are phlox pink with orange-yellow anthers. Tubers are large to medium, round, thick; skin smooth, red; eyes medium in depth; flesh white. Pontiac. Pontiac was developed by the United States Department of Agriculture from a cross between Triumph and Ka- tahdin. The plants are large, spreading, stems thick, dark green. The flowers are light reddish purple with tips nearly white. Tubers are large, oblong to round, blunt at ends, skin smooth or sometimes slightly netted, red; eyes medium in depth; flesh white. Pontiac is a high- yielding late variety of value where red varieties are in demand. It is especially adapted to peat soils. Temperature and Soil Requirements In general, relatively cool conditions (60° to 75° F) are considered most favor- able for plant and tuber development. Potato culture is most successful in the northern tier of states, but good crops of early varieties are grown in the south in the spring and fall. California growers have demonstrated that under proper cul- tural practices exceptionally high yields may be obtained in certain districts of California where the summer daytime temperatures frequently exceed 100° F. Under high temperature conditions the soil temperature is kept relatively low either by frequent irrigation on the sand and silt types, or by the continuous sub- irrigation practiced in the peat districts where the water level is held 18 to 24 inches from the soil surface until the crop approaches maturity. Almost complete absence of leaf spot fungus diseases where the temperature is so high doubtless con- tributes to high yield. The potato is probably as cosmopolitan in regard to soil requirements as any of our agricultural food crops, but like any other crop it thrives best on certain types of soil. The most favorable soils for po- tatoes are fertile, well drained and of rather loose texture. High fertility and the application of adequate fertilizer are necessary for maximum yields. In Cali- fornia potatoes are usually grown in sandy loam, silt loam, or peat soil types. Poorly drained soils, soils with an im- pervious subsoil and little depth of sur- face soil, or heavier types such as clay. [8] should always be avoided. Tubers grown under such conditions are frequently de- formed, subject to growth cracks and not attractively colored. Indifferent results may also be expected on light, shifting sands. Acidity of soils. Caution should be taken in the addition of sulfur to potato soils with the objective of lowering the pH (increasing the acidity) to control scab. Normal fertilizer practices, such as the continued use of ammonium sulfate, so increase soil acidity that the addition of sulfur may be more detrimental than helpful. Although the potato plant will grow in quite acid soils, there seems to be general agreement that the best yields will be se- cured with a soil pH of between 6.0 and neutral (pH 7) or above. Fair yields, however, can be expected with a pH slightly lower than 6.0. When the pH in the soil is 5.5 or less, nitrification is mark- PoM0 Seed edly reduced and much of the ammonium fertilizer will remain in the soil unused. Under these conditions low yields are the result. Investigators generally agree that some degree of scab control can be obtained by making the soil acid to the extent of a pH 5.5 to 5.3. A pH of 5.5 or below means that the acidity will be high enough to interfere with the normal growth of many plants. If attempt at scab control is made by lowering the pH (increasing the acid- ity), some reduction in yield will occur. In older potato fields in which the pH of the soil has been reduced by ammonium sulfate, probably no more than 100 or 200 pounds of sulfur per acre should be added at one time. Once the soil be- comes badly infested with scab, or the pH is reduced sufficiently for control, a re- duction in marketable potatoes will result, either from poor quality or from low yields. Every year some growers are disappointed in their yields. They do not grasp the main fact of potato growing: without ^he use of good seed, high yields are impossible. Qualify Although seed tubers of a certain defi- nite type— smooth, good sized, and attrac- tive in appearance— are often selected, none of these factors has any consistent correlation with producing qualities. The appearance and shape of tubers depends chiefly on the soil and climate of the lo- cality where they were grown. Naturally, tubers are preferred that are free of knobby second growths, rots, and tuber- borne diseases. Because the buying public will never understand that misshapen or undersized tubers {if from healthy plants) may be perfectly satisfactory as seed stock, it is usually unwise to offer them for sale. Conversely, tubers of ideal size, type and symmetry may be infected with virus disease not detectable until they produce plants. As a result of the seed-potato certifica- tion agencies that have been established in practically all of the states producing seed, it is possible for those who desire to plant good seed to procure it either through the seed-certification agencies or from individuals whose seed has been certified. Storage conditions have considerable effect on the quality of seed. Potatoes kept in a warm place and allowed to de- velop long sprouts before planting are usually unable to produce good stands of uniformly vigorous plants. When cold storage is used a temperature of 36° to 49° F is most favorable. Good ventilation is necessary to carry off surplus moisture. This tends to reduce both decay and sprouting of potatoes. The work of Smith (1937) may be consulted for a critical [9] treatment of storage factors affecting seed value. There is little evidence that irrigation, or that soil and air temperature condi- tions alone, under which seed potatoes are grown, affect their yielding ability. Cerffffcaffon Certification of California potatoes grown for seed purposes is administered by the California State Department of Agriculture. High quality seed stock can be produced in several districts of the state, and constantly increasing interest is being taken in certified seed by the seed growers and their customers. The grower of certified seed must com- ply with the standards and regulations governing certified seed production, par- ticularly the isolation of seed fields from table stock. He must always be on the alert for varietal mixtures and diseases. The extent of spread of virus diseases in interior California varies consistently with the time of planting. In general, aphids, which carry virus diseases, are prevalent during April, May, and June. With the advent of high temperatures dur- ing the summer months, aphid prevalence and activity decrease significantly. Many growers now plant their seed fields be- tween July 1 and August 1, and their ex- perience has demonstrated that virus spread in such late planted stock is rela- tively insignificant. Much of the California potato acreage is planted with seed grown in other states, particularly Minnesota, Wisconsin, Ore- gon, Idaho, Montana, and Washington. In each of these states, potato-seed certi- fication is practiced, and usually high quality seed may be obtained if the Cali- fornia grower specifies he will accept only seed which has passed inspection and has been certified by the proper authorities. Certified seed is in no sense resistant to disease, and seed dealers and potato growers should not expect that certifica- tion guarantees a profitable crop. Dormancy and How to Shorten It It is well known that potato tubers re- quire a rest period after digging before they will sprout readily. The rest period varies greatly between varieties, some re- quiring a much longer rest period than others. The problem of dormancy is in general not of importance in growing the spring and summer crop since most of the seed has been in storage for some time and seedpieces sprout readily after plant- ing. However, a considerable quantity of seed harvested in June and early July is used in planting a fall crop in late July or August. Each year numerous questions are received from growers desiring infor- mation on methods of shortening the rest period of seed potatoes. In general, dormancy may be shortened in varying degrees by mechanical, physi- cal, and chemical means. Anything that upsets the normal rest period will shorten dormancy. Mechanical means would be cutting or bruising the seed potatoes. A physical method of shortening dormancy is to subject tubers to extremes of temper- ature, either high or low, or to give them a cold shock immediately followed by rapidly rising temperatures. Experiments in shortening the dor- mant period of White Rose variety indi- cate that the most effective storage period and temperature is 1 month to 6 weeks at a temperature of 70° F. If potatoes have been harvested for as long as 6 weeks ahead of the desired planting date, it is recommended that they be stored at 70° F and planted without chemical treatment. More than 200 different chemicals have been tested experimentally in attempts to produce prompt sprouting of dormant tubers without causing injury to the eyes. The chemical treatment which has proved most successful under California conditions is ethylene chlorhydrin. Ethylene chlorhydrin is a liquid, which is sold by the pound either in a pure form [10] or in a 40 per cent solution. It may be used either as a dip on cut seedpieces or as a gas on whole or uncut potatoes. The gas treatment is more widely used. Dipping cut seedpieces. Prepare a 1.2 per cent solution of pure ethylene chlor- hydrin (by volume) in water. Dip the seedpieces and place them immediately in an airtight enclosure for 16 to 24 hours. Then remove and plant. Gassing whole or uncut potatoes. Any method which employs ethylene chlorhydrin as a gas requires that un- usual precautions be taken to protect the operator from fumes. A system which can be used with com- parative safety makes use of an airtight treatment room. An atomizer, such as a paint spray gun, is mounted on the ceiling in the center of the room. Liquid ethylene chlorhydrin is conveyed to the gun by compressed air from pressure tanks on the outside of the room, and is atomized into the atmosphere in the room. The only exposure to the operator using this system will be when he enters the chamber to obtain samples. Oxygen- breathing apparatus should be worn at such times, since amount of oxygen in the room will be reduced as well as the poisonous ethylene chlorhydrin fumes being present. Mechanical ventilation of the treat- ment chamber is absolutely necessary, and the minimum quantity of ventilation necessary is about 500 air changes. Treating small lots. For treating small lots of seed potatoes, use 22 grams of pure ethylene chlorhydrin per 100 pounds of potatoes. This is the equivalent of 1 pound or 0.40 quart of pure liquid per ton of potatoes. Place the ethylene chlorhydrin in a shallow container above the potatoes, or wet sacks or other cloth with the liquid and hang on wires above the potatoes. The gas from the solution is heavier than air and will settle toward the floor. The potatoes must be gassed in airtight containers or rooms for 3 days. Then open and allow to aerate before cutting and planting. If the 40 per cent solution is used in place of pure ethylene chlorhydrin, use 2% times the quantities given above. Ethylene chlorhydrin should be used with extreme caution— the fumes are toxic if inhaled for prolonged periods. The so- lution is also absorbed directly through the skin. Cutting Seed There is no satisfactory substitute for careful hand cutting of seed potatoes. However, several mechanical devices for cutting seed potatoes have been designed and patented from time to time, particu- larly within the past few years, with the objective of keeping the cutting surfaces sterilized to prevent the spread of bac- terial ring rot. Some are for power and some for hand operation. Among these are the Colorado rotary potato cutter and the double-edged stationary potato- cutting knife. While mechanical cutters reduce the time element to a minimum, they are nevertheless mechanical and a small per- centage of no-eye seedpieces may be ex- pected. Many different types of cutting bins, knife arrangements, systems of supplying seed to cutters and removing the cut seed, have been devised with the idea of in- creasing individual cutting efficiency. The choice should be left to the individual grower. Seedpiece Size Many comparisons have been made on the relative value of cut seedpieces versus whole seed, and on the best seedpiece size. Whole seed potatoes, usually called "drop seed" or "single seed" (from 1 to 2 ounces in weight) , may be safely used if they come from healthy plants. There is danger in selecting small tubers from the bin unless all the plants in the field where the crop was grown are relatively free from virus disease; otherwise the possi- bility remains that many of the small [11 tubers came from diseased plants and will produce diseased progeny. Disadvantages of whole seed are the difficulty of planting with a machine, the danger of disease perpetuation, the slowness of germination if the seed is immature, and the excessive number of sprouts produced if the seed has been in storage for a long time. The point should be emphasized that small potatoes, 1 ounce or over, need not be discriminated against if the stock is from disease-free fields. Numerous tests have been conducted to determine the proper size of the cut seedpiece, and several conflicting opin- ions are held. It has been found that small seedpieces produce fewer sprouts and fewer but larger potatoes; on the other hand, large seedpieces produce more sprouts and more but smaller potatoes. The size of the seedpiece to be used de- pends on fertility of the soil, distance between rows, and distance between the seedpieces in the row. A seedpiece which weighs 1% to 2 ounces is probably the best average size. Care of Cut Seed Seed potatoes may be cut and planted immediately, or they may be cut in ad- vance of planting if proper conditions are provided to facilitate the suberization, or corking over, of the cut surface. At a temperature of from 60° to 70° F, with an atmospheric humidity of 85 per cent, the formation of cork cells or periderm tissues seems to progress most rapidly. Exposure for 10 days to such conditions seems to be sufficient to insure good su- berization, after which cut seed may be held for several weeks at a temperature of 36° to 40° F if desired. The ideal method of handling is to cut the seed and plant immediately. If the seed is to be planted at once after cutting, the pieces are apt to stick together and its handling in the planter may be facilitated by dusting with some absorbent such as land plaster, slaked lime, gypsum or cement. Put a cupful of the absorbent in the sack and shake well before emptying seed pieces into the planter. Seed Treatment There is a tendency on the part of some growers to neglect seed treatment, par- ticularly if the seed has a clean external appearance. Nevertheless it is considered advisable to treat all potato stock, cut seed and whole seed, before planting. Rhizoctonia and common scab as well as other disease-producing microorganisms may be lodged on the surface without being visible to the naked eye. Seed treatment will kill only the disease germs carried on the seed itself. When potatoes are planted in infested land, in- fection of the potato crop from the soil cannot be prevented by any treatment of the seed. The principal benefit derived from seed treatment is to prevent infest- ing the soil with disease-producing organ- isms and to control infection from the seed in the new crop. Several different treatments for seed potatoes are in use, no one of which can be considered best under all circumstances. Choice of the treatment is up to the grower, and will depend largely on the facilities he has available. 1. Corrosive sublimate (mercuric chloride). Corrosive sublimate is dis- solved in water at the rate of 1 ounce in 8 gallons. The necessary quantity can be dissolved in a gallon of warm water and later diluted to the proper strength. Use wooden, crockery, or concrete container. Soak the seed potatoes in this solution for 1% hours. Both rhizoctonia and scab are controlled by this treatment. The solution gradually weakens and should be rein- forced, after 4 lots of potatoes have been dipped, by adding % ounce of corrosive sublimate for each 8 gallons of water in the original solution. The solution should also be brought up to its original volume. After dipping 8 lots of seed, the solution should be discarded and a fresh one pre- pared. [12] Fig. 2. Treating cut potato seedpieces with a fungicide. Caution.— Corrosive sublimate is a deadly poison and extreme care should be exercised in its use. Corrosive sub- limate solutions are very corrosive to metal and should be used only in wooden, crockery or concrete contain- ers. Potatoes treated with corrosive sublimate should not be used for food. Care should be exercised in the dis- posal of the solution after treatment. 2. Hot corrosive sublimate. Use the same strength solution as in No. 1, bring it to a temperature of 126° F and soak the seed in it for exactly 2 minutes. This is a very powerful solution and directions should be followed explicitly. 3. Acidulated corrosive sublimate. Soak the seed potatoes for 5 minutes in a solution composed of 6 ounces of cor- rosive sublimate, 1 quart of commercial hydrochloric (muriatic) acid (31 per cent acid) , and 25 gallons of water. This treatment requires no temperature con- trol and is very effective against scab and rhizgctpnia. Potatoes should not be left in the solution to exceed 5 minutes lest damage to the eyes results. 4. Hot formaldehyde. Hot formalde- hyde does not have many of the disad- vantages of corrosive sublimate. It does not lose strength during continued use, it may be used in metal containers, it is not highly poisonous, and tubers treated with it may be used as food. Hot formaldehyde is used as follows: Add commercial formaldehyde (37 to 40 per cent solution) to water at the rate of 1 pint to 15 gallons. Heat to 124° to 126° F and dip the tubers (in sacks) for 3 to 4 minutes. Time and temperature controls should be quite accurate. The solution may be heated by means of an open fire, steam coils, or gas burners. Commercial machines for treating potatoes by this method are on the market. 5. Proprietary organic mercury com- pounds. Several different compounds of this nature are on the market. Among the best known of these materials are Ceresan, New Improved Ceresan, Semesan (sev- eral types), and Merko. Manufacturer's directions should be followed. [13] Cuitunf Methods Preparing the Soil Proper preparation of the land includes deep plowing and thorough pulverization of the soil prior to planting the crop. It is impossible to produce maximum yields of well-shaped tubers from poorly pre- pared soil. Whether plowing should be done in the fall or in the spring is largely determined by the physical character of the soil, its exposure and the previous crop grown. Heavier soils are benefited by fall plow- ing and exposure to the action of winter weather and rains. If manure or cover crops are to be turned under, plowing should come early enough for the material to rot thoroughly before planting time. Fall-plowed land may be quickly fitted for planting by one or two thorough diskings. Planting The date of planting depends on the district where the crop is grown, the most profitable season of marketing, and the hazard from frost. It varies from Novem- ber in the Edison district of Kern County and the Colma district below San Fran- cisco, to July and August in the interior valleys where a late crop is grown. Gen- erally speaking, with the commercial crop, the earlier the planting date without undue risk of frost, the more satisfactory the development of the plants and of the crop, and the less danger of injury from heat and tuber moth. The time of plant- ing, although it affects both the produc- tion and the value of the crop, has to be determined largely by local conditions. Planting is usually by machine, of which several types are available. They may vary in the number of rows planted, from 1 to 4 ; in the number of operators required, from one to several ; and in the \w Fig. 3. A four-row picker-type potato planter in operation. ri4i mechanical design such as the assist-feed or picker type. The type of planter se- lected depends on the preference of the individual grower, his acreage and the help available. Every effort should be made to secure a planter that will give accurate, uniform spacing of the seed. Rate of planting in California varies from 12 to 20 sacks of seed potatoes per acre, depending on the spacing between rows and between seedpieces in the row, and on the size of the individual seed- pieces. The soil should contain enough mois- ture at planting time to germinate the seed promptly and to support its devel- opment until the sprouts are well above the surface of the soil. Occasionally, the land must be irrigated in preparation for the spring plantings. Irrigation is always necessary before planting the fall crop. In extensive depth-of-planting tests at Shafter and Davis, Lorenz (1945) con- cluded that all factors considered— yield, size of tuber, and injury from sunburn- planting the seedpieces at a depth of 4 to 6 inches is more desirable than either more shallow or deeper plantings. fertilizing The potato is an intensively cultivated crop and high yields per acre are desir- able. In order to accomplish this end the soil must be maintained in a condition of high fertility. Soil needs. Experiments in Kern County have shown that one crop of po- tatoes will remove from 125 to 200 pounds of nitrogen, about 60 pounds of phosphoric acid and about 300 pounds of potash per acre. Few, if any of the soils in California, will produce maximum yields without the addition of varying amounts of plant nutrients in one form or another. The addition to the soil of organic fertilizers and humus such as manure and cover crops, will, as in tlie case of other crops, prove beneficial. However, potato culture in California has come to depend primarily on the addition of inorganic nutrients or "commercial fertilizers" to the soil. Extensive fertilizer tests have been con- ducted with potatoes in Kern, Fresno, San Bernardino, Riverside, Los Angeles, and Lassen counties and in the Cuyama Valley and Tulelake district. The prac- tices which have been found applicable in one locality or on one soil type will not always prove economical when ap- plied in other districts. In Kern County application of nitrogen has always given responses up to about 100 pounds of nitrogen (which are pres- ent in 500 pounds per acre of ammonium sulfate). In the lighter, more heavily ir- rigated soils or where potatoes are grown throughout the winter, somewhat higher amounts are justified. At Fresno it ap- pears that at least 140 pounds of nitrogen per acre is justified on the spring crop, while half this amount is sufficient for the fall crop. In all tests in the central valley there has been a slight response to phos- phorus. The application of a fertilizer containing some phosphorus is recom- mended. At Tulelake, in Lassen County, and in the Cuyama Valley, both nitrogen and phosphorus give a response, and a good recommendation in these areas is the ap- plication of a fertilizer which will supply 80 pounds each of nitrogen and phos- phoric acid per acre. On the peat lands of the Delta region, it is a common prac- tice to apply 500 to 1,000 pounds per acre of a 10 : 10 : 10 fertilizer on the older fields, while on new or burned-over peat land less nitrogen is needed, so about the same application of an 0:10:10 fertiHzer is used. Fertilizer recommendations for the different potato-growing areas, based on existing experimental evidence, are summarized in table 4. What to use. Experiments to deter- mine the best source of nitrogen have shown that ammonium sulfate alone has consistently given the highest yields. Or- ganics, such as dried blood used alone or half and half with ammonium sulfate. [15] Table 4. SUMMARY OF FERTILIZER RECOMMENDATIONS FOR THE PRINCIPAL POTATO PRODUCTION AREAS OF CALIFORNIA Fertilizer Area Parts of Nitrogen : Phosphorus : Potassium Rate of application, pounds per acre Upper San Joaquin Valley Riverside-San Bernardino Tulelake Delta 20-0-0 or 16-20-0 or 17-7-0 20-0-0 or 16-20-0 or 17-7-0 16-20-0 or 11-48-0 10-10-10 (on older fields) 600 600 400 500 to 1,000 500 to 1,000 600 600 Miscellaneous mountain areas. . Miscellaneous coastal areas 0-10-10 (new or burned -over land) 16-20-0 20-0-0 have not given better results than straight ammonium sulfate. Nitrate of soda and calcium nitrate gave but little better re- sults than no fertilizer, and ammonium nitrate seems of benefit only for the am- monium that it contains. Uramon in some cases is as beneficial as ammonium sul- fate but in other cases is inferior. If placed too close to the seedpiece, uramon may reduce germination. Cyanamide has proved toxic, as little as 50 pounds of nitrogen per acre from this chemical showing toxicity throughout the entire growing season. Fertilizer should be applied at the time of planting the seedpieces. The method of application may vary somewhat but the most satisfactory and probably most uni- versally used is by means of an adjust- able fertilizer attachment carried on the planter. The fertilizer should be applied in a band about one inch below and two inches to the side of the seedpiece. Cultivating The chief purposes of cultivation after potatoes have been planted are to control weeds, prepare the land for the distribu- tion of moisture, and keep the tubers covered to prevent sunburn and reduce potato tuber moth infestation. A thorough study of root development of the potato was made in Nebraska by Weaver. He found that while potato roots penetrate into the third and fourth foot of soil, the majority are found in the top foot or eighteen inches. This indicates the advis- ability of rather shallow cultivation after the plants are well established. Cultivation should be reduced to a minimum that will accomplish the desired results. It should be kept as far away from the rows as pos- sible. Where potatoes are grown on ridges or beds, cultivation should be completed early in the life of the plant and the soil left undisturbed thereafter. Root pruning by cultivation will not force the roots to go deeper. If the roots near the soil sur- face are injured, the most efficient utili- zation of available nutrients will not be realized and reduced yields may be ex- pected. Irrigating Almost every conceivable irrigation practice is followed in the various dis- tricts of California. The methods include culture under winter rainfall conditions at Colma, dry-farming summer culture in Marin County, irrigation on alternate days in the Shafter-Wasco district, and almost continuous subirrigation in the Delta area. Overhead irrigation is prac- ticed to some extent in certain sections, particularly in San Diego County. Tubers begin to form when the plants [16] are 5 or 6 inches tall and are completed by the time the flower buds are formed. Conditions during this period determine the number of tubers set. From the begin- ning period until the plants begin to ma- ture, the tubers should grow at a regular rate. If the soil becomes too dry, normal tuber development is retarded, the skin of the tuber "sets" and subsequent irriga- tions may be responsible for growth cracks, rapid expansion in the region of eyes and knobby second growth. Although the potato is a shallow-rooted plant, a few roots penetrate deeply, but they are not numerous enough to perme- ate the soil thoroughly at greater depths. It is necessary to keep available soil mois- ture in the surface foot of the soil if best results are to be obtained. A total appli- cation of 30 inches of irrigation water for the production of the early potato crop, and 20 inches for the late crop, is recom- mended. Harvesting The yield of marketable potatoes in- creases rapidly as the plants approach maturity, even increasing somewhat after the vines have turned yellow. Because early potatoes often bring high prices, a considerable portion of the crop in the Colma district, and the spring crop in central and southern California, are har- vested before fully mature. Early potatoes in most of the other areas of the state, and all the late potatoes, are allowed to reach maturity or to grow until the vines are killed by frost. The skin on immature potatoes is very thin and is easily rubbed off. As the potatoes approach maturity, the skin be- comes thicker, tougher, and more firmly attached to the tuber. The advancement of this change in the coat of the tuber is im- portant in determining when to start dig- ging early potatoes. If harvested too im- maturely, some of the skin may be injured Fig. 4. A two-row potato digger. Note the roller instead of rear wheels. [17] during digging, grading, sacking or loading operations. Such skinned areas quickly darken and often are the starting places for decay. Potatoes are dug by machine, generally of the one- or two-row type. Most diggers are now equipped with a roller extending the width of the machine to replace the pair of rear wheels formerly used. The roller breaks down the clods and smooths out the soil disturbed by the plow end of the digger. The result is a smooth some- what compact surface upon which the potatoes are dropped. This greatly facili- tates picking up. After digging, the potatoes are picked from the ground into sacks by hand. Workers are paid for this operation on a piece-work basis, generally so much per "half sack." Handling and shipping tests have shown that as quickly as possible follow- ing digging, the potatoes should be picked up and hauled to the packing sheds. Tight- meshed picking sacks should be used, and they should be protected from sun and wind en route from field to shed, by a heavy canvas cover to reduce surface browning. Any practice which reduces moisture loss from the surface of the po- tato also reduces browning. The full ex- tent of the injury from drying of skinned areas is hard to detect at the time of pack- ing, because only severely injured pota- toes turn brown in this short time. Brown- ing continues to develop during shipment to market. Grading At the packing shed, the potatoes are first put through machines equipped with revolving brushes and water sprays. The potatoes then move on a divided belt con- veyer past graders who sort the stock so that finally each division of the belt con- tains but a single grade. The tubers drop from the end of the grading belt into sacks, usually of 100 pounds. Operators check, weigh, and sew the sacks and truck them into the cars for shipment. Storing A relatively small percentage of the potato crop in California is placed in stor- age. This is in contrast to the procedure in the main late potato-producing states. In California much of the table stock is matured during extremely hot weather and is rapidly placed on the market. The early crop is frequently harvested from April to July when immature. A portion of the late crop, harvested from October to December, is stored. This crop is produced mainly in the Tulelake area, in the San Joaquin Valley, in cer- tain coastal districts, and in Riverside County. Winter temperatures in these dis- tricts, except Tulelake, rarely fall below freezing, and growers often have difficulty in maintaining the necessary low storage temperatures unless they resort to cold storage— a rather expensive item for table stock. Such difficulty is not experienced in the Tulelake district, where the winter temperature often is well below freezing. The quantity of seed potatoes for stor- age has increased considerably during the past few years. Seed stock is produced in many districts and requires different stor- age methods. The seed crop is stored both in bulk and in sacks, with increasing quantities being held under artificially maintained low temperatures. The length of storage period for seed stocks varies with the season of production and the dis- trict in which the seed is sold for planting. Studies of the effect of storage on po- tatoes for both seed and table use have shown that varieties often respond differ- ently under identical storage conditions. (Wright, eL a/., 1936.) Temperature, Cooking quality, pala- tability and composition are markedly in- fluenced by storage temperature, whether the potatoes are used for boiling, baking, French frying, steaming, or chip making. In general, storage temperatures at or below 40° F result in an increase in unpleasant sweetness, in a change from a fluffy and white consistency to one that is soggy and slightly yellow. If tubers are [18] stored for six to eight weeks at 40° F and the temperature is then raised to 70° F for three or four weeks, they recover their original quality. It is recommended that table stock be stored at or near 50° F with adequate ventilation to prevent the occur- rence of blackheart. Sprouting of table stock can be held back for long periods of time by treating with the methyl ester of alpha-naphthal- eneacetic acid applied in Pyrax Talc dust, or by using impregnated shredded paper. Both of these are commercial products. The dust may be shaken in the sacks or over and around the sacks. The shredded paper is thrown loosely around the sacks. For seed stock the most satisfactory storage conditions are obtained at ap- proximately 40° F with sufficient venti- lation to ensure adequate oxygen and to prevent blackheart. If the storage period is relatively short, the temperature might be raised to 50° F. The danger of the higher temperature is that excessive sprouting may result if the storage period is prolonged. Potatoes Moy Have Serious Diseases Because of the dry summer climate and low humidity in the interior valleys, the California potato crop is not subjected to some of the defoliation diseases which are serious in areas experiencing summer rainfall. The most serious potato diseases in California are scab, rhizoctonia, and virus diseases. Scab Round, brown, rough areas are seen on the surface of the tuber. These areas may be few or numerous enough to cover the entire surface. The fungus causing this scab thrives under high temperatures and in alkaline or very slightly acid soil. To control scab use clean seed, dis- infect seed thoroughly, and plant in clean soil. Methods of seed treatment are given on page 12. Crop rotation should be prac- ticed when no new potato lands are avail- able. When planting new land, treat all seed very carefully, even though it may not show scab. This will help prevent in- festation of the soil. The practice of adding sulfur to soils to increase their acidity and thereby dis- courage scab disease is one to be followed with caution. Yield decreases where soil is more than mildly acid. (See page 9.) Rhlzocionia disease A fungus disease causing small, black particles of fungus tissue up to % inch in diameter on the potato. Presence of the disease is indicated by the leaves, which tend to roll upward and may turn yellow. Small aerial tubers may develop in the axils of the leaves on severely affected plants. Young sprouts in the soil are rotted or burned off below the surface. The roots may be corroded by the fungus. Crop rotation and seed treatment are the principal methods of keeping this dis- ease under control. Virus diseases At least ten virus diseases of potatoes have been recognized in California. They cause the foliage to be more or less mot- tled, dwarfed, crinkled and distorted, and the vigor and yield of the plants to be reduced. These diseases are transmitted in the seed potatoes, and are spread from plant to plant by aphids and other insects. No one can tell by looking at potatoes whether or not they are affected with virus diseases. Inspection of the growing plants is necessary for identification. Virus diseases are controlled by planting seed which is free from the disease. [19] M0st injurious Pests of Potato Assistance in tiie preparation of this section was received from A. E. Michelbacher, M. W. Allen, and E. S. Sylvester, of the Division of Entomology and Parasitology. Potato Tuber Moth Caterpillars of the potato tuber moth burrow through the tubers as long as they are available, either in the field or in stor- age. The full-grown caterpillars are white or pinkish and not over % inch long. Pupation occurs in white silken cocoons on the plant, in the surface soil, in sacks or storage bins. The adult moths are small and gray with silvery bodies and minute dark specks on the forewings. Each female may lay from 150 to 200 oval, pearly white eggs, any place on the potato plant or in storage. To prevent infestation of the potatoes in the field, hill up well around the grow- ing plants, remove the potatoes from the ground as soon as dug, and destroy all volunteer potato plants. Packinghouse sanitation should be practiced. Plant only clean seed. Potato Flea Beetle These tiny, flealike beetles are shiny black or brown. They perforate the leaves of the plant early in the spring. Their larvae burrow into or feed on the tubers, causing pimply potatoes. Early-planted potatoes are damaged most, and small fields are damaged more than large plant- ings. The only practical control is directed against the adults. If infestation warrants, dust with 5% DDT. Wireworms Wireworms have smooth, round, shiny bodies, varying from pale yellow to dark brown. They are about 1 inch long. They live in the soil and the larval stage may last from one to three years. The adults are called click beetles because when placed on their backs they jump into the air with a clicking sound. Control measures are extremely diffi- cult. Clean culture and crop rotation are practical measures on large acreages. To fumigate the soil with ethylene dibromide before planting is an effective control, al- though somewhat expensive. Fig. 5. Above, work of the potato tuber moth. Below, injury by the potato flea beetle. [20] Potato Aphid Large uniform green or pinkish aphids infest the young shoots and terminal growth of the potato plants. They are sucking insects and in their feeding they carry virus diseases from plant to plant. The green peach aphid, a most trouble- some species in much of the potato-grow- ing area, can be controlled by applications of DDT dust. These should be applied at intervals of 7 to 10 days during the season of infestation, beginning when plants are four to six inches high. The organic phosphate parathion would give excellent control when applied as a dust, but the hazards of applying this ma- terial must be considered. It is extremely toxic to animals, including man. Pof af o Psyllid This insect sucks the cell sap from the leaves, and injects a toxic substance into the plant which causes the condition known as "psyllid yellows." The plant becomes stunted, the leaves curl and turn yellow, and the condition results in poor development of the tubers. Damage is caused by the immature forms, or nymphs, which are yellowish orange to green in color, and flat and elliptical in shape. The nymphs secrete quantities of white wax. Best control is obtained by dusting thoroughly with sulfur. Roof-Knof Nemaf ocfe Root-knot nematode is rarely a prob- lem of early potatoes in California. The late commercial crop is sometimes dam- aged, and in some areas seed potatoes are subject to injury. Nematode injury gives the potato a warty, rough appearance outside. Inside, the tubers have scattered black spots about % inch below the surface. The nematode is contained in these black spots. Two cultural practices, crop rotation and summer fallow, should be followed in controlling nematodes. Chemical control measures are possible with fumigation. Fig. 6. Left, potato psyllid nymphs and white dots of excrement. Below left, wireworms live in the soil and bore into potato tubers. Right, potato aphids carry virus diseases as they feed. 21] but the grower should carefully consider the cost to determine if such control is feasible. Crop rotation involves rotating pota- toes with some less-susceptible crop on the same land. The grains, particularly oats and barley, are most satisfactory. Summer fallow allows the soil to lie fallow for a period in the summer, during which it is worked two or three times to dry it out thoroughly. Drying the soil tends to kill out the nematodes. This prac- tice works best in lighter, sandy soils. There are three chemicals on the market at present which give satisfac- tory results as fumigants. Shell D-D or Dowfume-N, containing the active ingre- dient, 1,3-dichloropropene, are used at the rate of 20 to 25 gallons per acre. Dowfume-W 40, in which the active in- gredient is ethylene dibromide, is used at the rate of 20 gallons per acre. The total cost of treatment with these chemi- cals, for material and application, is about $40 an acre. Planting of the land should not take place for two weeks after appli- cation of the chemical. The fumigants are most effective on light soils. In order that the information in our publications may be more intelligible it is sometimes neces- sary to use trade names of products or 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. 22] REFERENCES Barger, W. R., G. B. Ramsey, R. L, Perry, and John H. MacGillivray. 1942. Handling and shipping tests with new potatoes from Kern County. Calif. Agr. Exp. Sta. Bui. 664:1-22. Barger, W. R., and L. L. Morris. 1943. Tighter picking sacks reduce surface browning of early "White Rose potatoes. Calif. Agr. Exp. Sta, lithographed publication. Doneen, L. D., and John H. MacGillivray. Suggestions on irrigation of commercial truck crops. Calif. Agr. Exp. Station, lithographed publication. EssiG, E. 0., and W. M. Hoskins. 1944. Insects and other pests attacking agricultural crops. Calif. Agr. Ext. Cir. 87:197. Harrington, F. M. 1933. A comparison of irrigated and nonirrigated seed potatoes. Mont. Agr. Exp. Sta. Bui. 279: 1-11. Henderson, W. J. 1944. The Colorado rotary potato cutter. Colo. Agr. Ext. Serv. Bui. 381A:1-10. LORENZ, 0. A. 1945. Effect of planting depth on yield and tuber set of potatoes. Amer. Potato Jour. 22(11): 343-349. Paschal, J. G., G. H. Lane, and W. A. Krentzer. 1946. The double-edged stationary potato cutting knife. Colo. Agr. Exp. Sta. Bui, 493:1-11. Porter, D. R. 1935. Relation of virus diseases to potato production in California. Calif. Agr. Exp. Sta. Bui. 587 : 1-32. Out of print. Smith, Ora. 1937. Influence of storage temperature and humidity on seed value of potatoes. N. Y. (Cornell) Agr. Exp. Sta. Bui. 663:1-31. Smith, Ralph E. 1940. Diseases of truck crops. Calif. Agr. Ext. Cir. 119:1-112. Out of print. Thomas, J. E., and A. J. Riker. 1945. Sprouting of potatoes inhibited by plant hormones, Amer, Potato Jour. 22(4) . Weaver, John E. Root development of field crops. 291 pp. McGraw-Hill Book Company, New York, N. Y. Werner, H. O. 1929. Effect of cultural methods and maturity upon the seed value of eastern Nebraska potatoes. Neb. Agr, Exp, Sta, Res, Bui, 45:1-28. 1929. Relative productivity of seed potatoes grown under various controlled environmental con- ditions. Jour. Agr. Res. 38(7): 405-410. Wright, R. C, W. M. Peacock, T. M. Whiteman, and E. F. Whiteman. 1936. The cooking quality, palatability, and carbohydrate composition of potatoes as influenced by storage temperature. U. S. Dept. Agr. Tech. Bui. 507:1-20. 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. C. W. Rubel, Acting Director, California Agricultural Extension Service, 15m-6,'49(B3493) DO YOU HAVE QUESTIONS about crops . . . soils . . . fertilizers irrigation . . . or special problems affecting your own farm operation h^ YOUR FARM ADVISOR CAN GET THE BEST INFORMATION He is ready to talk over your farming problems with you and help you solve them. There is no charge for this service. 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