UNIVERSITY OF CALIFORNIA 
 
 COLLEGE OF AGRICULTURE 
 
 AGRICULTURAL EXPERIMENT STATION 
 
 BERKELEY, CALIFORNIA 
 
 CIRCULAR 287 
 May, 1925 
 
 POTATO PRODUCTION IN CALIFORNIA 
 
 By J. T. KOSA 
 
 CONTENTS page 
 
 Foreword : 2 
 
 Present status in California 2 
 
 Potato growing districts of California 5 
 
 Varieties 9 
 
 Selection of seed potatoes 14 
 
 Factors affecting seed quality , 14 
 
 Appearance :. 14 
 
 Region of production 14 
 
 Irrigated vs. dry land 15 
 
 Maturity of seed 15 
 
 Storage conditions 15 
 
 Infection by diseases 16 
 
 Preparation of seed for planting 17 
 
 Planting 18 
 
 Irrigation practice 20 
 
 Fertilization practice 21 
 
 Harvesting 25 
 
 Insects of the potato 26 
 
 Stalk borer 26 
 
 Tuber moth 26 
 
 Aphids 28 
 
 Nematode 28 
 
 Diseases of the potato 29 
 
 Common scab 30 
 
 Rhizoctonia 32 
 
 Wilts 33 
 
 Stem-end rot : 34 
 
 Leak 35 
 
 Mosaic 35 
 
 Leaf roll, net necrosis, spindle sprout 37 
 
 Internal browning and heat necrosis 38 
 
 General disease control measures 40 
 
 Seed treatment 40 
 
 Crop rotation 42 
 
 Control of insects 42 
 
 Use of healthy seed 42 
 
 Production of certified seed 42 
 
UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION 
 
 FOREWORD 
 
 The Irish potato is one of our staple food crops, in the production 
 of which California ranks as one of the leading states. As a quick 
 maturing annual crop having high acre-value, it has played an im- 
 portant part in the development of districts now almost entirely 
 given over to other lines of agriculture. However, as new land is 
 becoming scarce, the industry at present is being centered within 
 certain districts which have proved their especial adaptability to this 
 crop. The increased prevalence of certain diseases, the necessity in 
 many places of using commercial fertilizers in some form, the advan- 
 tages to be gained through the selection of proper seed, the require- 
 ments of the markets as to variety, type, and quality of potatoes 
 grown, and the increasing competition from other producing regions 
 all make it necessary for the successful potato grower to have a con- 
 siderable fund of knowledge of the potato plant, as well as of the 
 culture of the crop. It is the purpose of this circular to discuss in 
 detail certain phases of potato production, especially in the light of 
 new information gained by scientific study in recent years. 
 
 PRESENT STATUS OF THE POTATO INDUSTRY IN CALIFORNIA 
 
 Table 1 shows the acreage, production, and car lot shipments of 
 potatoes from California in recent years. Generally speaking, these 
 have been years of heavy production in the country as a whole, and 
 potatoes from other states have been shipped into California in 
 increasingly large quantities. The rapid growth of the population 
 of California cities has led to increased consumption of the crop by 
 local markets, which partly explains the decrease in car lot shipments. 
 
 TABLE 1 
 Acreage, Yield, and Car Lot Shipments of Irish Potatoes from California 1 
 Acreage Yield, bushels Car lot shipments 2 
 
 1920 95,000 3 13,015,000 10,108 
 
 1921 74,000 10,360,000 8,252 
 
 1922 76,000 9,880,000 7,765 
 
 1923 52,000 7,800,000 5,690 
 
 • * Weather, Crops, and Markets, 4:677, 1923. 
 
 2 For the crop year, May 15 to May 15. From Market News Service, U. S. D. A. 
 8 Yearbook, U. S. D. A., 1920. 
 
Circ. 287] 
 
 POTATO PRODUCTION IN CALIFORNIA 
 
 June Ju/y Aug 5ept. Oct Nor. Dec. Jon. Feb. Mar Apr Mac/. 
 
 Fig. 1. — Car lot shipments of potatoes, by weeks, from Pacific Coast districts, 
 for the season 1922-1923. 
 
 600 
 
 soo- 
 
 Shipping Season /9£J -<B 4. 
 
 Northern California 
 
 Southern Co/ifornio 
 
 Idaho 
 
 Oregon <s Washington 
 
 June July Aug Jept Oct Nor Dec Jon red. Mar Apr May 
 
 Fig. 2. — Car lot shipments of potatoes, by weeks, from Pacific Coast districts, 
 for the season 1923-1924. 
 
4 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 Competition with other states for outside markets, as well as for 
 the home markets, has been the outstanding feature of potato mar- 
 keting in recent years. This factor will have to be kept constantly 
 in mind in future developments. The tendency is toward earlier 
 crops and earlier shipping of the California crop, thus avoiding to 
 some extent direct competition with the main crop in the Northwest. 
 To show the relation of California shipping seasons to those of com- 
 peting states, figures 1 and 2 have been constructed from data sup- 
 plied by the Market News Service of the United States Department 
 of Agriculture. These furnish us with a guide to the usual seasons 
 of shipment and the volume of shipments from the principal potato 
 districts on the Pacific coast — a region more or less independent of 
 the rest of the country so far as production and consumption of 
 potatoes are concerned. 
 
 Early in the season, during April and May, there is a considerable 
 movement of "new potatoes" from the San Francisco district to 
 California markets and to markets of the Northwest. This is followed 
 early in June by a much heavier shipment of early potatoes from the 
 lower San Joaquin Valley (Shafter district), which goes to local, 
 Northwestern, and Southwestern markets. Shipments from this 
 district cause the first "peak" in the movement of potatoes from 
 northern California. Then in July come the peak shipments from 
 the Los Angeles district, principally from the San Fernando and 
 San Gabriel valleys. In August active movement begins from the 
 Stockton district in northern California, and continues heavy through- 
 out the fall months, gradually declining to a moderate, steady volume 
 during the winter. A number of other, smaller districts in the 
 northern part of the state harvest and ship their crop from October 
 through the winter. 
 
 From March to the middle of August, the California potato 
 districts have to compete with each other to some extent, but more 
 particularly with the "old potatoes" of the previous year's crop, still 
 being shipped from the Northwestern states. The first is taken care 
 of in some measure by the difference in season of shipment from the 
 different California districts, the short periods of "peak shipment" 
 from the several districts following each other in more or less regular 
 order from April to August. Competition with "old potatoes" from 
 the Northwest must, however, be considered during this period. 
 While the home-grown new potatoes and the shipped-in old potatoes 
 appeal more particularly to different classes of trade, still the supply 
 of one kind has an influence on the price of the other. Hence, in 
 
ClRC. 287] POTATO PRODUCTION IN CALIFORNIA 5 
 
 attempts to forecast probable market conditions for the California 
 spring and summer crops, the size of the crop of the preceding year 
 in the Northwest and the quantity being held over for spring ship- 
 ments, are factors to be considered. 
 
 From the middle of August until the following spring, export 
 of California potatoes to other states practically ceases, except for 
 small quantities shipped to Arizona and New Mexico. Texas formerly 
 furnished a market for considerable quantities during this period, 
 but with the revival of potato production in Colorado and its expan- 
 sion in Utah, these sections with a shorter haul now supply much of 
 Texas' needs for late potatoes. During fall and winter, the Califor- 
 nia markets are being supplied to a large extent by the heavy ship- 
 ments from Idaho, Washington, and Oregon. The Yakima Valley of 
 Washington, with a combined freight rate of 36% cents by rail and 
 boat to San Francisco, and southwestern Idaho with a rate of 56V2 
 cents, furnish the main competition for fall and winter markets. 
 While California growers shipping at this season have an advantage 
 in somewhat lower rates than these to the main markets, still it is not 
 large. Another factor entering here is the market demand for certain 
 types of potatoes. The bulk of the shipments from the Northwest are 
 of the "Russet" or "Netted Gem" variety, which is generally con- 
 sidered an especially good baking potato and which the consuming 
 public has become accustomed to use largely during the fall and 
 winter. On the other hand, California stock available at this season 
 is mostly of the long white type usually considered to excel other sorts 
 as boiling potatoes. Potato growers have to consider the type of 
 potato in demand by consumers at different seasons. 
 
 CALIFORNIA POTATO GROWING DISTRICTS 
 
 A number of more or less isolated districts have developed exten- 
 sive potato production within the state, each district having certain 
 advantages as to soil, climate, and shipping season. Cultural 
 methods, varieties, time and method of marketing differ widely in 
 different districts. The rather exacting requirements of the potato 
 plant as to soil and climate render it unwise to attempt commercial 
 potato production in new or untried districts until it has been deter- 
 mined that conditions are suitable for the crop, and that season of 
 maturity and marketing facilities are such that the product can be 
 disposed of advantageously. Arranged in approximate order of 
 shipping season, the principal districts are discussed briefly below. 
 
6 
 
 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 San Diego County. — Several small districts in this county special- 
 ize in winter potato production. Seed harvested in August is planted 
 in November, and the crop is harvested from February to April. 
 British Queen has been the principal variety used because of its 
 vigorous growth during the cool winter months. White Rose is also 
 being used. The crop of this district is usually dug when only partly 
 developed. The product is marketed in lugs and shipped by express 
 or by truck to California markets. The use of earlier varieties, such 
 as Irish Cobbler, Idaho Rural, and Bliss Triumph, might increase the 
 yields obtained in this district. 
 
 Fig. 3.— Thrifty field of White Eose potatoes at Shafter, California, planted 
 with certified seed at the rate of 16 sacks per acre. Note 2 -inch pipes in head 
 ditch for irrigation. Yield 195 sacks per acre. 
 
 Colma, San Francisco County. — The mild winters on the hillsides 
 near the ocean permit the culture of winter potatoes. The Garnet 
 Chili is the standard variety, seed being grown in Oregon and planted 
 at Colma from November to February. The later plantings are on 
 the lower, more level fields, which produce a winter crop of cauli- 
 flower before being planted with potatoes. The crop matures from 
 the first of April to the first of July. The earlier harvests are of 
 partly grown tubers that go to local and nearby markets in lugs, 
 but as the crop becomes more mature, carlot shipments are made to 
 more distant regions, especially to the Northwest. The installation 
 of vacuum fumigation plants should aid in expanding the shipments 
 to states having tuber-moth quarantines. The Garnet now used is 
 a late variety and perhaps could well be supplemented with similar 
 varieties that mature in a shorter time. 
 
ClRC. 287] POTATO PRODUCTION IN CALIFORNIA 7 
 
 Shafter, Kern County. — This district has developed rapidly in 
 recent years. The sandy soils and warm spring weather are favor- 
 able for the production of standard varieties for shipment in early 
 summer. White Rose is the variety used, seed being planted in Feb- 
 ruary and maturing in June or July. Late plantings are likely to be 
 injured by heat or by tuber-moth. Shipments are mostly sacked, in 
 car lots, though there are some extra early shipments in lug boxes. 
 
 Los Angeles County. — The San Fernando and San Gabriel valleys 
 are the main districts. White Rose is the variety generally grown 
 in this county. The seed is grown locally the preceding fall and 
 planted from February to April. Certain favored localities are able 
 
 Fig. 4. — Potatoes as an intercrop in a two-year-old vineyard at Shafter. 
 
 to plant in December and these " winter potatoes" are harvestd in 
 the spring and marketed in lugs. Car lot shipments of sacked stock 
 begin about July 15 and continue for about six weeks. Local markets 
 consume most of the crop. A second or fall crop is also grown, 
 planted in late July or early August, and harvested during the early 
 winter. Much of the fall crop is used for seed. Maturing in the cool 
 fall weather makes these potatoes equivalent for seed purposes to 
 those " northern grown," provided ample moisture is available for 
 their development during the latter part of the growing season. 
 
 Sacramento-San Joaquin Delta. — The newly reclaimed peat or 
 tule land of this section has been the most important potato district 
 in California. Exploitation of the raw land by Oriental growers 
 has led to a rapid decline in yields. The indications are that intelli- 
 gent methods of production will restore and maintain potato produc- 
 tion permanently in this section. The chief problems are control of 
 diseases and use of fertilizers according to modern scientific prin- 
 
8 UNIVERSITY OF CALIFORNIA— EXPERIMENT STATION 
 
 ciples. Though the Burbank variety was formerly grown it is rapidly 
 being replaced by the" Pride Burbank," also known as "Wisconsin." 
 Planting commences in March and continues until June. Harvesting 
 begins in July and continues until January. The tendency now is 
 toward earlier planting and shipping to avoid competition with the 
 Northwest. Large yields are possible on the peat and muck soils of 
 this district. The quality of the potatoes grown in this region, known 
 on the markets as "Rivers," is quite variable, and seems to depend 
 largely on the skill with which the water supply is handled. 
 
 Fig. 5. — Potatoes as an intercrop in eight-year-old walnut grove. 
 
 Pajaro Valley and Salinas Valley. — These are the oldest potato 
 districts in California. The Burbank variety is grown mostly. The 
 crops from these districts are late, shipments being in fall and early 
 winter. 
 
 Marin and Sonoma Counties. — This is also an old potato district, 
 centering around Sebastopol and Tomales. The crop is grown on 
 rather rough land, without irrigation. Several varieties are grown 
 but most of the commercial crop is of the British Queen variety, 
 which is well adapted to this coastal district, but is not a popular type 
 
ClRC. 287] POTATO PRODUCTION IN CALIFORNIA 9 
 
 on the large markets. Climatic conditions favor high quality, 
 especially for seed purposes, but soil conditions appear to be respon- 
 sible for the rather low yield. 
 
 Other Districts. — There are several other smaller districts. In 
 Humboldt County the British Queen variety is grown for shipment. 
 Modoc, Lassen, and Inyo counties on the east side of the Sierras grow 
 a considerabe quantity of the Netted Gem variety for shipment during 
 fall and winter. 
 
 VARIETIES 
 
 Only a few really distinct varieties of potatoes are grown in Cali- 
 fornia, though a confusing array of names is applied to them. 
 Below are given in some detail the characteristics of the leading 
 varieties. The best known name is recommended for varieties that 
 seem to be identical. General adoption of such standard varietal 
 names will help to avoid confusion in buying seed in future. The 
 importance of knowing the characteristics of the leading varieties 
 should be emphasized, if for no other reason than that of recognizing 
 and discarding varietal mixtures. 
 
 In order to gain more definite information as to the yielding 
 possibilities and adaptability of various varieties to different sections 
 in California, some variety tests were conducted during 1923 and 
 1924 by the Division of Truck Crops, at the University Farm at 
 Davis, in cooperation with the Agricultural Extension Service 1 in 
 Los Angeles County, and at Shafter in Kern County. 2 The results 
 are presented in table 2. In so far as possible, two or more lots or 
 strains of each variety were tested, and in such cases only the average 
 yields for the group are given in the table. It is well recognized 
 that varietal tests involving only one stock of a variety are unreliable, 
 and may even be misleading. 
 
 Differences in time of maturity, type of potato, and adaptability 
 to market requirements were observed that are fully as important as 
 the gross yield produced by the different varieties. The difference 
 between the early and midseason or late varieties is brought out by 
 the early and late harvests of the tests at Davis. All varieties were 
 planted February 24, and at the time of the first harvest, June 21, 
 the Bliss Triumph, Earliest of All, Irish Cobbler, and Idaho Rural 
 were practically mature, while in the other varieties tuber develop- 
 ment was still in the early stages. 
 
 1 Conducted by L. C. Holmes and F. H. Ernst, in cooperation with A. J. Mueller 
 and Bert C. Bougher in 1923 and 1924, respectively. 
 
 2 In cooperation with W. B. Camp, U. S. Dept. Agr. Cotton Expt. Station. 
 
10 
 
 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 White Rose. — This is the main variety for southern California. 
 It is a medium-early variety, being relatively earlier than the Bur- 
 bank or British Queen, but later than Irish Cobbler or Idaho Rural. 
 The plants are of medium size, erect, with many well developed lateral 
 branches from the main stem, a feature which gives the plant a 
 spreading appearance late in the season. The stems are bright green 
 in color. The flowers are white. The tubers are white, have few eyes, 
 and are rather long and flat, being rounded at the apical end but often 
 pointed at the stem end. The White Rose can be distinguished from 
 the Burbank in that the tuber is flatter and wider in the central 
 part, while that of the Burbank is usually more cylindrical and about 
 the same width throughout its length. Soil conditions, however, 
 have a great deal to do with shape and smoothness of any tuber. 
 When grown under favorable soil and climatic conditions, the White 
 Rose has smooth evenly shaped tubers of fancy appearance for 
 market. Experiments indicate that the natural rest period of the 
 tubers of this variety is shorter than in most other sorts, making it 
 well adapted to the practice of growing two crops a year. 
 
 TABLE 2 
 Yields Produced by Various Varieties of Potatoes an Spring Crop Trials 
 
 
 Davis-1924 
 
 Van Nuys-1923 
 
 Alhambra-1924 
 
 Shafter-1924 
 
 
 Dug June 21 
 
 Dug. Aug. 1 
 
 Dug July 25 
 
 Dug July 18 
 
 Dug June — 
 
 Variety 
 
 No. 
 
 of 
 
 plots 
 
 Pounds 
 per 
 acre 
 
 No. 
 
 of 
 
 plots 
 
 Pounds 
 per 
 acre 
 
 No. 
 
 of 
 
 plots 
 
 Pounds 
 per 
 acre 
 
 No. 
 
 of 
 
 plots 
 
 Pounds 
 per 
 acre 
 
 No. 
 
 of 
 
 plots 
 
 Pounds 
 per 
 acre 
 
 Bliss Triumph 
 
 Earliest of All 
 
 1 
 
 5 
 
 2 
 2 
 19 
 5 
 2 
 1 
 1 
 2 
 1 
 
 13,850 
 
 13,550 
 
 13,000 
 
 11,825 
 
 9,820 
 
 9,330 
 
 7,900 
 
 7,600 
 
 6,850 
 
 5,415 
 
 1,470 
 
 3 
 
 14,500 
 
 
 
 
 
 
 
 
 
 1 
 1 
 
 12,022 
 12,320 
 
 
 
 Irish Cobbler 
 
 3 
 
 16,150 
 
 1 
 
 1 
 
 10 
 2 
 
 34,825 
 30,550 
 23,000 
 23,880 
 
 
 
 Idaho Rural 
 
 1 
 
 4 
 
 1 
 
 17,714 
 
 White Rose 1 
 
 1 
 3 
 1 
 4 
 2 
 9 
 2 
 6 
 8 
 1 
 1 
 
 23,750 
 14,720 
 11,450 
 18,525 
 16,800 
 14,700 
 10,390 
 15,640 
 11,504 
 19,600 
 11,350 
 
 3 
 
 1 
 
 10,541 
 
 4,700 
 
 15,611 
 
 12,713 
 
 American Wonder 2 
 
 Early Red Rose 
 
 Green Mountain. 
 
 11,158 
 
 3 
 1 
 2 
 2 
 3 
 1 
 1 
 
 32,854 
 34,100 
 25,750 
 17,940 
 24,315 
 27,175 
 21,000 
 
 1 
 
 
 
 King 3 
 
 
 
 
 Eurbank 
 
 
 
 1 
 
 12,333 
 
 Garnet Chili 
 
 
 
 
 Russet Burbank 4 
 
 1 
 
 5,530 
 
 
 
 British Queen 
 
 
 
 
 
 Rural New Yorker 
 
 
 
 1 
 
 1,334 
 
 
 
 Brown Beauty 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 1 California type — includes Great Divide, Wisconsin Pride, American Giant. 
 
 2 Some lots received under this name were the same as Burbank. 
 
 3 Includes Candian Queen and Chicago Market. 
 
 4 Includrs Netted Gem. 
 
ClRC. 287] POTATO PRODUCTION IN CALIFORNIA 11 
 
 The "Great Divide" grown in southern California appears to be 
 identical with the White Rose, and the variety now being grown 
 so extensively in the Sacramento-San Joaquin district under the 
 names Pride Burbank, Wisconsin, Wisconsin Pride, and Late Pride, 
 likewise seems to be nearly or quite the same as the White Rose. 
 The Late Pride is an old established variety in Wisconsin and Min- 
 nesota, whence many carloads of seed are shipped to California and 
 grown under the names mentioned above. The name "White Rose" 
 probably is of later origin than Late Pride, but the former name 
 has become so well established in California that it is thought best 
 to retain it as the name for the entire group as grown in this state. 
 The variety, "American Giant," much grown in New Jersey, also 
 seems to be identical with this group. However, the White Rose 
 grown in the East is distinctly different from the variety grown 
 under that name in California. 
 
 Burbank. — This was long the standard variety in the Delta dis- 
 trict, but it is now giving way to the White Rose type. It is still 
 grown extensively in some of the coastal and interior districts of 
 northern California. Three strains are grown, Low Top Burbank, 
 Pride of Multnomah, and High Top Burbank. The tubers of these 
 strains are alike, but the tops increase in size in the order in which 
 they have been mentioned. The plants are more erect and have 
 larger, coarser leaves than the White Rose. The flowers are white, 
 the cyme is purplish — another distinction from the White Rose, in 
 which the cyme is light green. The tubers are long, slightly flattened 
 or cylindrical, slightly pointed at both apical and basal ends, and 
 have a large number of eyes. It is a late variety. 
 
 Russet Burbank. — This variety is more generally known as Netted 
 Gem, or Idaho Russet, since such large quantities are shipped from 
 Idaho. It is little grown in California except at the higher elevations 
 east of the Sierras, though it grows very well in other sections. It 
 has been found to produce especially well in the peat soils of the 
 Delta district though it has not yet been grown there on a large scale. 
 The season is late. The plants and tubers resemble Burbanks, except 
 that the tubers are covered with a dense, brownish corky netting. 
 This netting is less marked when the potatoes are grown in peat or 
 muck soils. A serious defect in both the Netted Gem and the white 
 varieties of Burbanks, is the tendency to produce a knobby second 
 growth on the tubers when raised on heavy uncongenial soils, or with 
 an irregular moisture supply. 
 
12 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 Garnet Chili. — This is probably the oldest variety now grown in 
 America. In California it is grown in the Colma district near San 
 Francisco. In time of maturity it is relatively a late variety, but 
 when planted in the fall it produces a fair crop in early spring — 
 hence so far as market is concerned it is considered an early potato. 
 The plants are tall, erect, rather spindling in habit of growth. The 
 stems are purplish, and the flowers are also purple. The tubers are 
 slightly elongated or nearly round, and tend to have deeply sunken 
 eyes. The skin is smooth and of a light pink color early in the season, 
 but as maturity approaches, the skin becomes rougher and darker in 
 color. The Garnet Chili, which is the only pink skinned variety of 
 any importance in California, is grown for the early market. Whether 
 or not it has any peculiar advantage as a winter grower in the coastal 
 frost-free districts has not been determined. It is not recommended 
 for trial elsewhere. 
 
 British Queen. — This is a standard variety for coastal districts 
 from San Francisco north to Washington. It is more strictly limited 
 by its climatic requirements than most other varieties. Only in a 
 constantly cool humid climate will the plant and tubers develop 
 normally. In the interior valleys and in the warmer parts of the 
 south, tubers do not develop satisfactorily. Under conditions prevail- 
 ing in such localities white sprouts appear from the apical end of 
 the young tubers, and may develop laterally in the soil for some 
 distance, stopping at intervals to form other small tubers or pro- 
 ceeding to the surface of the soil to form vegetative shoots. This 
 abnormal development of tubers results in a crop of little market 
 value. Under irrigated conditions in warm sections, the plants may 
 grow rapidly all season with the formation of very few marketable 
 potatoes. On the other hand, where there is a long, cool growing 
 season this variety thrives and produces well. The plant is a rela- 
 tively vigorous grower, tall, erect, dark green. The blossoms are 
 usually profuse and are purple in color. By these purple flowers, 
 mixture of British Queens in other varieties is often noticeable in 
 all parts of the state. The tubers are slightly elongated, slightly 
 flattened, smooth, with both stem and apical ends quite square and 
 blocky. 
 
 The market demand for the British Queen is not great, and it 
 usually sells at a much lower price than other standard varieties. 
 This is said to be due to the tendency of the potatoes to cook to 
 pieces when boiled, or to burst when baked. The culture of this 
 variety is therefore not recommended, except for limited acreages in 
 the regions where it is adapted. Growers, in sections where climatic 
 
ClRC. 287] POTATO PRODUCTION IN CALIFORNIA 13 
 
 conditions prevent its maturing properly, have suffered much loss 
 through the buying of seed of this variety under misleading names. 
 Loss also regularly occurs through the admixture of a small per- 
 centage of this seed with that of other varieties, especially of those 
 which mature and are dug before the British Queen has formed 
 tubers of marketable size. 
 
 While this variety is of British origin, it is different from the 
 English variety known as British Queen, but seems to be identical 
 with the standard English variety Up-to-date. In the eastern states 
 it is grown under various local names, and in Oregon it is known as 
 Purple-flowered White Rose — a very misleading name to California 
 growers accustomed to the California White Rose, a very different 
 sort of potato. 
 
 Irish Cobbler. — The Irish Cobbler is the standard early variety of 
 the East but is little grown in California. Tests show that it yields 
 well both in northern and southern California, and matures two to 
 three weeks before the White Rose. The tubers are white and nearly 
 round, and the eyes are relatively deep, especially when grown on 
 heavy soils. The plants are very dwarf and stocky, the blossoms are 
 purple when first open, fading to white under intense sunlight. This 
 variety is notably free from the tendency to produce knobby second 
 growths or vegetative sprouts from the growing tubers. 
 
 Bliss Triumph. — The Bliss Triumph is the standard extra-early 
 variety of the Middle West and Gulf States, but is little grown in 
 California. It might well be grown in some places for early market 
 as a substitute for the Garnet Chili. The tubers are nearly round, 
 and are reddish in color. It yields well and matures early. 
 
 Idaho Rural. — This variety is also known as "Charles Downing" 
 and seems to be similar to a variety called "Earliest of All." The 
 plants are relatively small, with finely divided leaves and white 
 flowers. The tubers are somewhat elongated, flattened, and smooth. 
 It has yielded well in northern and southern California, and while it 
 is not exactly the type of tuber in demand on our markets, it should 
 be further tested because of its earliness in maturity and its good 
 yields. It may be expected to produce marketable potatoes in soils 
 too heavy to produce most other varieties to advantage. 
 
 Late Rose. — This variety seems to be grown only in the Marin- 
 Sonoma County district. It is an exceedingly late-maturing variety, 
 having a very hardy, vigorous plant, bearing white flowers. The 
 tubers are rough, inclined to be knobby, and because of their pink 
 skin are not popular on most markets in their season, which is during 
 the late fall and winter. 
 
14 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 Green Mountain. — This is a standard main-crop variety of the 
 Northeastern states, and is grown to a slight extent in California 
 under such names as Gold Coin, Snow, and Carmen. The plants 
 are large and vigorous; blossoms white, tubers white, elongated, 
 slightly flattened, but rather rough in appearance. It will probably 
 outyield most other sorts, but does not appear to be a particularly 
 attractive market type when grown under irrigation. 
 
 SELECTION OF SEED 
 
 Much of the success of growing a crop of potatoes depends upon 
 securing "good seed." Just what constitutes good seed potatoes is 
 a difficult question to define, however. Few commercial potato 
 growers produce their own seed, and perhaps in the majority of cases 
 it is best that they do not. Therefore, the most of the potato acreage 
 is planted with seed purchased from another grower, a dealer, or a 
 seedsman. Seed so purchased is generally of unknown history and 
 quality, except as to its appearance when received, and sometimes 
 as to the reputation of the seed grower and the region where the seed 
 was grown. Appearance of the seed is of practically no value in 
 determining the quality of seed potatoes but the last two factors may 
 mean a good deal. The production of high-grade seed potatoes is 
 coming to be more and more a specialized business, and the need for 
 some assurance as to quality has led to the development of "certified 
 seed" production, a feature that will be discussed in detail at the 
 end of this bulletin. 
 
 FACTOES AFFECTING SEED QUALITY 
 
 (1) Appearance. — While we are usually told to select tubers of 
 a certain definite type, smooth, good sized, and attractive looking, 
 none of these things have any connection with its producing qualities. 
 The appearance and shape of tubers depends much on the soil and 
 climate where grown. Experiments have shown that there is no 
 correlation between show-stock qualities and ability to produce. 
 Naturally, tubers are preferred that are free of knobby second 
 growth, rots, and tuber-borne diseases, such as scab and Rhizoctonia. 
 
 (2) Region of Production. — There is a firmly rooted idea that 
 seed from northern districts is more vigorous than that from more 
 southern localities. Certainly it is true that potatoes grown under 
 cool, humid climatic conditions are better than those grown in hot 
 sections. Seed grown in southern California as a fall crop, in the 
 
ClRC. 287] POTATO PRODUCTION IN CALIFORNIA 15 
 
 cool season of the year, may equal or surpass in producing quality 
 that grown in the Northwest. While climate no doubt has some direct 
 effect on the vigor of seed potatoes, it also has been found that high 
 temperatures and low humidity mask the effect of certain diseases, 
 especially mosaic, so that it is not possible to rogue out the diseased 
 plants from the seed fields effectively where the temperatures are 
 high, humidity low, and sunlight intense. These diseases are mani- 
 fested most clearly in cooler, more humid sections. Thus the climatic 
 factor may have much to do with the elimination of diseases, such as 
 mosaic, which under severe climatic conditions weaken the plants 
 without being readily visible. There is another factor in location, 
 too. Some places are relatively free from aphids and other insects, 
 and therefore enjoy a natural advantage in production of healthy 
 seed. Where the potato aphis occurs in large numbers it is impossible 
 to keep up disease-free seed stocks even when some roguing is done. 
 
 (3) Irrigated vs. Dry Land Seed. — No constant relation between 
 this factor and seed quality has been established, except that in some 
 sections plants affected by certain diseases, especially spindle tuber, 
 that would eliminate themselves under dry-farming conditions, are 
 enabled to survive under irrigation, thus producing more diseased 
 stock. Probably this factor can be disregarded in California. Pota- 
 toes subjected to high temperature and lack of water just prior to 
 harvest may result in inferior stock. 
 
 (4) Maturity of Seed. — It is generally believed that potatoes har- 
 vested in the immature condition are more vigorous for seed than 
 potatoes that have fully matured in the ground before harvest, and 
 there is much experimental data to justify this belief. Hence, crops 
 intended for seed are often planted later than a market crop would 
 be in the same section, so that vines will still be green when frosted 
 in the late fall. There are probably several factors responsible for 
 the advantages of immature seed. 
 
 (5) Storage Conditions. — These have a considerable effect on 
 quality of seed. Potatoes kept in a warm place and allowed to 
 develop long sprouts before planting will not produce good stands 
 or vigorous plants. Much loss is due to decay of seed pieces in the 
 ground, a fact which is often traceable to the wilted or shrivelled 
 condition of the seed stock at the time it was planted. Storage rooms 
 should be well ventilated, otherwise blackheart may occur. Where 
 cold storage is used, a temperature of 36° to 40° F. is most favorable. 
 Good ventilation is especially necessary to carry off surplus moisture, 
 thus inhibiting both decay and sprouting of the potatoes, which are 
 
16 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 likely to occur in moist storage houses. Bins provided with false 
 bottoms and with bottom and top ventilation, have proved most satis- 
 factory for bulk storage. 
 
 (6) Infection with Diseases. — Potatoes that show a large amount 
 of scab or of Rhizoctonia on the surface should not be planted. These 
 diseases in slight attacks are not serious if proper disinfection (in 
 mercuric chlorid) is practiced. Potatoes showing internal discolor- 
 ations, such as black-heart, internal browning, heat necrosis, or the 
 brown ring below the skin associated with wilt diseases, should not 
 be used. However, these diseases, which present more or less evi- 
 dence of their presence on or in the seed tubers, have less effect on 
 the quality of potatoes for seed, than the group of " virus' ' diseases. 
 
 This includes mosaic, curly dwarf, leaf roll, and spindle tuber. 
 The unduly pointed ends of potato tubers indicate the last named 
 disease. The first three, however, cannot be detected in the tuber. 
 Many cases have occurred where attractive-looking seed stock has 
 produced plants practically 100 per cent of which were affected 
 with mosaic or curly dwarf, and consequently resulted in a very 
 poor crop. These diseases are the most serious factors in getting 
 good seed potatoes. The only way to avoid them is to make sure that 
 they are absent in the fields where the seed was grown. These diseases 
 can be detected only in the growing plants. Many growers who 
 realize the loss caused by these diseases have learned to inspect during 
 the preceding season the field from which they expect to purchase 
 seed. Thus, selection of seed becomes a matter of selecting a healthy 
 field in a section known to produce vigorous healthy potatoes. Such 
 a practice, however, is not possible for all growers ; neither are many 
 growers sufficiently acquainted with the diseases to judge a field cor- 
 rectly. Hence we have had developed a system of seed certificaton, 
 whereby the fields are inspected by a disinterested specialist, who 
 certifies as to their health and seed-quality, insofar as these qualities 
 can be judged. 
 
 As an illustration of the loss caused by mosaic disease, the results 
 of a test carried out in 1923 in cooperatioon with the Agricultural 
 Extension Service of Los Angeles County, are given in table 3. 
 Several lots of White Rose potatoes were obtained from different 
 growers and planted at Van Nuys. 3 The per cent of plants affected 
 by mosaic in each lot was recorded. 
 
 It will be seen that the lots having much mosaic produced low 
 yields, while lots having little mosaic produced the highest yields. 
 
 3 In cooperation with Mr. A. J. Mueller. 
 
ClRC. 287] POTATO PRODUCTION IN CALIFORNIA 17 
 
 There were evidently other factors affecting the results in this test, 
 but the difference in highest and lowest yield demonstrates the effect 
 of mosaic-infected seed on yields. 
 
 TABLE 3 
 
 Kelation of Per Cent Mosaic to Yield of Different Strains of 
 White Rose Potatoes 
 
 Per cent Yield, 
 
 Lot No. Mosaic pounds per acre 
 
 104 29,550 
 
 185 12.7 25,950 
 
 100 11.6 24,515 
 
 105 18.7 23,660 
 
 101 4.0 23,190 
 
 110 9.2 19,675 
 
 102 4.0 18,365 
 
 103 63.0 17,200 
 
 107 : 66.0 16,790 
 
 PREPARATION OF SEED FOR PLANTING 
 
 Many experiments have been made on the question of whole vs. 
 cut seed-pieces. In general, it may be said that small tubers (from 
 one to two ounces in weight) may be safely used for seed purposes 
 if such small tubers come from healthy plants. It is dangerous to 
 select small tubers for seed purposes from the bin unless one is sure 
 that all the plants in the field where the crop is grown were free 
 of virus diseases — otherwise the chances are that most of these tubers 
 come from diseased plants and will produce a diseased progeny. An 
 advantage of whole seed or "drop seed," as it is usually called, is 
 that it seldom decays without sprouting, as cut seed sometimes does, 
 and hence gives better stands. Also the slight commercial value of 
 the small tubers makes them a cheap source of seed. Disadvantages 
 of whole seed are the difficulty of planting them with a machine, 
 which generally necessitates hand planting; the danger of disease 
 transmission; the slowness of germination if immature or the exces- 
 sive number of sprouts produced if the seed has been in storage a 
 long time. Drop seed, much used abroad, has never become popular 
 in America. The point should be made here that small potatoes — 
 one ounce in weight or over — need not be discriminated against if the 
 stock is from healthy fields — hence seed stock does not have to be 
 graded for size as does market stock. Tubers 1 to 2 ounces can be 
 planted whole, from 2 to 4 ounces cut in half cross-wise, and larger 
 tubers cut in the usual way. 
 
18 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 As to size of seed piece, many experiments have been conducted, 
 and many conflicting opinions are held. In deciding on the proper 
 size to which the seed is to be cut, it is necessary to consider also 
 the distance apart at which the seed is to be planted. If a wide 
 spacing (14-18 inches) is to be used, the seed pieces should be rela- 
 tively large — averaging 2 ounces each. If relatively close planting 
 distance (10-12 inches) is to be used, then smaller seed pieces, aver- 
 aging 1 to 1% ounces, will serve best. Experiments have generally 
 shown that yields increase with the amount of seed planted to the 
 acre, whether the greater amount is used by cutting to larger size 
 or by using small pieces planted closer together. 
 
 The larger the seed piece, the more stalks there will be, and con- 
 sequently the greater number of tubers set in each hill. However, 
 if there are too many stalks in a hill and consequently too many 
 tubers are set, few of them will reach marketable size. Hence, the 
 objection to seed pieces that are too large, aside from the greater cost. 
 The optimum size for any given planting distance depends on the 
 variety, the fertility of the soil, and whether or not the seed is fully 
 out of the rest period when planted. Seed pieces from potatoes that 
 are still in the dormant condition will usually produce only one or 
 two stalks each; pieces from potatoes that have already begun to 
 sprout will produce several; while pieces from potatoes that have 
 been kept dormant in cold storage for several months beyond the 
 end of their natural rest period will generally produce an excessive 
 number. The practice of the best growers in California is to use 
 from 8 to 10 sacks (110 pounds each) to the acre. This amount of 
 seed will generally give best results if cut to 1 to 1% ounce pieces 
 and planted 11 to 12 inches apart in the row. In a record-breaking 
 yield of 57,000 pounds an acre grown at Stockton in 1924, 22 sacks 
 of seed were planted to the acre. Both plant-food and water must be 
 supplied abundantly where very heavy seeding is practiced. 
 
 PLANTING 
 
 The date of planting depends upon the district where the crop is 
 grown, the most profitable season of marketing, and the hazard from 
 late frosts. It varies from November, in coastal districts like Colma 
 and Carlsbad, to July in interior locations where a late crop is grown. 
 Generally speaking, the earlier the planting date (without running 
 undue risk of frost), the more satisfactory the development of the 
 plants and of the crop, and the less the danger of injury from 
 tuber moth. We have already seen how the tendency in recent years 
 
CIRC. 287] POTATO PRODUCTION IN CALIFORNIA 19 
 
 in the Delta district is toward earlier planting and the use of earlier 
 varieties than formerly, in order to mature and market a larger por- 
 tion of the crop before heavy shipments from the Northwest commence 
 in September. Though the time of planting is a factor that affects 
 both the production and the value of a crop, it has to be determined 
 largely by local conditions. 
 
 For planting a machine is generally used. Several types are on 
 the market; namely, one-man machines, two-man machines, one-row 
 planters, and two-row planters. The type of planter used depends 
 on the preferences of the individual grower, the acreage he has to 
 plant, and the amount of help available. More nearly perfect stands 
 are likely to result where a two-man planter is used, but this is more 
 expensive than the one-man or "automatic" type. Some of the large 
 growers obtain good results with three of the one-row automatic 
 dropping type of machines, hitched abreast and drawn by a tractor. 
 
 It is desirable to have enough moisture in the soil at planting time 
 to germinate the seed promptly and to support its development until 
 the plant is several inches high. Most California potatoes are grown 
 in soils moistened by winter rains. It is occasionally necessary, how- 
 ever, to flood the land in preparation for the spring plantings and is 
 always necessary before planting the fall crop. When natural moist- 
 ure is to be held in the ground for planting rather late potatoes, it 
 is necessary to give a shallow cultivation occasionally to keep down 
 weeds that would otherwise sap the moisture from the soil very 
 quickly. If the w T eeds are kept down, most soils will hold their moist- 
 ure near enough to the surface for late spring planting. 
 
 Within a few days after planting, it is good practice to smooth 
 dow T n the planter ridges by cross-harrowing twice with the section 
 harrow. This destroys many germinating weeds directly in the row. 
 A second harrowing may be given just before the sprouts emerge, if 
 development of weeds or the crusting of the surface by late rains 
 warrants it. Usually as soon as the plants are up sufficiently to mark 
 the rows, row cultivation with a two-row cultivator is begun. Some- 
 times, it is necessary to give the field a c< blind" row cultivation before 
 the sprouts emerge, if weeds are developing rapidly. In the early 
 row cultivations, soil is thrown toward the plants slightly, but the 
 main "hilling up" is deferred until the rows are furrowed for the 
 first irrigation. 
 
20 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 IRRIGATION PRACTICE 
 
 Almost every conceivable variation of irrigation practice is fol- 
 lowed in various districts of California, ranging from culture under 
 winter rainfall conditions at Colma through dry-farming summer 
 culture in Marin County, to irrigation every 5y 2 or 6 days in the 
 Shafter district, and almost continuous irrigation through the main 
 part of the growing season in the Sacramento-San Joaquin district. 
 The amount of water required to grow a crop and the best frequency 
 of irrigation vary so much with local climatic and soil conditions, that 
 a general discussion is impossible. 
 
 The ideal condition is a uniformly ample supply of moisture 
 throughout the season, especially from the time tubers are formed 
 until shortly before harvest or maturity of the tops. The potato is 
 particularly sensitive to irregularities in moisture supply during this 
 period. Tuber formation begins when the plants are 5 or 6 inches 
 high, and is completed by the time the flower buds are formed. Con- 
 ditions during this period determine the number of tubers formed, 
 or the "set." From the beginning of the flowering period until the 
 plants are dead, the tubers grow at a regular rate, if uniformly favor- 
 able conditions exist. Hence conditions during this period determine 
 the development of the tubers, especially as to size, smoothness, and 
 shape. The rate at which potato plants exhaust moisture from the 
 soil is proportional to their leaf area, hence the need for water is 
 greater as the plants grow larger. Also, varieties characteristically 
 having large vines with a large leaf area use more water than do 
 those having a smaller foliage system. The use of water is further 
 influenced by temperature and by humidity of the air — higher tem- 
 perature and lower atmospheric humidity increase the transpiration 
 rate. It is quite possible that large plants under mid-day conditions 
 in summer may give off moisture faster than their roots take it up, 
 even from a moist soil. This condition leads to incipient wilting and 
 is apparently connected with several troubles that potatoes are sub- 
 ject to under irrigated conditions, such as knobby tubers, vegetative 
 sprouts from half-grown tubers, heat necrosis, and internal browning. 
 
 In Utah, where the water requirements for potatoes are probably 
 not as great as in central and southern California, Harris 4 found that 
 where water was applied once a week during the growing season, 
 applications of 1 acre-inch at each application gave better results 
 than 2V2, 5, or 7 inch applications. In a set of experiments designed 
 
 4 Harris, F. S., The irrigation of potatoes. Utah Expt. Station Bull. 157 : 1-20, 
 1917. 
 
ClRC. 287] POTATO PRODUCTION IN CALIFORNIA 21 
 
 to find at what stage of development a single irrigation would be of 
 most benefit to the crop if only one application could be made for the 
 whole season, the best yields were secured by applying the water 
 when the plants were in full bloom, rather than at earlier or later 
 stages. The largest yield secured from any of the treatments tested 
 was obtained by the application of one acre-inch each week. This 
 required only 13 acre-inches for the season and served to emphasize 
 the importance of regular moisture supply through the growing 
 season. 
 
 Another point requiring careful consideration is the proper time 
 to commence irrigating. Irrigating too early may favor the develop- 
 ment of an overly large plant. Such over development may accent- 
 uate moisture troubles later in the season. A fair supply of moisture 
 in the soil, however, during the tuberizing period — just before the 
 flower buds form — is necessary to secure a good "set" of tubers. 
 Allowing the plants to suffer for water after the tubers are already 
 formed results in rough, ill-shaped tubers. Any very decided check 
 from lack of water the latter part of the growing season, even for a 
 short period, followed by late irrigation, is almost certain to result 
 in knobby second growth and cracking of the nearly mature tubers 
 by "growth cracks." One rule is to delay the first irrigation until 
 plants show a slower rate of growth indicated by darker green color 
 of foliage, but have not yet reached the stage where there is even 
 slight wilting during the midday period. Thereafter, irrigations 
 should be frequent enough to keep the plants growing at a uniformly 
 rapid rate. The total amount of water required by potatoes is not 
 large if it is well distributed. Frequent irrigations appear to be 
 most desirable for the development of both plant and tubers. 
 
 FERTILIZATION PRACTICE 
 
 In many districts it has been found necessary to give attention 
 to soil fertility in potato production, through applications of animal 
 manures or chemical fertilizers or by plowing under green manuring 
 crops. However, only limited information exists as to the kinds of 
 fertilizers needed in the different districts or as to amounts that will 
 give profitable returns. 
 
 Some indication of the benefit of nitrogenous fertilizers for pota- 
 toes at Riverside, California, was obtained by W. M. Mertz. 5 A 
 summary of his results on plots given the same treatment for five suc- 
 
 s Mertz, W. M., Green manure crops in southern California. California Expt. 
 Sta. Bull. 292:1-32, 1918. 
 
22 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION 
 
 cessive years is given in table 4. However, only two crops of potatoes 
 were grown in this period, the results for the two years being aver- 
 aged in the table. The check plots in this experiment receiving 
 "minerals only" were given applications of acid phosphate and 
 potash salts. 
 
 TABLE 4 
 
 Eesults with Nitrogenous Fertilizers on Potatoes at Eiverside 
 
 Treatment Yield per acre Per cent gain 
 
 lbs. 
 
 Average of four plots receiving minerals only 9,732 
 
 Same plus 41 lbs. nitrogen per acre 9,982 2.6 
 
 Same plus 82 lbs. nitrogen per acre 12,256 26.0 
 
 Same plus 123 lbs. nitrogen per acre 11,504 18.3 
 
 Same plus 163 lbs. nitrogen per acre 13,103 34.8 
 
 The application of nitrogen giving the greatest gross increase in 
 Mertz' experiment, 163 pounds to the acre, would be equivalent to an 
 application of 815 pounds of sulfate of ammonia, more than is gen- 
 erally considered either safe or economical to apply. 
 
 Recently, a large number of experiments with commercial fertil- 
 izers on potatoes have been carried out by the Agricultural Extension 
 Service in Los Angeles County, under the direction of L. C. Holmes 
 and F. H. Ernst, in cooperation with potato growers in that county, 
 and have demonstrated large increases in yield of potatoes. These 
 experiments were conducted on a large scale under field conditions 
 on a number of different kinds of soil, and show strikingly the value 
 of nitrogenous fertilizers for potatoes in that section. A summary of 
 the results for 1922 is given below. 6 The different mixtures were all 
 calculated to supply about the same amount of nitrogen to the acre, 
 from 40 to 42 pounds. 
 
 Per cent increase 
 over checks 
 
 320 lbs. fish meal plus 80 lbs. sulfate of ammonia, per acre 67 
 
 500 lbs. cotton seed meal plus 80 lbs. sulfate of ammonia, per acre 61 
 
 500 lbs. fish meal alone, per acre 55 
 
 700 lbs. cotton seed meal alone, per acre .. 40 
 
 500 lbs. tankage alone, per acre 38 
 
 Average yield of 18 check plots receiving no fertilizer, 64 sacks per acre. 
 
 e Data from L. C. Holmes, formerly Assistant Farm Advisor, Agricultural Ex- 
 tension Service, Los Angeles County. 
 
Circ. 287] 
 
 POTATO PRODUCTION IN CALIFORNIA 
 
 3E 
 
 23 
 
 Fig. 6. — Effect of nitrogenous fertilizer on growth of potatoes at El Monte. 
 On right, no fertilizer; on left of tree row, sulfate of ammonia and fish meal 
 was applied. (F. H. Ernst.) 
 
 Fig. 7. — Effect of nitrogenous fertilizers on yield of potatoes in Los Angeles 
 County. Yield with sulfate of ammonia and fish meal was 141 sacks per acre. 
 without fertilizers the yield was 104 sacks per acre. (F. H. Ernst.) 
 
24 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 As to the source of nitrogen in potato fertilizers, rather quickly 
 available materials are preferred. Sulfate of ammonia and fish meal 
 used in combination seem to be most satisfactory. Nitrate of soda 
 is not favorable because it tends to increase the alkalinity of the soil, 
 and as it is easily soluble in water, may be leached away before it is 
 fully utilized by plants. Phosphorus, another element generally used 
 in complete fertilizers, has shown no significant increases in yield in 
 the Los Angeles experiments, and potash likewise showed no increase 
 on the heavier soils, especially where green manuring crops had been 
 plowed under. However, on the lighter, more sandy soils, potassium 
 gave some increase in yield. As to the form in which potassium is 
 used, there seems to be little choice. Many experiments have been 
 conducted to determine whether there was any difference in the 
 effects produced by potassium chloride (muriate of potash) and 
 potassium sulfate. No significant difference between results with 
 these materials has been obtained, either in this country or in Europe. 
 
 The use of chemical fertilizers has also been carefully investigated 
 by some of the large growers in the Delta district. The potatoes here 
 are grown on muck soils, which are generally deficient in minerals. 
 Applications of fertilizers carrying a high percentage of phosphorus 
 and potash have therefore become the practice. A favorite fertilizer 
 mixture has been the 0-8-10 formula. An 0-21-24 formula is also 
 used, this being obtained by mixing equal quantities of triple acid 
 phosphate and sulfate of potash. While muck soils contain a rela- 
 tively large amount of nitrogen, it is mostly in a form unavailable 
 to plants, hence, some growers have found it profitable to use a small 
 percentage of nitrogen in their potato fertilizers. Where only a small 
 amount of nitrogen is desirable, the 3-12-12 formula is recommended. 
 
 Many of the soils on which potatoes are grown, especially in the 
 Shafter district and in southern California, are very deficient in 
 organic matter and in nitrogen. One of the most economical methods 
 of building up the soils in this respect is the use of winter-growing 
 leguminous crops, plowing these under in early spring, prior to plant- 
 ing the potato crop. In a comparison of nine different winter- 
 growing legumes at Riverside, Mertz found that purple vetch (Vicia 
 atropurpurea) and bitter clover (Melilotus indica) were the most 
 satisfactory. These plants rank high in ability to grow during cool 
 weather and under poor moisture conditions. The purple vetch pro- 
 duced 20 tons of green manure to the acre, and was estimated to 
 return 228 lbs. of nitrogen to the soil, while the Melilotus produced 
 13.7 tons of green manure and furnished 152 pounds of nitrogen to 
 
ClRC. 287] POTATO PRODUCTION IN CALIFORNIA 25 
 
 the soil and increased the potato yield 54 per cent. As an illustration 
 of the beneficial effects of plowing under winter legumes for potato 
 production, the following summary from Mertz's results is given. 
 
 Pounds Per cent 
 
 per acre increase 
 
 Average yield on green manured legume plots .... 13,588 39.5 
 Average yield on non-legume plots, nitrates and 
 
 minerals 11,711 21.2 
 
 Average yield on non-legume plots, minerals only 9,732 
 
 It is seen that the plowing under of legumes increased yield more 
 than did heavy application of nitrogen in commercial fertilizers. 
 This is to be expected, for the green manure crops add much organic 
 matter to the soil, improve its texture and aeration, and leave the 
 soil in a more congenial condition for the development of potatoes. 
 However, great as is the benefit from plowing under green manures, 
 experiments in Los Angeles County show that additional nitrogen, in 
 commercial fertilizers, can be applied with advantage for potato 
 production. 
 
 The winter legumes should be planted in early fall; the more 
 growth they make before the colder part of the winter, the sooner 
 they can be plowed under in the spring, and the less do they exhaust 
 the soil moisture which is needed not only for the prompt decay of 
 the green manure itself but for the sprouting of the potatoes. 
 
 HARVESTING 
 
 The yield of marketable potatoes increases rapidly as the plants 
 approach maturity, even increasing somewhat after the vines have 
 turned yellow. Nevertheless, a considerable portion of the crop is 
 harvested when only partially matured, because early potatoes bring 
 better prices and because heat or tuber-moth may cause injury if 
 the crop is allowed to grow until the vines die down. All of the crop 
 in the extra-early districts, such as Colma and Carlsbad, is harvested 
 when only partly grown, as ' ' new potatoes, ' ' because of the favorable 
 market conditions. Most of the spring crop in central and southern 
 California is also harvested quite immature. 
 
 The skin on immature potatoes is very thin and is easily rubbed 
 off. As the potatoes approach maturity, the skin becomes thicker, 
 tougher and more firmly attached to the tuber. The advancement of 
 this change in the coat of the tuber is important in determining when 
 to start digging early potatoes. If harvested too immature, most of 
 
26 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION 
 
 the skin is knocked off in handling, sacking and loading. These 
 "skinned" areas quickly turn black and are often the starting places 
 for decay, or at least the potatoes are rendered very unsightly. 
 Immature potatoes, as indicated by the ease with which the skin is 
 rubbed off, are of very poor shipping and keeping qualities, especially 
 when dug in warm weather. As the potatoes become more mature, 
 the skin becomes thicker, and tougher, the potatoes stand handling 
 better, and may be expected to reach the market in better condition. 
 
 INSECTS OF THE POTATO 
 
 Stalk Borer. — Occasionally the potato stalk borer causes some 
 damage. The grower may recognize this insect in its larval stage, 
 as a white or yellowish grub, from % to % inch in length, burrowing 
 channels in the pith of the stalk. It pupates in the dead stalk where 
 the adult beetle also hibernates, emerging in the spring about the 
 time the new plants are coming up. 7 . A large percentage of the 
 plants was destroyed by the insect in some fields at Stockton in July, 
 1924. No satisfactory control measure is known. Burning of the 
 dead vines after the crop is harvested has been suggested, but would 
 not be practicable on muck or peat soils unless special arrangements 
 are made for extinguishing the fire. 
 
 Tuber-Moth. — This insect has been in California since the early 
 days, having been first reported at San Francisco in 1856. It has 
 gradually spread to all the important potato growing districts of the 
 state and has become one of the chief difficulties with which the 
 potato grower has to contend. The eggs are laid upon leaves and 
 stem and in the eyes of exposed tubers in spring and summer, or 
 upon the tubers before or at digging time. The larvae hatching 
 from these eggs burrow through the tubers, starting at the eyes, 
 where a little pile of black frass is noticeable if the larvae are within 
 the tuber. Affected tubers are rendered unfit for market and fre- 
 quently this injury is followed by decay. Further loss has been 
 caused the grower of early potatoes by quarantines against shipments 
 from tuber moth districts, put in effect by states of the Northwest, 
 to which California ordinarily ships a considerable quantity of early 
 potatoes. However, these states have been admitting potatoes, which 
 are certified to be free of tuber moth by the California State Depart- 
 ment of Agriculture. The recent development of a system of fumi- 
 gation with carbon bisulfide under vacuum will further aid in market- 
 ing early potatoes in the Northwest. 
 
 7 Essig, E. O., Injurious and beneficial insects of California, pp. 303-305. 
 
CIRC. 287] POTATO PRODUCTION IN CALIFORNIA 27 
 
 Control measures for tuber moth consist mainly in preventing the 
 infestation of the tubers. The following recommendations 8 have been 
 made. 
 
 1. Clean culture — Destroy weeds and volunteer potatoes which 
 harbor this insect. 
 
 2. Clean seed — Plant only seed known to be free from tuber moth 
 infestation. 
 
 3. Deep planting — Potatoes planted 5 to 6 inches deep will be less 
 affected. 
 
 /O 
 
 * ^ 
 
 Vi-, 
 
 Fig. 8. — Tuber moth injury to potatoes. On left, piles of black frasa are 
 noted at the eyes, where the larvae have bored into the tuber. On right, tunnels 
 of the larvae within the tuber. 
 
 4. High ridging — -After the tubers are set, the deeper they are 
 covered by ridging the soil toward the plants, the less likely they are 
 to be attacked. 
 
 5. — Do not allow the soil around the plants to crack open — High 
 ridging and careful irrigation will largely prevent this. A success- 
 ful practice has been to pass over the field with a light roller soon 
 after the last irrigation to close the cracks in the ridges. As the 
 plants are practically mature at this time, this treatment does not 
 affect the yield seriously, but does prevent infestation of exposed 
 tubers during the brief period intervening before the crop is dug. 
 
 6. Early harvesting — The larvae feed on the tops as long as they 
 are green, and afterwards will make special efforts to get at the 
 tubers. 
 
 s Calif. State Dept. Agric, Spec. Pub. 24:1-6, 1922. 
 
28 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 7. Never leave potatoes exposed over night as the moths fly and 
 lay their eggs mostly at night, hence potatoes left in the field are 
 likely to be covered with eggs, which will develop later in transit or 
 in storage. 
 
 8. Never place potato vines on top of the sacks or lugs containing 
 potatoes, for the larvae on the vines are likely to be transferred to 
 the tubers. 
 
 9. Place potatoes under refrigeration as soon as possible if they 
 are to be shipped. The low temperature of the refrigerator car checks 
 the development of tuber moths that may be in the potatoes. 
 
 Aphids. — Several species of aphids (green plant lice) frequently 
 attack potatoes in California. At times these insects are so numerous 
 that the under sides of the leaves are literally incrusted with them, 
 and the plants are thus greatly weakened. But usually, the attacks 
 are not severe enough to cause obvious damage. However, the various 
 species of aphis have been proved the principal means of spreading 
 throughout the field such "virus" diseases as mosaic, curly dwarf, 
 leaf roll, and spindle tuber. Hence, their control is especially im- 
 portant to the seed growers. Aphis feed on diseased plants, then 
 migrate to adjoining healthy plants carrying infection with them. 
 Plants infected in this way may not show it the same season, but the 
 tubers grown on them will produce diseased plants the following year. 
 
 The various kinds of aphis attacking potatoes can be controlled 
 by spraying with nicotine solutions. "Black Leaf 40," diluted 1 to 
 800 in water, is effective if carefully applied. Recently nicotine dusts 
 have been used very effectively against aphis. Dusts containing 2 per 
 cent of nicotine sulfate are effective. For application on a field scale, 
 horse-drawn power dusters, equipped with a canvas trailer to hold the 
 dust over the plants, have proved convenient. Potato plants should 
 be watched closely during the early part of the season and control 
 measures applied before the aphis become numerous. 
 
 Nematodes. — The garden nematode (Heterodera radicicola) or eel- 
 worm is a soil-infesting microscopic animal of a lower order than the 
 insects, and is the cause of a very common disease known as root 
 knot on the roots of many plants. In potatoes, the tubers are chiefly 
 affected, but characteristic galls have also been found on the fibrous 
 feeding roots. Many acres of sandy and muck soils in California 
 have become infested, but fortunately this nematode does not thrive 
 in the heavier soils. Tubers infested with nematodes show numerous 
 small raised galls over the surface, and when such potatoes are sliced, 
 
CJRC. 287] POTATO PRODUCTION IN CALIFORNIA 29 
 
 small discolored channels are found just beneath the skin. The 
 potatoes so affected are worthless for market or for seed purposes. 
 
 Nematodes have been widely distributed on affected nursery stock, 
 vegetable plants, and seed potatoes. Once a field becomes infected, 
 the insects are spread rapidly by farm animals, cultivating tools, and 
 irrigation water. The only satisfactory method of freeing infected 
 soils of nematodes is to grow immune crops for at least two years. 
 Members of the grass family, particularly the cereals, and the Iron 
 and Brabham varieties of cow peas, are suitable for this purpose. It 
 is, of course, necessary to keep down weeds that the nematode might 
 live on. Care should always be taken to see that seed potatoes, 
 especially those brought in from other districts, are free of nematodes. 
 
 Pig. 9. — Tuber infested with Nematodes. The galls are especially prominent 
 at the eyes of the tuber. 
 
 DISEASES 
 
 Various diseases take a considerable toll of the potato production 
 in California, and the control or prevention of these diseases is one 
 of the most important considerations in growing satisfactory crops 
 and in getting good yields. The following table shows the average 
 losses caused by the principal diseases in California as estimated by 
 D. G. Milbrath of the State Department of Agriculture, for 1922 and 
 1923. Of course in some sections the loss from certain diseases is 
 sometimes much greater than is indicated by the averages given in 
 this list. 
 
30 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 Per cent loss Per cent loss 
 
 Common scab 0.3 Fusarium root rot 0.5 
 
 Rhizoctonia 6.0 Fusarium dry rot of tuber 0.5 
 
 Wilt diseases 0.5 Mosaic (and curly dwarf) 5.0 
 
 Seed piece decay 2.0 Leaf roll 0.5 
 
 Leak of tuber 0.5 Internal browning of tuber 2.0 
 
 Jelly end rot 0.2 Heat necrosis of tuber 1.5 
 
 Early blight 1.0 Black heart of tuber 0.4 
 
 Late blight 0.1 
 
 Fortunately, not all of these diseases are likely to occur in the 
 same field or at the same time, for the conditions favoring some are 
 unfavorable to others. Below is given a brief description of the prin- 
 cipal diseases and their control, followed by a detailed description of 
 general control measures. 
 
 Common Scab (Actinomyces sp.). — This disease is caused by a 
 group of soil bacteria of very wide distribution, found even in virgin 
 soils. The numbers of the organisms and the seriousness of the dis- 
 ease increase rapidly under condition of more or less continuous 
 potato production. The disease, however, while widely distributed 
 in California, has not been the cause of a very great loss. The tuber 
 is the only portion of the plant attacked, the disease appearing first 
 in small brownish spots that gradually enlarge into rough, corky 
 patches, which may run together so that practically the whole sur- 
 face of the tuber is covered. The scabs are usually elevated slightly 
 around the margins, but may be sunken somewhat in the center. The 
 exact appearance of scab seems to vary a good deal, probably accord- 
 ing to the soil conditions where it occurs, or according to the particu- 
 lar species of the scab organism that happens to be present. 
 
 Scab is often introduced on seed tubers. This source of infection 
 can be practically eliminated by dipping the seed in formaldehyde 
 solution, but as this treatment is not so effective against other tuber 
 borne diseases as corrosive sublimate, the latter is preferred. Seed 
 treatment, however, is largely without effect if the soil is already 
 infected; hence, some method of rendering the soil unfavorable for 
 the development of the disease is needed. It has long been observed 
 that scab is more serious where lime has been applied. Since lime 
 makes the soil more alkaline, it was thought that the disease might be 
 checked by making the soil more acid. This has been done very suc- 
 cessfully in some eastern states, by the application of sulfur. The 
 sulfur is oxidized by soil bacteria to sulfuric acid, which changes the 
 soil reaction, either reducing the alkalinity or making it slightly acid, 
 
ClRC. 287] POTATO PRODUCTION IN CALIFORNIA 31 
 
 dependng on its original reaction. Fertilizers such as sulfate of 
 ammonia, have the same effect. A series of plots at the University 
 Farm at Davis were treated with various fertilizers in the spring of 
 1923 and again in 1924. Potatoes were grown both years on the same 
 plots. The fertilizers were applied broadcast and then harrowed in 
 just before planting. Table 5 gives the results of yield and the 
 per cent of tubers from each plot infected with scab, for the spring 
 of 1924, the White Rose variety, planted February 26, being used. 
 Part of each plot was dug June 18 and the rest on July 2, when the 
 plants were beginning to mature. 
 
 TABLE 5 
 
 Effect of Various Materials cn Occurrence of Scab on Potatoes 
 
 Yield in lbs. per acre Per cent 
 
 t K ^ scabby potatoes 
 
 Treatment — June 18 July 2 (average) 
 
 Inoculated sulfur, 300 lbs. per acre 5,850 8,970 29.2 
 
 Commercial sulfur, 300 lbs. per acre 6,260 9,670 27.2 
 
 Check, no treatment 5,730 8,440 47.0 
 
 Sulfate of ammonia, 225 lbs. per acre .. 6,360 9,870 34.8 
 
 Nitrate of soda, 300 lbs. per acre 6,500 9,540 47.0 
 
 Check, no treatment 6,370 9,440 51.4 
 
 Acid phosphate, 300 lbs. per acre 6,560 9,650 46.3 
 
 Sulfate of ammonia, 225 lbs. per acre ~) 
 
 Acid phosphate, 300 lbs. per acre i 7,140 9,470 27.1 
 
 Potassium chloride, 100 lbs. per acre J 
 
 While these results do not reveal any very definite effect of the 
 fertilizers upon yield of potatoes, they do indicate a marked effect on 
 the prevalence of scab. Comparing the two check plots, with 47.0 
 and 51.4 per cent of scab respectively, to the two sulfur plots with 
 29.2 and 27.2 per cent of scab respectively, it is seen that both forms 
 of sulfur considerably decreased the amount of scab and correspond- 
 ingly increased the percentage of clean potatoes. It should be 
 recalled, however, that these results were for the second year these 
 plots had received the same treatment. The first year, two crops were 
 grown; the spring crop, however, was a failure and no scab counts 
 were made. The fall crop that year gave differences in scab infection 
 similar to the above. On the other hand, two experiments carried 
 out in Los Angeles County in 1922 showed no effect from application 
 of sulfur for control of scab. Under conditions of severe soil infec- 
 tion application of sulfur, however, is likely to prove helpful, unless 
 the soil is excessively alkaline. 
 
32 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 Rhizoctonia (Corticum vagum). — This is probebly the most de- 
 structive fungous disease of potatoes in California. Like scab, it is 
 transmitted both on the surface of seed tubers and in the soil. Except 
 where potatoes are grown continuously, or where they follow some 
 other susceptible crop, soil infection is not so serious with Rhizoc- 
 tonia ; control, therefore, by seed treatment is more effective. Injury 
 as a result of infections from the organism already in the soil, how- 
 ever, often causes serious loss even where treated seed is planted, 
 especially where potatoes follow Lima beans, another host plant, in 
 southern California. 
 
 Fig. 10. — ' ' Black Scurf, ' ' the sclerotia or resting stage of Ehizoctonia on 
 surface of the tuber. 
 
 The disease appears on the surface of the tuber as small, black 
 specks, the sclerotia or resting state of the fungus. This stage does 
 little harm in itself, but detracts somewhat from the appearance of 
 the tubers. It is, however, the source of infection to the plants grown 
 the following year from infected seed. When such seed is planted, 
 the fungus starts growing and attacks the sprouts, leading to another 
 phase of the disease, in which the direct loss is great, resulting in 
 many missing hills in the potato field or in weak non-productive 
 plants. Upon digging up the seed piece, it is found that the tip of 
 the sprout has died before reaching the surface of the soil. A second 
 sprout may be sent up from the first one and may die in the same 
 way. Or the sprout may reach the surface and produce only weak, 
 spindly plants, because of injury to the lower part of the stem. In 
 this kind of attack, the disease is recognized as brown decayed areas 
 
ClRC. 287] POTATO PRODUCTION IN CALIFORNIA 33 
 
 on the white underground stems. Plants may also be attacked later 
 in the season. On the underground part of the stem are found super- 
 ficial brownish areas, often entirely girdling the stem, sometimes 
 accompanied by deep lengthwise cracks in the cortex. Plants attacked 
 in this way may turn yellowish, the leaves become considerably rolled, 
 and small greenish tubers appear in the axils of the leaves on the 
 above-ground stem. Without showing such pronounced symptoms, 
 the fungus may attack also the tuber-bearing stolons which branch 
 out from the underground stem, and thus prevent tuber formation. 
 
 The Rhizoctonia fungus thrives best at relatively low temperatures 
 and is most likely to attack potatoes planted in the cooler part of the 
 year. This has led some growers to omit the precaution of treating 
 the seed when planting a summer crop. It is advisable, however, to 
 treat the seed no matter what season of the year it is planted, and 
 regardless of whether or not the seed seems to be infected. It has 
 been shown that seed, which to the eye appears to be free of Rhizoc- 
 tonia, may have a considerable amount of disease in the resulting 
 crop unless disinfected before planting. 
 
 Soaking the seed in corrosive sublimate solution (1-1000) for IV2 
 hours is the usual method of disinfecting seed for the prevention of 
 this disease. Formaldehyde (in cold solution) is not effective against 
 Rhizoctonia. Rotation of crops is also important in preventing this 
 disease. 
 
 Wilts. — There are at least three organisms that cause a wilt disease 
 of potatoes more or less generally in California, though seldom to any 
 great extent. These are Fusarium radicicola, Fusarium oxysporium, 
 and Verticillium albo-atrum. The effects of these three diseases are 
 so nearly alike that they cannot be distinguished in the field. The 
 attacked plants wilt slowly and die, even after they have reached 
 nearly their full size. The first two diseases at least are favored by 
 high temperatures and other conditions unfavorable for the potato 
 plant. If the lower part of the stems of plants attacked by wilt are 
 split, it will be found that the woody portion is brown in color, in 
 contrast to the greenish white of healthy stems. Many of the tubers 
 produced by wilt-infected plants will also show a dark ring in the 
 region of the vascular bundles just below the skin, if clipped at the 
 stem end. However, there are other causes of similar darkening of 
 this vascular ring ; in fact, McKay 9 in Oregon found that less than 
 half of the tubers so affected contained wilt organisms, and on the 
 other hand, many potatoes that showed no discoloration, did contain 
 
 9 McKay, M. B., Transmission of some wilt diseases in seed potatoes. Jour. 
 Agric. Kesearch, 21: 821-848, 1921. 
 
34 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 the wilt organism. This shows that it is impossible to select wilt-free 
 seed simply by discarding tubers showing internal discoloration. 
 
 There are three methods of infection: from the soil through the 
 seed pieces ; from the soil through the roots ; and from the seed tuber 
 itself when infected seed is planted. Rotation of crops, wherein 
 potatoes are not grown more than once in four or five years on the 
 same land will aid in reducing loss from wilt. As a general practice, 
 it is best to discard all tubers that show any internal discoloration, 
 sa far as use of seed is concerned, but this by no means insures one 
 against the occurrence of wilt in the resultant crop. Disinfection of 
 the seed is of no avail against wilt, for the organism is within the 
 tuber and so is not reached by the disinfectant. The only reliable 
 means known for avoiding wilt is the use of seed from healthy plants 
 only. Under practical conditions this means the removal of wilt- 
 infected plants from the seed fields as soon as they occur. McKay 
 has show that the disease spreads from wilted plants to adjoining 
 healthy plants rather quickly; hence, the need for frequent and 
 prompt roguing of the seed fields. Growers of certified seed especially 
 should be on the watch for wilted plants and remove them promptly. 
 A single roguing toward the end of the growing season is of little 
 use. 
 
 Stem-end Rot (Jelly-end Rot). — This is a type of decay that quite 
 commonly attacks the stem end of long-tubered varieties. It takes 
 several different forms and is said to be caused by several different 
 organisms, principally different species of Fusaria. It is usually 
 noted in the field at harvest time, especially if the crop is fully 
 matured. Common forms of stem end rot show a dry, sunken, brown- 
 ish, or black discolored condition for a short distance back from the 
 stem end, or a soft, jelly-like, light brown colored rot extending an 
 inch or more from the stem end. In either case, the remainder of the 
 tuber appears sound and is separated by a definite boundary from 
 the decayed area. The rot may or may not make further advances 
 during storage. 
 
 In so far as stem-end rot is associated with the diseases that cause 
 wilt, it can be controlled by the means suggested in the preceding 
 section. However, some stem end rot occurs where the wilt diseases 
 are not apparent. Doubtless some of the surface and stem end rots 
 that occur after harvesting are caused by soil organisms that gain 
 entrance to the tuber through the cuts and bruises resulting from 
 rough handling. 
 
Circ. 287] 
 
 POTATO PRODUCTION IN CALIFORNIA 
 
 35 
 
 Leak. — The leak disease, which occurs in potatoes dug and shipped 
 during hot weather, has been a cause of some loss in the Delta dis- 
 trict. It is caused by soil organisms that gain entrance through 
 wounds made in digging the potatoes by hand, a practice formerly 
 quite general in the Stockton district. The disease is not common 
 in potatoes grown in newly reclaimed or burned over peat lands. 
 Hawkins 10 found that this was due to the absence from these soils 
 of the rot-producing organisms. He concluded as a result of his 
 investigations that sorting out all wounded potatoes (injured by 
 
 Fig. 11. — Plant on right shows mild infection of mosaic. Healthy plant 
 on left. 
 
 digging forks and by breaking off knobby second growths) would 
 insure the lot from damage by this disease. Fortunately, the disease 
 is decreasing in prevalence because of the increasing use of machine 
 diggers in the Delta district, and the planting of a type of potato 
 less likely to produce knobby second growths than the Burbank, 
 which formerly was grown exclusively in the Delta. 
 
 Mosaic. — This term is used to include what appears to be at least 
 three distinct diseases, known as mild mosaic, leaf-rolling mosaic, 
 and rugose mosaic, all having the characteristic symptoms of dwarf- 
 ing the plant to a greater or less extent, mottling and wrinkling the 
 leaves, and reducing the size of the tubers. Mosaic is one of the 
 
 !o Hawkins, L. A., Experiments in the control of potato leak, IT. S. Dept. 
 Agr., Bull. 577:1-5, 1917. 
 
36 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION 
 
 commonest and most threatening of diseases found in potatoes, not 
 only in California but throughout the country. It has already been 
 shown (p. 17) how the presence of mosaic diseases reduces the yield 
 of infected stock. In fact, it is now thought that most, if not all 
 of the troubles formerly attributed to degeneration and "running 
 out" of seed stocks, are due to mosaic and related diseases. 
 
 Certain definite characteristics of the mosaic diseases should be 
 clearly understood. While no micro-organism has been found as the 
 cause of these diseases, still they are infectious and spread from plant 
 to plant in the field, the chief agency for dissemination being the 
 aphis or plant lice. These insects, after sucking juice from a diseased 
 plant, may move to a healthy plant and infect it. No sign of such 
 secondary infection may appear, but the tubers from such plants will 
 produce diseased plants the following year. In this way, a lot of 
 potatoes that has only a few mosaic plants one year may show almost 
 complete infection the following year, if aphis have been numerous. 
 While the disease is transmitted from year to year in the seed tubers, 
 such infected tubers show no sign of the disease, and may, in fact, 
 have the appearance of very fancy potatoes. Cases have been re- 
 corded where prize-winning lots from fairs or shows were planted the 
 following year and gave almost 100 per cent infection of mosaic with 
 correspondingly poor yields. Mosaic is likely also to be transmitted 
 from year to year in volunteer potatoes which spring up from the 
 culls left in the field at digging time. Other crop plants — tomatoes, 
 tobacco, beans, and cantaloupes — have mosaic diseases, and it is pos- 
 sible that infection may be carried from one crop to another. Weeds, 
 especially perennial or winter-growing, are likewise suspected of 
 wintering over the disease. 
 
 There is no direct method of control for mosaic diseases, but de- 
 pendence is placed in use of healthy seed stock. Since there is no 
 sign of these diseases in the tuber, the only way to form any opinion 
 as to the health of the seed is to examine the growing plants in the 
 field the preceding season. To prevent the dissemination of these 
 diseases in the seed-field, careful roguing out of diseased plants early 
 in the season must be practiced, as well as the control of aphis, and 
 elimination of volunteer potatoes and of weeds, in the neighborhood, 
 that might carry the disease. It has been stated that it is useless to 
 try to free a stock of these diseases if 15 to 20 per cent of the plants 
 are affected. The efficiency of roguing is also limited by the grower's 
 ability to recognize the disease, for under certain climatic conditions 
 the plants may be affected without showing the symptoms clearly 
 
ClRC. 287] POTATO PRODUCTION IN CALIFORNIA 37 
 
 enough to permit thorough roguing. Goss 11 found that temperatures 
 above 70° tend to mask the symptoms of mosaic, while temperatures 
 below 70° increase their visibility. Another disease, spindle tuber, 
 has exactly the opposite temperature relations. Low moisture and 
 intense sunlight also seem to mask the disease symptoms. This indi- 
 cates that roguing may be more effectively done, and therefore more 
 healthy and productive seed may be produced, in some sections than 
 in others. It is further indicated that roguing early in the season, 
 before hot weather sets in, will be more effective for the elimination 
 
 Fig. 12. — Plant affected with severe mosaic, or curly dwarf. 
 
 of mosaic than later roguing. Still another helpful practice in control 
 of mosaic is indicated by Oortwijn Botjes, 12 who states that secondary 
 infection, which takes place in the field during the growing season, 
 advances down the stem slowly toward the tubers. Thus there is a 
 possibility of obtaining disease-free seed by digging early, before the 
 infection from aerial parts has reached the tubers. This probably 
 explains in part at least the well known advantages of using immature 
 seed. 
 
 Leaf Boll, Net Necrosis, Spindle Sprout. — Leaf roll is a disease of 
 the virus type having many of the characteristics of mosaic, so far 
 as transmission is concerned. Dwarfing and rigidity of the plant, 
 and rolling of the leaves along the midrib, are its effects. Frequently 
 
 11 Goss, E. W., Effect of environment on potato degeneration diseases. Nebr. 
 Station Besearch Bull. 26:1-39, 1924. 
 
 12 Oortwijn, Botjes, J. Tidschr. Plantenz, 29: 113-126, 1923. (Abst. in Bot. 
 Abst., 13: 54, 1924.) 
 
38 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 leaf roll plants produce tubers that show a definite necrosis when the 
 end is clipped, and sometimes tubers from such plants produce only 
 slender spindling sprouts. However, these symptoms do not always 
 accompany leaf roll, and apparently can also occur from other causes. 
 Leaf roll is a very serious disease in some parts of the country, but is 
 not seen very often in California. On the other hand, a spindle sprout 
 trouble accompanied by a necrosis of the seed tuber, was a very 
 serious cause of loss in southern California in the spring of 1924. 
 Many seed pieces did not sprout at all, others sent out little secondary 
 tubers by the direct method, while still others produced fine spindling 
 sprouts that never developed into normal plants. In some cases the 
 stand of normal plants was only 25 to 50 per cent. This trouble 
 apparently was not connected with the leaf-roll disease, but was 
 thought to be traceable to injury of the seed tubers in the field the 
 previous year, by a combination of high temperatures and lack of 
 water while the crop was maturing. Investigations on the cause and 
 nature of this trouble are under way, but meantime it seems advisable 
 to use seed grown in the cooler, more humid sections, and, in the 
 absence of rainfall, to continue irrigation of fields intended for seed 
 purposes as late in the autumn as possible. 
 
 Internal Browning and Heat Necrosis. — These are diseases of the 
 tuber that appear when the maturing crop is exposed to hot weather. 
 Frequently a considerable portion of the larger tubers are affected, 
 in the summer crop of the warmer sections of the state. Internal 
 browning appears as streaks or isolated brown dead spots in the 
 middle portion of the interior of the tuber. 
 
 Heat necrosis probably is similar in its nature and cause to 
 internal brow T ning. It differs from the latter in that the dead tissue 
 consists of small pithy brown spots distributed throughout the interior 
 of the tuber. The disease seems to be connected with periods of 
 unusually hot weather accompanied by low humidity or lack of 
 moisture in the soil. This combination of conditions may cause the 
 vines to give off moisture faster than the roots can obtain it from 
 the soil. It is most likely to occur on sandy soils, which do not hold 
 much moisture and accordingly dry out rapidly. More frequent light 
 irrigations under these conditions should be helpful in preventing 
 the trouble. It may also be suggested, that when a crop is practically 
 mature but cannot be harvested before internal browning is likely 
 to occur, that the foliage of the plants be partly killed by spraying 
 with some toxic material. A 1 per cent solution of sodium arsenite 
 will kill potato tops more or less completely. 
 
CiRC. 287] 
 
 POTATO PRODUCTION IN CALIFORNIA 
 
 39 
 
 Fig. 13. — Two rows on right show perfect stand obtained from normal healthy 
 seed. On the left the stand is poor, apparently due to injury of the seea the 
 previous year by lack of water and high temperatures. (F. H. Ernst.) 
 
 r 
 
 T 
 
 Fig. 14. — On left, abnormal tuberization characteristic of the British Queen 
 variety when grown in warm sections. On right, formation of vegetative sprouts 
 from nearly mature tubers. This occurs on many varieties when moisture supply 
 is irregular and temperatures very high as the crop matures. 
 
40 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 GENERAL DISEASE CONTROL MEASURES 
 
 Seed Treatment. — Disinfection of the seed before planting is a 
 necessary step in potato culture, aimed primarily at control of scab 
 and Rhizoctonia. Three treatments are in use : cold formaldehyde, 
 cold mercuric chlorid, and hot formaldehyde. The first mentioned 
 has been abandoned because it is not effective against Rhizoctonia. 
 The second is the standard method at present, although hot formalde- 
 hyde is giving promising results in the East. 
 
 For the standard treatment, a commercial grade of mercuric 
 chlorid (corrosive sublimate) is used at the rate of 4 ounces to 30 
 gallons of water. It is convenient to use a stock solution prepared 
 at the rate of 4 ounces to 1 gallon of water, which is added to 29 
 gallons of water in the dipping tank just before starting the seed 
 treatment. As mercuric chlorid is practically insoluble in cold water, 
 the usual recommendation is to prepare the stock solution with boiling 
 hot water. However, it has been found that mercuric chlorid will 
 dissolve much more rapidly in the presence of soluim chlorid or 
 ammonium chlorid. The proper amount of mercuric chlorid should 
 be weighed out, placed in a stone crock or wooden vessel with an 
 equal quantity of common salt, and the warm water added. If four 
 times as much salt is used, complete solution is obtained in cold water 
 in two or three minutes. This saves some trouble in preparing the 
 stock solution, and the presence of the salt does not affect the efficiency 
 of the treatment. 
 
 The solution made by adding 1 gallon of stock solution to 29 
 gallons of water may be conveniently used in clean, tight wooden 
 barrels of 50 to 60 gallons capacity. The barrels used for the treat- 
 ment should be placed on a platform so that the solution can be 
 drained off quickly at the end of each treatment and transferred to 
 another barrel. A drain plug should be inserted in the bottom of 
 each barrel for this purpose. Seed should be dumped from the sacks 
 into the barrel, though it is sometimes treated in the sacks. This is 
 objectionable, however, because the sacks absorb the mercuric chlorid 
 from the solution very rapidly, thus weakening it and lessening its 
 efficiency. In the past the usual recommendation has been to discard 
 the solution after treating four successive lots. However, it is cheaper 
 and more effective to maintain the strength of the same solution by 
 adding one-half ounce of mercuric chlorid for every two sacks dipped, 
 and sufficient water to bring the solution up to its original volume. 
 If a stock solution (4 ounces mercuric chlorid to 1 gallon of water) 
 
Circ. 287] 
 
 POTATO PRODUCTION IN CALIFORNIA 
 
 41 
 
 has been prepared, then 1 pint of this stock may be added to the 
 dipping vat for every 2 sacks dipped. 
 
 As mercuric chlorid reacts with any sort of metal, only wooden, 
 agateware, glass, or stoneware vessels should be used in preparing the 
 solution. Dirt adhering to the surface of the potato also tends to 
 weaken the solution at a rapid rate. 
 
 Potatoes should always be disinfected before they are cut. They 
 should be left in the solution for 1% hours, as a shorter treatment is 
 not effective. 
 
 Fig. 15. — Knobby second growths produced on partly developed tubers. 
 Irregular moisture supply, especially lack of water during the middle of the 
 season, causes this condition. It is not transmitted through the seed. Certain 
 varieties are more subject to this trouble than others. 
 
 The hot formaldehyde has not yet been tried in California. A 
 solution made by dissolving commercial formalin in water at the rate 
 of 2 pounds (1 quart) to 30 gallons of water is used. The solution is 
 heated to 122° F. and is kept between 118° and 122° F. while the seed 
 potatoes are treated. The potatoes are immersed for exactly two 
 minutes, and upon removal are dumped in piles, which are kept cov- 
 ered for one hour with a heavy canvas to hold in the fumes, thus 
 making the treatment more effective. This method is well adapted 
 to the treatment of large quantities of seed in a short time, but 
 requires more skill and care to avoid injuring the potatoes than the 
 mercuric chlorid method. 
 
42 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION 
 
 Crop Rotation. — Potatoes are likely to be more healthy and pro- 
 ductive if grown in rotation with other crops. From three to five 
 years between potato crops is advisable. Some of the worst diseases 
 live over in the soil, and others are transmitted from year to year in 
 the volunteer potatoes which spring up from the culls left in the field. 
 
 Control of Insects. — In some seasons, aphis are particularly 
 numerous on potatoes, especially in the early part of the season. 
 While they do direct damage by weakening the plants, they do even 
 more damage by spreading mosaic and other virus diseases from plant 
 to plant. The seed grower, especially, should be on the lookout for 
 aphis in the potato fields, and be prepared to control them with nico- 
 tine dusts or nicotine sprays. 
 
 Use of Clean Healthy Seed. — This is the important factor in con- 
 trol of certain diseases. While any grower can produce good healthy 
 seed by taking proper precautions, still it is more likely to be 
 actually done by the seed specialist. Such potatoes are produced at 
 greater expense than the usual market crop, but if they are actually 
 freer from disease than ordinary stock, they are worth more for 
 seed purposes. 
 
 PRODUCTION OF CERTIFIED SEED POTATOES 
 
 With the increasing prevalence and number of diseases of pota- 
 toes, and the greater risk in purchasing seed stocks about which 
 nothing was known by the purchaser, has come the realization that 
 the production of satisfactory seed requires special skill and care. 
 Consequently, there has been developed in California and other seed 
 growing states during the past ten years a system of seed certifica- 
 tion. To accomplish this, fields entered for certification are inspected 
 twice during the growing season and once after harvest by a repre- 
 sentative of the State Department of Agriculture to determine as 
 accurately as possible whether the potatoes come up to the required 
 standards for certified seed. The principal requirements are: 
 
 1. True to name. Not more than 5 per cent varietal mixture 
 
 allowed at first inspection. 
 
 2. Of a type true for the variety. 
 
 3. Freedom from diseases. Not more than 10 per cent diseased 
 
 plants and not more than 5 per cent of any one group — 
 mosaic, leaf-roll, wilt and blackleg, and Rhizoctonia. 
 
 The first inspection should be made about blossoming time, at 
 which stage it is easiest to detect varietal mixtures. Afterwards the 
 grower is supposed to rogue out all off-type and diseased plants. At 
 
Circ. 287] 
 
 POTATO PRODUCTION IN CALIFORNIA 
 
 43 
 
 a second inspection, later in the season, the permissible number of 
 diseased plant is much smaller. Seed passing all inspections is sold 
 under official certification tag. 
 
 Fig. 16. — ll Witch's Broom," a disease characterized by excessive number of 
 slender stalks, and production of few or no tubers. Such plants should be rogued 
 from the seed fields. 
 
44 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION 
 
 Certified seed has in general justified the extra trouble and ex- 
 pense in producing it. Naturally seed buyers are interested in know- 
 ing whether, aside from all theoretical considerations, certified seed 
 actually yields more than common stock. In Missouri, where average 
 yields from several certified and uncertified strains were taken, there 
 was a gain of 20 bushels an acre for the certified seed. In Connecticut 
 certified seed averaged an increase of 62 bushels to the acre. In 
 Michigan, where 2,100 acres were planted with certified seed in 1923 
 the average increase over plantings with uncertified seed was 78 
 bushels an acre. These illustrations indicate that the principles on 
 which seed potatoes are certified have a firm foundation. It is, never- 
 theless, true that some lots of certified seed have given unsatisfactory 
 results. Apparently, such cases can be traced to two causes (aside 
 from fraud) : either the field from which the seed came was attacked 
 by aphis, which spread such diseases as mosaic, or climatic conditions 
 were such that mosaic-infected plants could not be recognized and, 
 accordingly, were not rogued out. 
 
 Careful roguing is absolutely necessary in order to produce high 
 grade potatoes. The fields should be rogued not once but a number 
 of times during the season. Many certified seed growers have made 
 the mistake of waiting until the plants are nearly mature before 
 roguing, overlooking the fact that plants infected with mosaic were 
 thus allowed a long period to transmit the disease to their healthy 
 neighbors. Early roguing will reduce the number of plants to be 
 rogued out, and, to a still greater extent, will reduce the amount of 
 disease in the next year's crop. 
 
 In making the first roguing, it is convenient to carry a small pail 
 of kerosene, in which to dip diseased plants as they are dug. In later 
 roguings when the plants are larger, the rogued out plants should be 
 placed in a sack, taken from the field, and destroyed, the object being 
 to destroy any aphids that might be on these diseased plants and 
 which would certainly infect others if allowed to escape. In roguing, 
 the entire plant should be removed, including the stem, seed piece, 
 and tubers, if any. Diseases like mosaic permeate all parts of the 
 plant, and if any portion is left in the soil, new infections may result. 
 
 About 600,000 sacks of seed are required to plant the potato acre- 
 age in California each year. About 60,000 sacks of certified seed were 
 produced in 1923; hence it is seen that there is still plenty of room 
 for expansion in production of certified seed.