TECHNICAL BULLETIN NO. 99 APRIL, 1929 UNITED STATES DEPARTMENT OF AGRICULTURE WASHINGTON, D. C. A MONOGRAPHIC STUDY OF SWEET- POTATO DISEASES AND THEIR CONTROL By L. L. HARTER, Senior Pathologist, Office of Horticultural Crops and Diseases^ and J. L. WEIMER, Senior Pathologist, Office of Forage Crops and Diseases, Bureau of Plant Industry ^ CONTEKTS Page Page Introduction i Diseases caused by fungi—Continued. Farm practices employed in growing Minor storage diseases—Continued. sweet potatoes .-_ 2 Botrytis cinérea 74 Seriousness of the disease situation. 4 p] picoccum 74 Diseases caused by fungi 5 Gibberella saubinctii 74 Major field diseases. 5 Fusarium culmorum 75 Stem rot or wilt 5 Fusarium acuminatum 75 Black rot 19 Trichoderma köningi . 75 Foot rot 27 Sclerotinia 76 Texas root rot 33 Other fungi- 76 Scurf -. 36 Transportation and marketing diseases.. 78 Sou rot 40 Disease caused by nematodes 79 Mottle necrosis. 45 Field disease 79 Eootletrot 47 Root knot-_ 79 Sclerotial blight 48 Physiological diseases and diseases of un- Rhizoctonia rot SO known cause -.- 80 White rust 53 Field diseases 80 Ix?af blight Ö6 Mosaic 80 Minor field diseases _ 57 Growth cracking... 82 Septoria leaf spot 57 Fasciation ,- 83 Alternarla leaf spot 58 Intumescence -- 84 Rust - 59 Sun scald 85 Cercospora leaf spot 60 Storage disease 86 Sclerotinia 61 Internal breakdown 86 Macrosporium foot rot 61 Disease control - - 87 Other fungi reported 61 Field diseases 87 Slime molds 62 Exclusion from foreign countries 88 Major storage diseases 62 Inspection and certification 88 Soft rot and ring rot 63 Eradication 89 Black rot.-- - 66 Resistant and susceptible varieties... 97 Java black rot 67 Discussion. 102 Dry rot 6S Summary of control measures 103 Foot rot.... - - 70 Storage diseases _ — 104 Charcoal rot _ ~-- - - - "0 Elimination _. _ -. - - 104 Scurf 71 Digging and handling 105 Surface rot 72 The storage house and its manage- Minor storage diseases 73 ment - - 105 Mucor racemosus 73 Fumigation of potatoes in storage 106 A Iternaria __ 73 Summary 107 Pénicillium 74 Literature cited 108 INTRODUCTION The sweet potato (Ipofrwea hatatas Poir.) ranks as one of the most valuable and dependable food crops in the tropical and semitropical world, and next to the potato {Solammn tuberoswrn L.) it is the most 1 The writers acknowledge the assistance contributed in various ways by some of their coworkers. They wish to thank J. I. Lauritzen for the use of certain experimental data and W. A. Whitney and C. B. Sumner tor help in the preparation and arrangement of the illustrations, reading of the manuscript, and verification of citations. 6178°—29 1 2 TECHNICAL BULLETIN 9 9, U. S. DEPT. OF AGRICULTURE important vcfietalilu in the United States. The sweet potato ^rows as a perennial in ti()i)ical countries, but not in tlie United States, wlicrc it lia.s been developed and conimcrcialized probably to a greater de<irce tlian in any other country. P'or this reason the crop has been adjusted to meet wider variations in cultural conditions than the natural habits of the plant would suggest were j)ossihle. In many tropical countries the sweet-potato planting is more or less permanent, the plants maintaining themselves for several years with- out being reset and without much cultivation. There is very little accurate information on tlie origin of the sweet l)otalo. Two theories are advanced: (1) That it is of Asiatic origin; and (2) that it originated in tropical America. I)e Candollc {21),- although not committing himself definitely, seems to bo of the opinion that it is of trojjical Amei'icîin origin. The same view is lield by (îroth {-5,9), who states that the evidence presented warrants fully the conclusion that the sweet ])otato is a native of tro])ical Americii. Accoiding to (irf)th, Columbus discovered the sweet potato in Cuba on his iii-st voyage to America. The sweet ])()tato is cultivated piimarily for the fleshy root, which in sonic of the varieties often attains a weight of several pounds. The cells in the roots are gorged with starch, and a little sugar is also present. An examination of the statistics reveals the fact that the acreage of sweet ]M)talocs gi'adually increased in the ITnited States from 1SÍ»!) to and including 1!)22, showing that the crop is becoming of more economic importance among the food crops of the country. Follow- ing 1922, there was some decrease in the acreage. In 1899, 42,517,412 bushels (/A7) were ]iroduced in the United States, with a value of !i!19,8(;9,840. In 1922, 109,394,000 bushels (7.9,?) were grown, which had an estimated total value of $84,29r),()()0. More than 1,117,000 acres wei'e jdanted in that year—probably the largest acreage in the histoiy of the Ignited States. No figures are available showing the relative value of this cro]:) in countries outside the United States. In this country the largest acreage of sweet potatoes is grown in the Southein Stales, although some of the Northern States (especially New Jersey, Delawai'e, Maryland, Iowa, Kansas, California, Illinois, Ohio, AVashington, and Indiana) also produce a considerable quan- tity. Pennsyhania, Nebraska, Colorado, Utah, and a number of other States grow sweet ])otatoes for home consum]ition. The acre- agi! by States in 1924 is indicated graphically by Figure 1. FARM PRACTICES EMPLOYED IN GROWING SWEET POTATOES In view of the relationshiii existing between the methods of handling the sweet-potato crop in the United States and the general j)revaleTice of its diseases, a brief consideration of the farm prac- tices employed is essential. In no country in the world is the sweet potato cultivated and handled as it is in the United States, and, so far as the writers are able to determine, the destructive diseases are not so generally present in any other country. This correlation ^ Italic Timnbers in iKimithosos refer to " I-itcrature cited," i>. 10,S. STUDY OF SWEET-POTATO DISEASES AND THEIE CONTROL à between the prevalence of the diseases and the methods of handling the crop probably is not accidental. The sweet potato is grown in the United States as an annual; that is, a new crop is planted each year. This is obligatory, since sweet potatoes are very sensitive to frosts, and in no part of the United States will they survive the winter, with the possible excep- tion of southern Florida. The roots that are to be used for seed purposes the following season are dug in the fall just preceding or immediately following the first frost and are stored during the winter. In the spring they are removed from storage and bedded in soil or sand, where young plants are produced for the initiation of the next crop. In many sections of the country it is the general practice to lay the seed potatoes in the bed about one-fourth to one-half inch apart and more or less parallel with one another. The Fio. Í.—OiitUne map of the Unit«! States, showing by bhiok dots the distribution of svvet^t-potato acreage in 1924. Kj^ch dot represi'uta 20,000 bu.shels. (Data prepared by tile Bureau uf Agricultural liconomics) spacing of the potatoes in the bed not only permits the liealthj' development of the young sprouts but also, at least to some extent, prevents the spread of diseases, if any are present, from one potato to another. In other sections it is the practice among some growers to dump the potatoes into a ditch, level them off with a rake, and then cover them with soil. When the latter method is followed the potatoes are usually very thick in the bed, and the communication of black rot or other diseases from potato to potato or from one plant to another is very prevalent. In the northern border of the sweet-potato belt hot water, steam, hot air, or fermenting manure are used as the sources of the heat supplied to the bed. The beds are frequently covered with canvas or with glass sash to retain the heat and moisture in the soil. In from three to five weeks the plants are large enough to pull from the mother potato and set in the field, where they are planted from 4 TECHNICAL BULLETIN 9 9, U. S. DEPT. OF AGRICULTURE 16 to 20 inches apart in rows about 31/2 feet apart. In some sections the plants are set on ridges and in others they are not. They are cultivated about the same as any other crop that is set out in rows, usually by means of plows and cultivators. In parts of the United States where the season is sufficiently long, another form of propagation, known as slip seeding or vine cutting, is sometimes practiced. This method of propagation consists of cut- ting the vines into sections from. 6 to 12 inches in length and then planting each section in the ground to form a single hill. Vine cutting does not obviate entirely the necessity of beading potatoes, but it does reduce considerably the quantity that must be bedded. In order to propagate by vine cuttings, especially where the plants may be killed prematurely by frost, the potatoes are bedded as early as weather conditions permit. The plants produced from the potatoes are set in the field, and the cuttings are made from them. An acre, more or less, depending upon the acreage to be planted for the major crop, is set out. When the vines of these plants are of sufficient length they are cut into sections and .set in the field.