448 YEARBOOK OF AGRICULTURE 1953 labor became scarce, a trend was started toward the use of machine- shelled seed. Poor stands often resulted. Work started by Luther Shaw in North CaroUna in 1939—and later con- Preventing the firmed by research in Virginia, South Carolina, Georgia, Florida, and Ala- bama—showed that machine-shelled Diseases of seeds treated with a seed protectant before planting produced stands com- parable to those obtained by using Peanuts hand-shelled and treated seed. Although seed usually are treated at the time they are shelled because of Coyt Wilson convenience, the treatment may be applied with equal efTectiveness just Prevention, rather than cure, is the before planting. rule with the diseases of peanuts. Several good seed protectants are Some beneficial preventive measures available commercially. Eight materi- are the application of fungicides to seed als were tested in Alabama in 6 years. to prevent seed rot, the use of fungi- Best results were had with Ceresan M cidal dusts or sprays on grovv^ing plants (7.7 percent ethyl mercury /?-toluene to prevent leaf spot, and the treatment sulfonanilide). Phygon (90 percent 2, of soil with chemicals before planting 3-dichloro-i ,4-naphthoquinone), Ara- to control nematodes and soil insects. san (50 percent tetramethyl tbiuram- Prevention involves more than the disulfide), and Spergon (98 percent use of chemicals, however. Manage- tetrachloro-jÉ?-benzoquinone) were ment practices that promote vigorous somewhat less effective. Reasonably growth enable the plants to escape in- good results were obtained with Yellow fection or to survive after infection. Cuprocide (yellow cuprous oxide con- Outstanding are rotations that in- taining 47 percent metallic copper) clude one or two crops of corn, oats, or and Dow 9-B (50 percent zinc trichlo- similar crops before each peanut crop; rophenate). Seedox (50 percent 2,4,5- adequate mineral fertilization; plant- trichlorophenyl acetate) and Merc-O- ing on light-textured, well-drained Dust (a compound of indefinite com- soils; and reduction of mechanical position containing mercury and form- damage by cultivating implements. aldehyde) were ineffective. Similar results have been obtained in other SEED ROT is the most serious of States. the diseases that affect stands of pea- Although the mercurial treatments nuts. It is caused by mildly pathogenic such as Ceresan M are most effective, or saprophytic organisms that live in they are more dangerous to use. Over- the soil—mainly species of Fusarium, dosage results in poor germination. Rhizopus, Mucor, Diplodia, Pénicillium, The radicle of the germinating seed and Aspergillus. Their entrance is facili- does not elongate but becomes thick- tated by broken seed coats and by ened and stubby. If a seedling is pro- other mechanical injuries produced duced, it is stunted and never makes by the sheller. Seed rot is most destruc- normal growth. Therefore—and be- tive under conditions that retard ger- cause of the danger to warm-blooded mination—cool, damp weather, ab- animals that might eat treated seed— normally deep planting, and water- most States recommend one of the logged soils. Most seed rot occurs organic materials such as Arasan or within the first week after planting. Spergon in preference to the mercurial Beginning about 1940, when farm treatments. PREVENTING THE DISEASES OF PEANUTS 449 The organic seed treatments have a tissues and give them a dark gray or tendency to be somewhat erratic in black color on the inside. performance. In tests lasting 6 years Another form of seedling blight is at Auburn, Ala., Ceresan M was the dry rot, caused by Rhizoctonia solani. most consistent of four treatments. It is like charcoal rot in the early Spergon was the best of the four in stages. Lesions develop on the stem 1949 but the poorest in 1947. Dow 9-B near the soil line. The plant may be was practically as good as Arasan or girdled and killed; if the infection Ceresan M in 1946 but was consider- does not spread, the plant may partly ably poorer than either in 1948. recover. The lesions are not likely to Arasan gave less protection than Ce- extend as far below the surface as resan M in 1944, 1946, 1948, and 1949. those caused by Sclerotium baiaticola. Seed protectants are applied as dust There are no specific control meas- treatments at the rate of 2 or 3 ounces ures for seedling blights. Losses may to 100 pounds of seed. Slurry treat- be reduced by planting treated seed ments—liquid suspensions of the fungi- of good quality on a well-prepared cide—have not been popular on pea- seedbed. nuts, probably because of the tendency of the seed coats to peel after the slurry OF THE DISEASES of growing plants, treatments are applied. The tendency leaf spot generally is the most destruc- appears to be more common with tive disease during the growing season. Spanish-type seed than with seed of It is recognized by the brown or runner peanuts. black and somewhat circular spots on the leaves. As the disease pro- SEEDLING BLIGHTS may be destructive gresses, the spots enlarge until the in some localities, but generally they entire leaf is affected. Defoliation do little damage in established stands. follows. The detrimental effects of Damping-off is not a serious disease of leaf spot are threefold: The yield of peanuts. nuts is reduced; the quality of the One of the most common types of peanut hay is lowered; and the fallen seedling blight in peanuts is caused leaves provide organic matter on by Sclerotium baiaticola. This organism which inoculum of other fungi, such causes a disease known as charcoal as Sclerotium rolfsii, is produced. rot in a number of other species of Two species of fungi cause leaf spot. plants during periods of high temper- Each produces characteristic symp- atures. Infections result in the forma- toms. tion of lesions on the succulent stems. Early leaf spot, caused by Cercospora The initial infections usually are near arachidicola, produces spots that are the soil line, but the lesions may ex- light tan at first. With age the spots tend downward some distance into become reddish brown to black on the soil. If plant growth is retarded the lower surface and light brown on by dry weather and if the temperature the upper surface of the leaf. A yellow is high—above 75° F.—the lesion is halo surrounds each spot. Late leaf likely to girdle the stem and kill the spot, caused by C. personata, produces plant. The stem assumes a dull brown dark-brown or black spots on both color and becomes quite dry. Some- surfaces of the leaf. The spots usually times the progress of the disease is are somewhat smaller than those of stopped and the seedling is stunted early leaf spot and there is no distinct rather than killed. Then the plant is halo. The fungi can be distinguished made more susceptible to other dis- by microscopic examination of the eases later in the season. Charcoal conidia. rot in peanuts can be recognized by The conidia of Cercospora arachidicola the many small, irregular, black are colorless to pale olive green and sclerotia that develop in the affected often curved. According to measure- YEARBOOK OF AGRICULTURE 1953 ments made by W. A. Jenkins in carbamate fungicides have not come 1938, they are 37-108 by 2.7-5.4 into general use on peanuts, probably microns and contain 3 to 12 septations. because of their higher cost. Increases The conidia of C. personata are much in yield resulting from dusting depend shorter and considerably thicker; upon the fertility of the soil on which they measure 18-60 by 5-11 microns, the crop is grown, the prevalence and with I to 8 septations. They are gen- severity of leaf spot, and the type of erally cylindrical and seldom curved. peanuts being grown. Highest returns Both leaf spots attack the leaves, have been obtained on runner pea- petioles, pegs, and pods of Spanish, nuts growing on fertile sandy loam bunch, and runner peanuts. No host soils in the Southeast. Increases of plants other than Arachis hypogaea are 1,000 pounds an acre have been re- known for them. ported by the Alabama Agricultural Primary infections of leaf spot are Experiment Station. The increases caused by ascospores formed in the generally amount to 500 pounds an spring on overwintered peanut leaves. acre or less. Secondary infections result from co- nidia. Although the fungus has been SOUTHERN BLIGHT, caused by Sclero- reported to be seed-borne, this method tium rolfsii^ is more destructive on Span- of overwintering is of minor impor- ish and bunch types of peanuts than on tance. The principal means of dis- runners. It may appear on the plants semination is by wind-borne inocu- at any time during the growing season lum. The ascospores or conidia ger- but is more likely to cause damage in minate within a few hours and pene- late summer and early fall as the trate the leaf directly through the plants approach maturity. The fungus, epidermal cells or through stomata. soil-borne, attacks the plants near the In both species, the mycelium is inter- soil line. It causes wilting and eventual cellular at first. Branched haustoria death of the part of the plant above are soon formed by the mycelium of the infection. It may attack the cen^ Cercospora personata and the host cells tral stem and affect the entire plant, are not killed outright. C. arachidicola or the infection may be limited to one does not form haustoria; the host cells or more branches. Diseased plants are killed in advance and the invading wilt; the leaves gradually turn brown germ tubes enter dead cells. Infections or black and eventually fall ofí'. The on leaves are visible within 8 to 23 fungus destroys the succulent tissues days after inoculation. in the stem; the vascular bundles that Leaf spot is controlled by fungicides are left give a shredded appearance to applied as dusts or sprays to the the diseased area. When the humidity foliage. Most commonly used are is high, light-tan to reddish-brown dusting sulfur or dusting sulfur con- sclerotia are formed in abundance on taining approximately 3.5 percent the infected tissues and the surround- metallic copper. Specific recommenda- ing ground. The sclerotia are usually tions vary slightly in difi'crent States; spherical and about the size of mus- three to five applications beginning tard seed but may be larger and irreg- about 90 days after planting often are ular in shape. In dry weather the recommended. Usually applications sclerotia are not so conspicuous. are repeated every lo to 14 days; 15 Sclerotium rolfsii attacks hundreds of to 25 pounds of dust per acre are species of plants. In the absence of applied each time. suitable hosts it lives quite well as a Bordeaux spray—6 pounds of cop- saprophyte in the soil. Crop rotation per sulfate and 2 pounds of hydrated is therefore of limited value for con- lime in 100 gallons of water—is effec- trol. Most grasses are highly resistant, tive, but ofí'ers no particular advan- but among them usually are enough tages over fungicidal dusts. The dithio- volunteer plants that are susceptible PREVENTING THE DISEASES OF PEANUTS to enable the fungus to survive in the branch. The affected part wilts sud- soil. Southern blight appears to be denly; the leaves become pale green more destructive on peanuts following and turn dark brown or black rather peanuts or cotton, however, than on quickly. Death is usually complete peanuts following corn. within 2 or 3 days after wilting be- Occasional reports of strains of pea- comes evident. If the entire plant is nuts resistant to S. rolfsii have been diseased, the wind usually blows the made, bnt it has not been possible leaves and stems away, leaving the to incorporate the resistance into ground completely bare. The taproot the commercial varieties that are is usually shredded at or near the soil available. line. Collar rot of runner peanuts is a There is no control for collar rot. disease complex that sometimes resem- bles southern blight. The disease has NEMATODES have become increas- been reported only from Georgia, ingly important on peanuts. Several Florida, and Alabama. The cause has species attack peanuts, among them not been definitely established. The the root knot nematodes {Meloidogyne fungi that can be isolated from dis- arenaria and M. hapla)^ meadow nema- eased plants are either saprophytic or tode {Pratylenchus leiocephalus) ^ and the weakly pathogenic, and efforts to pro- sting nematode (Belonolaimus gracilis). duce the disease by inoculating healthy The root knot nematode attacks peanut plants with these fungi usually both roots and fruits and causes the are unsuccessful. Diplodia theobromae formation of galls. The feeding roots apparently is always associated with are deformed. The knots on the roots the disease. Other fungi that usually usually are similar in size and shape are present include various species of to the nodules formed by nitrogen- Fiisarium, Pénicillium, Trichoderma, and fixing bacteria. On the fruits and Rhizopus nigricans. Sclerotium rolfsii and peduncles the galls are corky and S. bataticola sometimes are isolated. variable in shape. Symptoms on the above-ground part of the plant are COLLAR ROT may result when plants not characteristic. Plants are stunted; that have been wounded are invaded leaves are likely to show chronic by any of those fungi. L. W. Boyle in symptoms of wilting; and the color is Georgia has demonstrated that young light green. In advanced stages leaves seedlings of peanuts are subject to are necrotic along the margins, and sunscald. The injured areas on the considerable leaf shedding results. stem or branches become lesions or Heavy infestations may reduce yields cankers through which the fungi gain to 500 pounds or less to the acre. entrance. If the lesion is small, the The symptoms of meadow nematode plant may continue to grow for several injury consist of lesions on peanut weeks before the disease becomes shells and lateral feeding roots. The severe enough to cause death. damage it does to plant tissues pro- Other types of injury also may pre- vides avenues of entrance for soil dispose the plants to attack—injury fungi that cause peg rot. by cultivating implements, wind- The most characteristic symptom of blown sand, and chewing insects. It is the sting nematode is an unusually possible that infections also develop long taproot with relatively few lateral in lesions produced by the charcoal roots. Those that are present are short, rot and rhizoctonia dry rot organisms. stubby, and deformed. This nematode Collar rot may appear at any time causes damage by puncturing the roots during the growing season. It usually with a needlelike "sting" and with- is most noticeable during July, August, drawing the juices. The nematode and early September. It may involve does not enter the roots but feeds ex- the entire plant or only a single ternally, The area punctured by the YEARBOOK OF AGRICULTURE 1953

Sting is attacked by soil fungi. Small tions by fungi. Many soil fungi attack dead spots develop; the result is root the slender gynophore near the soil pruning, which accounts for the elon- line as the fruit approaches maturity gated taproot and the deformed and many gain entrance to the tissues. lateral roots. Their action weakens the gynophore so If the nematode infestation is light, that when the plant is pulled or lifted control can usually be accomplished from the soil the gynophore breaks, by rotation. Such crops as corn, oats, leaving the pod in the soil. Early de- and grain sorghum are resistant to the foliation of plants by leaf spot, leaf- root knot nematode but not to the eating worms, or other causes also sting nematode. Control by rotation is causes the gynophores to be more not successful if infestations are heavy. brittle and to break more easily. If infestations are heavy, nematodes Injuries to the "shell" of the de- can be controlled by fumigating the veloping fruit by soil insects permit soil before planting, A fumigant, such soil-borne fungi to gain entrance. Dis- as Dowfume W-40, should be applied coloration or decay of the seed follows. to the drill at the rate of 7 gallons to If harvest is delayed beyond matur- the acre about 3 weeks before planting. ity of Spanish or bunch peanuts, the After fumigation the soil should be left seeds begin to germinate. undisturbed until planting time. Care Miscellaneous fungi, including Scle- should be taken to plant the seed di- rotium rolfsii and ^S*. bataticola and species rectly above the area in which the OÎ Rhizoctonia, Diplodia, Rhizopus, Asper- fumigant was applied. The plows used gillus, and Pénicillium, may be isolated for opening the furrows should be as from developing pegs and mature small as possible to prevent mixing of fruits. Apparently any one of them fumigated and unfumigated soil. may cause one form of peg rot under The root knot nematode, Aieloid- proper conditions. ogyne arenaria^ attacks several plants The control of peg rot is a problem other than peanuts, including sweet- of crop management. The plants potato, Ramie, coffeeweed, tomato, should be kept growing as vigorously okra, onion, peach, and several weeds. as possible until harvesttime. Defoli- ation should be prevented by control- PEG ROT, strictly speaking, is not a ling leaf spot and leaf-eating worms. disease of the growing peanut plant. The crop should be harvested as soon The term inx:ludes all losses that result as mature. With runner peanuts, that from rotting, sprouting, and other is quite a problem because the fruits types of damage to the developing nuts are formed and mature in cycles. Thus or to loss of nuts in the soil at harvest it is necessary to decide when peak by breaking of the gynophores. production has been reached, to sac- The peanut flower is formed in the rifice some of the earliest formed fruits, axil of the leaf on the stem. After pol- and to harvest before the later fruits lination, the petals wither, and the are fully mature. The crop should be peg (or gynophore) elongates and harvested with machinery adapted for grows into the soil, where the fertilized the purpose so as to minimize unneces- ovary at the tip of the peg enlarges to sary breaking of the pegs. form the pod or peanut fruit. The pea- nut fruit thus develops in an environ- SEVERAL OTHER diseases of growing ment populated with soil micro-organ- peanuts occur occasionally in the isms and is subject to attack by them United States. during all of its development. Peanut rust, Puccini a arachidis, has Soil insects such as wireworms and been found occasionally in Florida, the larvae of the southern corn root- Alabama, and Texas. A leaf spot worm do considerable damage to the caused by an unidentified species of developing pods and facilitate infec- Phyllosticta appears on young peanut PREVENTING THE DISEASES OF PEANUTS 453 leaves early in the spring, but infec- Seeds w4th concealed damage may tions do not spread to older leaves. contain a variety of fungi, primarily Botrytis late blight of peanuts has been Diplodia theohromae. Others are species observed on overmature peanuts dur- of Fusarium^ Aspergillus, Pénicillium, ing cool, damp days of early fall in Rhizopus, and Rhizoctonia. Occasion- Georgia. It is sometimes destructive on ally, Sclerotium rolfsii and S. bataticola peanuts grown in greenhouses in win- are found. The disease has been ter. Fusarium wilt of peanuts has been produced by inoculation only with reported from Southeastern States Diplodia theohromae and Sclerotium ba- many times, but apparently it has not taticola. Although initial infections caused heavy damage. Virus diseases usually occur before digging, con- and bacterial wilt—destructive in the cealed damage makes its most rapid East indies and in Africa—are not development during the curing proc- serious in the United States. ess. The moisture content of the seed is the principal conditioning factor, as As THE PEANUT DEVELOPS in the temperatures during the curing period soil, a variety of fungi become estab- are usually near the optimum for lished in the shell, the peg, and fungal development. Concealed dam- occasionally the seed. When peanuts age develops most rapidly in partly are harvested, they usually are placed cured peanuts that contain 15 to 35 in stacks or small piles or windrows percent moisture. The disease does not and allowed to cure. During the develop appreciably in seeds con- curing process the fungi continue taining less than 10 percent moisture. their activity and frequently become Neither soil type nor fertilizer practice established in the tissues of the seed has any measurable effect on develop- or on the interfaces of the cotyledons. ment of the disease. Activity of the fungi during curing, Concealed damage can be controlled together with the activity of the by rapid curing of peanuts as soon as enzymes within the peanut seed, they are harvested and the use of results in various types of seed deterio- resistant varieties. ration. In commercial circles all these Freshly dug runner peanuts usually troubles are grouped together under contain 40 to 55 percent moisture. one heading—damage. If peanuts are allowed to wilt before Concealed damage is first evident being placed in symm.etrical, well- as a slight yellow discoloration on the capped stacks with provisions for air interfaces of the cotyledons. A mat circulation at the base, curing usually of fungal mycelium develops between is rapid enough to prevent develop- the cotyledons as the disease pro- ment of concealed damage. If peanuts gresses. The tissues become rancid are stacked before wilting or if they and discolored. There is no evidence are allowed to become brittle before of the disease on the outside until stacking, curing is retarded and the its late stages. If conditions remain disease is more pronounced. Peanuts favorable, the entire seed becomes that are cured in windrows instead of shriveled and blackened, assumes an stacks usually contain very little con- oily appearance, and is unfit for cealed damage, but this method of human consumption. The percentage curing is more hazardous because of of free fatty acids in the oil increases shattering in unfavorable weather. markedly. The disease is more com- The Dixie Runner variety has a mon in runner peanuts than in great deal of resistance to concealed Spanish or bunch peanuts, and has damage, and that resistance has been been more prevalent in the south- incorporated into a number of other eastern peanut belt than in the strains of peanuts. By using resistant Virginia-North Carolina area and varieties and by exercising reasonable the Texas-Oklahoma area. care at harvest, most growlers can keep 454 YEARBOOK OF AGRICULTURE 1953 losses from concealed damage to a minimum. Blue damage is a discoloration that often occurs on the seed coat of Span- ish peanuts cured during periods of warm, damp weather. The small spots Ways To Combat usually have a "bull's-eye" appear- ance, with colors ranging from blue through black. Larger spots may be Disorders of irregular in outline with no evident center. The discoloration docs not pro- duce detectable changes in chemical Tomatoes constituents, taste, or germination of seed. Quite often the discoloration S. p. Doolittle does not extend into the cotyledons, but sometimes the cotyledons may be conspicuously discolored. Every tomato grower is faced with Experiments at the Georgia Agri- the hazard of loss from disease. Infec- cultural Experiment Station showed tions by fungi, bacteria, or viruses are that the discoloration results from an responsible for much of the loss, but interaction between pigments in the extremes of temperature and moisture seed coat and oxalic acid produced by or excess or deficiency of mineral ele- Sclerotium rolfsii. Under favorable con- ments in the soil at times may damage ditions S. rolfsii grows saprophytically the crop. on the peanut shells during the curing The symptoms thus caused can be process. The fungus secretes oxalic classified as wilts, leaf spots, fruit spots acid, which diffuses into the tissues in or rots, and abnormalities of growth advance of the hyphae. When curing of the foliage or fruits. conditions are unfavorable for growth Fusarium wilt, caused by the fungus OÎS. rolfsii, no discoloration occurs. Fusariura oxysporum f. lycopersici, is seri- ous on tomatoes. Its first symptom is OTHER STORAGE disorders, including a yellowing of the oldest leaves, usually seed rot, rancidity, and reduced on a single stem. Beginning at the base. germination, may be the result of the yellowed leaves wilt and die until improper curing or of improper all the leaves are killed. Often a single storage. Peanuts should contain less shoot dies before others are severely in- than 10 percent moisture at the time jured, but eventually the entire plant they are stored. Some deterioration is afí"ected. There is no soft decay of occurs at moisture contents between the stem, but if one cuts it lengthwise 6 and lo percent, but the rate is near the ground, he can see in the fairly slow. The relative humidity of woody portion next to the outer the storage environment should be 65 "bark," or cortex, a brown discolora- percent or less. Under those condi- tion of the water-conducting tissues— tions, well-cured peanuts will keep fibrovascular bundles—which is char- well in storage for several months. acteristic of the disease. The fungus that causes fusarium wilt CoYT WILSON is assistant dean of the can persist in the soil for many years. School of Agriculture of Alabama Poly- Infection occurs through the roots. tec knie Institute and assistant director of From them the fungus passes into the the Alabama Agricultural Experiment Sta- water-conducting vessels of the stem. tion. He was graduated in agriculture by Apparently it produces a toxic sub- Alabama Polytechnic Institute, He has a stance that causes the wilting of the doctoras degree in plant pathology from the foliage and eventual death of the University of Minnesota, plant. The fungus is most active at