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.