Preventing the Diseases of Peanuts

Coyt Wilson.

Prevention, rather than cure, is the rule with the diseases of peanuts.

Some beneficial preventive measures are the application of fungicides to seed to prevent seed rot, the use of fungicidal dusts or sprays on growing plants to prevent leaf spot, and the treatment of soil with chemicals before planting to control nematodes and soil insects.

Prevention involves more than the use of chemicals, however. Management practices that promote vigorous growth enable the plants to escape infection or to survive after infection. Outstanding are rotations that include one or two crops of corn, oats, or similar crops before each peanut crop; adequate mineral fertilization; planting on light-textured, well-drained soils; and reduction of mechanical damage by cultivating implements.

SEED ROT is the most serious of the diseases that affect stands of peanuts. It is caused by mildly pathogenic or saprophytic organisms that live in the soil mainly species of Fusarium, Rhizopus, Mucor, Diplodia, Penicillium, and Aspergillus. Their entrance is facilitated by broken seed coats and by other mechanical injuries produced by the sheller. Seed rot is most destructive under conditions that retard germination cool, damp weather, abnormally deep planting, and waterlogged soils. Most seed rot occurs within the first week after planting.

Beginning about 1940, when farm 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 Carolina in 1939; and later confirmed by research in Virginia, South Carolina, Georgia, Florida, and Alabama showed that machine-shelled seeds treated with a seed protectant before planting produced stands comparable to those obtained by using hand-shelled and treated seed.

Although seed usually are treated at the time they are shelled because of convenience, the treatment may be applied with equal effectiveness just before planting.

Several good seed protectants are available commercially. Eight materials were tested in Alabama in 6 years. Best results were had with Ceresan M (7.7 percent ethyl mercury p-toluene sulfonanilide). Phygon (90 percent 2, 3-dichloro-1,4-naphthoquinone), Arasan (50 percent tetramethyl thiuramdisulfide), and Spergon. (98 percent tetrachloro-p-benzoquinone) were somewhat less effective. Reasonably good results were obtained with Yellow Cuprocide (yellow cuprous oxide containing 47 percent metallic copper) and Dow 9 B (50 percent zinc trichlorophenate). Seedox (50 percent 2,4,5- trichlorophenyl acetate) and Merc-O- Dust (a compound of indefinite composition containing mercury and formaldehyde) were ineffective. Similar results have been obtained in other States.

Although the mercurial treatments such as Ceresan M are most effective, they are more dangerous to use. Over-dosage results in poor germination. The radicle of the germinating seed does not elongate but becomes thickened and stubby. If a seedling is produced, it is stunted and never makes normal growth. Therefore and because of the danger to warm-blooded animals that might eat treated seed most States recommend one of the organic materials such as Arasan or Spergon in preference to the mercurial treatments.

The organic seed treatments have a tendency to be somewhat erratic in performance. In tests lasting 6 years at Auburn, Ala., Ceresan M was the most consistent of four treatments. Spergon was the best of the four in 1949 but the poorest in 1947. Dow 9 B was practically as good as Arasan or Ceresan M in 1946 but was considerably poorer than either in 1948. Arasan gave less protection than Ceresan M in 1944, 1946, 1948, and 1949.

Seed protectants are applied as dust treatments at the rate of 2 or 3 ounces to 100 pounds of seed. Slurry treatments liquid suspensions of the fungicide have not been popular on peanuts, probably because of the tendency of the seed coats to peel after the slurry treatments are applied. The tendency appears to be more common with Spanish-type seed than with seed of runner peanuts.

SEEDLING BLIGHTS may be destructive in some localities, but generally they do little damage in established stands. Damping-off is not a serious disease of peanuts.

One of the most common types of seedling blight in peanuts is caused by Sclerotium bataticola. This organism causes a disease known as charcoal rot in a number of other species of plants during periods of high temperatures. Infections result in the formation of lesions on the succulent stems. The initial infections usually are near the soil line, but the lesions may extend downward some distance into the soil. If plant growth is retarded by dry weather and if the temperature is high above 75 F. the lesion is likely to girdle the stem and kill the plant. The stem assumes a dull brown Color and becomes quite dry. Sometimes the progress of the disease is stopped and the seedling is stunted rather than killed. Then the plant is made more susceptible to other diseases later in the season. Charcoal rot in peanuts can be recognized by the many small, irregular, black sclerotic that develop in irregular affected tissues and give them a dark gray or black color on the inside.

Another form of seedling blight is dry rot, caused by Rhizoctonia solani. It is like charcoal rot in the early stages. Lesions develop on the stem near the soil line. The plant may be girdled and killed; if the infection does not spread, the plant may partly recover. The lesions are not likely to extend as far below the surface as those caused by Sclerotium bataticola.

There are no specific control measures for seedling blights. Losses may be reduced by planting treated seed of good quality on a well-prepared seedbed.

OF THE DISEASES of growing plants, leaf spot generally is the most destructive disease during the growing season. It is recognized by the brown or black and somewhat circular spots on the leaves. As the disease progresses, the spots enlarge until the entire leaf is affected. Defoliation follows. The detrimental effects of leaf spot are threefold: The yield of nuts is reduced; the quality of the peanut hay is lowered, and the fallen leaves provide organic matter on which inoculum of other fungi, such as Sclerotium rolfsii, is produced.

Two species of fungi cause leaf spot. Each produces characteristic symptoms.

Early leaf spot, caused by Cercospora arachidicola, produces spots that are light tan at first. With age the spots become reddish brown to black on the lower surface and light brown on the upper surface of the leaf. A yellow halo surrounds each spot. Late leaf spot, caused by C. personate, produces dark-brown or black spots on both surfaces of the leaf. The spots usually are somewhat smaller than those of early leaf spot and there is no distinct halo. The fungi can be distinguished by microscopic examination of the conidia.

The conidia of Cercospora arachidicola are colorless to pale olive green and often curved. According to measurements made by W. A. Jenkins in 1938, they are 37-108 by 2.7-5-4 microns and contain 3 to 12 septations. The conidia of C. personata are much shorter and considerably thicker; they measure 18-60 by 5-11 microns, with 1 to 8 septations. They are generally cylindrical and seldom curved. Both leaf spots attack the leaves, petioles, pegs, and pods of Spanish, bunch, and runner peanuts. No host plants other than Arachis hypogaea are known for them.

Primary infections of leaf spot are caused by ascospores formed in the spring on overwintered peanut leaves. Secondary infections result from conidia. Although the fungus has been reported to be seed-borne, this method of overwintering is of minor importance. The principal means of dissemination is by wind-borne inoculum. The ascospores or conidia germinate within a few hours and penetrate the leaf directly through the epidermal cells or through stomata. In both species, the mycelium is intercellular at first. Branched haustoria are soon formed by the mycelium of Cercospora personata and the host cells are not killed outright. C. arachidicola does not form haustoria; the host cells are killed in advance and the invading germ tubes enter dead cells. Infections on leaves are visible within 8 to 23 days after inoculation.

Leaf spot is controlled by fungicides applied as dusts or sprays to the foliage. Most commonly used are dusting sulfur or dusting sulfur containing approximately 3.5 percent metallic copper. Specific recommendations vary slightly in different States; three to five applications beginning about 90 days after planting often are recommended. Usually applications are repeated every 10 to 14 days; 15 to 25 pounds of dust per acre are applied each time.

Bordeaux spray-6 pounds of copper sulfate and 2 pounds of hydrated lime in 100 gallons of water is effective, but offers no particular advantages over fungicidal dusts. The dithiocarbamate fungicides have not come into general use on peanuts, probably because of their higher cost. Increases in yield resulting from dusting depend upon the fertility of the soil on which the crop is grown, the prevalence and severity of leaf spot, and the type of peanuts being grown. Highest returns have been obtained on runner peanuts growing on fertile sandy loam soils in the Southeast. Increases of 1,000 pounds an acre have been reported by the Alabama Agricultural Experiment Station. The increases generally amount to 500 pounds an acre or less.

SOUTHERN BLIGHT, caused by Sclerotium rolfsii, is more destructive on Spanish and bunch types of peanuts than on runners. It may appear on the plants at any time during the growing season but is more likely to cause damage in late summer and early fall as the plants approach maturity. The fungus, soil-borne, attacks the plants near the soil line. It causes wilting and eventual death of the part of the plant above the infection. It may attack the central stem and affect the entire plant, or the infection may be limited to one or more branches. Diseased plants wilt; the leaves gradually turn brown or black and eventually fall off. The fungus destroys the succulent tissues in the stem; the vascular bundles that are left give a shredded appearance to the diseased area. When the humidity is high, light-tan to reddish-brown sclerotia are formed in abundance on the infected tissues and the surrounding ground. The sclerotia are usually spherical and about the size of mustard seed but may be larger and irregular in shape. In dry weather the sclerotia are not so conspicuous.

Sclerotium rolfsii attacks hundreds of species of plants. In the absence of suitable hosts it lives quite well as a saprophyte in the soil. Crop rotation is therefore of limited value for control. Most grasses are highly resistant, but among them usually are enough volunteer plants that are susceptible to enable the fungus to survive in the soil. Southern blight appears to be more destructive on peanuts following peanuts or cotton, however, than on peanuts following corn.