Cylindrocladium black rot (CBR) in peanut is caused by the soil borne fungus Cylindrocladium parasiticum. CBR is present in all peanut production areas in North Carolina, although it is uncommon in fields that do not have a long history of peanut production. Without proper management, this disease can cause devastating losses.
The first above-ground symptom of CBR often is wilting on hot afternoons in mid-to-late August. Later, the entire plant may turn light green or yellow and eventually may die. Late in the season, particularly following moist weather, the fungus may produce numerous brick-red, pinhead-sized fruiting structures on crowns, lower stems, and pods of infected plants.
The taproot of symptomatic plants is black and rotted. Lateral roots have similar symptoms, or may be missing entirely. The rotted roots are very brittle and infected plants often break off at the soil line when tugged. CBR also rots pods, resulting in heavy yield losses. Seeds that are not rotted often are covered with cinnamon-brown speckles. These speckles are microsclerotia produced by the fungus. Microsclerotia also are produced inside the infected roots.
The CBR fungus is most active in cool, moist soil and many root infections occur soon after planting. Root wounding from nematode infections increases severity of CBR infection.
Poor rotations are a major source of CBR problems. Peanut, soybean, and other hosts support growth and reproduction of the CBR fungus. Decomposing roots, pods, and seeds release thousands of microsclerotia into the soil, where they can overwinter and infect peanut or other susceptible hosts.
Rotations of four years or more to non-hosts are necessary to prevent or reduce CBR problems. Cotton, corn, sorghum, and small grains are not hosts of the CBR fungus and are excellent rotation crops. On the other hand, the CBR fungus infects soybeans, increasing its population between peanut crops. On soybeans, the disease is known as red crown. Soybeans can be infected without a grower being aware of the problem as the symptoms of red crown often are less severe than CBR on peanut.
Planting a resistant cultivar often is all that is needed to avoid CBR losses in well-rotated fields. The virginia-type cultivars Bailey and Sullivan and several runner-type cultivars have high partial resistance to CBR. Avoid planting highly susceptible cultivars such as Gregory or CHAMPS in fields with a history of CBR.
Always purchase fungicide-treated seed from a reputable dealer to prevent the possibility of seed transmission of CBR.
Planting on a bed helps to promote soil warmth and drainage and will reduce the chances of infection in the spring. Nematode control also is important for reducing CBR damage.
CBR can be confused with spotted wilt, so be sure to obtain a positive diagnosis if there is any doubt. Make a note of problem fields and attempt to estimate the percentage of plants with symptoms. This information will help with rotation, fumigation, and cultivar choices the next time peanuts are planted.
Fumigation with metam sodium may be necessary in fields with a history of greater than 10% disease the last time peanuts were planted. Fumigation works by killing the fungus in soil. Root knot and ring nematodes can make CBR problems worse and fumigation also provides some control of these nematodes.
Inject metam sodium 42% at 7.5 gal/acre 12 inches below the top of the bed (or 8 inches below the original soil surface) at least two weeks before planting. Apply after soil temperatures reach 60°F at a 4-inch depth, and temperatures of 60°F or higher are forecast for the next 5 days. Wait to fumigate if an inch or more of rain is forecast within 3 days because rain can greatly diminish the effectiveness of fumigation. Avoid disturbing the soil after fumigation or untreated soil could be mixed with the fumigated soil. Herbicides can be incorporated before bedding and fumigation. Special precautions are required by law when applying soil fumigants. See the soil fumigant toolbox for requirements and other information.
In-furrow applications of some fungicides can suppress CBR, but these usually are not necessary if a resistant cultivar is planted.
Publication date: Oct. 30, 2018
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