COLLAR ROT OF PEANUT CAUSED BY LASIODIPLODIA-THEOBROMAE

In August and September of 1993, a collar rot disease of peanut was ob served in several fields in Virginia and North Carolina. Only a few sc attered plants exhibited symptoms and signs of the disease in Southamp ton County and Suffolk, Virginia, and Northampton County North Carolin a. The disease was severe at two farm sites in Dinwiddie County, Virgi nia where the affected areas exceeded 0.4 ha in size. Numerous plants were either chlorotic, wilted, or dead. Symptomatic plants exhibited b lackened stem cankers and pods. Diseased stems and tap roots were easi ly shredded to reveal slate-gray to black internal tissues. Black, eru mpent pycnidia were observed on stem lesions at the soil surface. Imma ture conidia were single-celled and hyaline. Mature conidia were two-c elled and dark brown. Morphological features of the fungus on diseased plants and potato dextrose agar were consistent with descriptions of Lasiodiplodia theobromae (Diplodia gossypina). The fungus was isolated from discolored seed and asymptomatic seed from fields having plants which exhibited severe symptoms. Seed treatment with captan 1.125 g pentachloronitrobenzene 0.375 g + carboxin 0.25 g a.i./kg reduced reco very of the fungus from seed, but did not eradicate the pathogen. This treatment on naturally infested seed provided significant early-seaso n disease suppression and improved yield significantly in 1994. Season -long disease suppression and a significant yield increase were obtain ed in plots planted to fungicide-treated, commercial seed from non-inf ested fields. At-plant and mid-season applications of fungicides in 19 94 and 1995 did not improve disease suppression over that of fungicide -treated, commercial seed. Overall, Virginia-type cultivars of peanut were more susceptible to collar rot than runner-type cultivars. Among the Virginia-type cultivars, NC-V 11 exhibited moderate susceptibility and the 79-X breeding line from Florida exhibited resistance. Georgia Browne and Southern Runner were the most resistant of the runner-type cultivars.