Disease Note.

Black Dot of Potato Caused by Colletotrichum coccodes in Nebraska. Alexander D. Pavlista, University of Nebraska, Panhandle Research and Extension Center, Scottsbluff 69361. Eric D. Kerr, University of Nebraska, Panhandle Research and Extension Center, Scottsbluff 69361. Plant Dis. 76:1077. Accepted for publication 10 June 1992. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/PD-76-1077A.

Tubers of potato (Solanum tuberosum L.) cv. Monona with dark, grayish lesions on the surface were collected from storage in Box Butte County, Nebraska, in December 1988. Isolates on potatodextrose agar were identified as Colletotrichum coccodes (Wallr.) S.J. Hughes, the cause of potato black dot. Pathogenicity was confirmed by fulfilling Koch's postulates on Monona tubers. Vines of cv. Russet Burbank potatoes grown in north central Nebraska in 1989 and 1990 were girdled by stem lesions in August. Small black sclerotia were abundant in stem lesions above and below ground level. Earlier in the season, foliage of infected plants showed yellowing and wilting. Amethyst coloration of inner tissues was observed in stem bases and near leaf attachments. Sclerotia and amethyst coloration were also observed on tuber stolons and underground stems where the periderm was sloughed away. Stolon attachments with amethyst coloration and black sclerotia have been found on stored tubers from western and northern Nebraska. The incidence and severity of black dot in Nebraska potato fields have not been investigated, and the disease may be misdiagnosed as Verticillium wilt or early blight without careful examination. Extremely dry, hot weather during the 1988-1990 seasons may have favored development of severe black dot symptoms.