Distribution of Colletotrichum coccodes in Idaho and Variation in Pathogenicity on Potato. A. W. Barkdoll, Former Postdoctoral Fellow, Department of Plant, Soil and Entomological Sciences, University of Idaho Research and Extension Center, Aberdeen 83210. J. R. Davis, Professor, Department of Plant, Soil and Entomological Sciences, University of Idaho Research and Extension Center, Aberdeen 83210. Plant Dis. 76:131-135. Accepted for publication 11 June 1991. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/PD-76-0131.

Tubers and soil in potato-growing areas of Idaho and soil in native vegetation sites were surveyed for the presence of Colletotrichum coccodes. Colonization of tubers by C. coccodes was highly correlated with colony-forming units found in the soil (r = 0.69, P = 0.001). The fungus was not detected in native vegetation sites. Isolates of C. coccodes from each area were tested for variation in pathogenicity and symptom expression on Russet Burbank potato. All isolates reduced yield in the greenhouse by up to 14%. Some produced leaf lesions and severe wilt. Wilt was negatively correlated with tuber yield (r = –0.64, P = 0.001) and specific gravity (r = –0.60, P = 0.001). In the field, one isolate reduced total tuber yield and yield in the >280-g category. Those isolates that produced the most wilt in the greenhouse produced the most in the field.