Incidence of Colletotrichum dematium on Prickly Sida, Spotted Spurge, and Smooth Pigweed and Pathogenicity to Soybean. K. S. McLean, Graduate Student, Department of Plant Pathology and Weed Science, Mississippi State University, Mississippi State 39762. K. W. Roy, Professor, Department of Plant Pathology and Weed Science, Mississippi State University, Mississippi State 39762. Plant Dis. 72: 390-393. Accepted for publication 2 December 1987. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/PD-72-0390.

Field collections of prickly sida (Sida spinosa), spotted spurge (Euphorbia maculata), and smooth pigweed (Amaranthus hybridus) from various soybean (Glycine max) fields in Mississippi were assayed for Colletotrichum. Isolates from weeds were identified as either C. dematium var. truncata or C. dematium. Mean incidence of C. d. var. truncata on prickly sida, spotted spurge, and smooth pigweed was 19, 6, and 10%, respectively. Mean incidence of C. dematium on prickly sida and spotted spurge was 2%, and on smooth pigweed was 6%. In greenhouse pathogenicity tests, isolates of C. d. var. truncata from prickly sida and spotted spurge caused damping-off, seedling blight, and reduced root volume of soybean seedlings. The same isolates colonized the two weeds and, although they caused no symptoms, they produced conidia on dead weed tissue. Dispersal of conidia from the dead weed tissue to emerging soybean seedlings and the subsequent infection of the seedlings was demonstrated. Isolates of C. d. var. truncata from smooth pigweed and C. dematium from prickly sida, spotted spurge, and smooth pigweed were not pathogenic on soybean.

Keyword(s): cotton, soybean anthracnose, tomato.