Comparison of Seeds and Crop Residues as Sources of Inoculum for Pod and Stem Blight of Soybeans. D. M. Garzonio, Former Graduate Research Assistant, Department of Plant Pathology, Seed and Weed Sciences, Iowa State University, Ames 50011. D. C. McGee, Associate Professor, Department of Plant Pathology, Seed and Weed Sciences, Iowa State University, Ames 50011. Plant Dis. 67:1374-1376. Accepted for publication 24 June 1983. Copyright 1983 American Phytopathological Society. DOI: 10.1094/PD-67-1374.

Soybean seed lots of three cultivars (in 1979) and of one cultivar (in 1980) infected with different amounts of Phomopsis sp. and Diaporthe phaseolorum var. sojae (collectively referred to as Phomopsis), the causal organisms of pod and stem blight, were planted in fields near Ames, 1A, with cropping histories of continuous soybeans, corn rotated with soybeans, or continuous corn. No relationship was found between the amount of seedborne inoculum and severity of Phomopsis infection on seedlings or on mature plants. Marked differences, however, occurred among cropping practices, with the most severe infection in the continuous-soybean field, less in the corn-soybean-rotation field, and least in the continuous-corn field. Soil potassium content and plant lodging were eliminated as possible explanations for this disease pattern. Transmission of Phomopsis from viable artificially inoculated seeds to soybean seedlings was detected in a continuous-corn field, but inoculum from that source could not be distinguished from that from other sources in a continuous-soybean field. Transmission of Phomopsis from nonviable artificially inoculated seeds to adjacent viable seedlings was not demonstrated.