for 10 yr, corn rotated with soybeans for 5 ABSTRACT yr, or continuous corn for 10 yr. The Garzonio, D. M., and McGee, D. C. 1983. Comparison of seeds and crop residues as sources of fields were within 11 km of each other. inoculum for pod and stem blight of soybeans. Plant Disease 67:1374-1376. Soil types were all of the Clarion-Nicollet-Webster association. Individual Soybean seed lots of three cultivars (in 1979) and of one cultivar (in 1980) infected with different plots consisted of four 4.8-m rows 0.8 m amounts of Phomopsis sp. and Diaporthe phaseolorum var. so/ae (collectively referred to as apart with 100 seeds per row. Phomopsis Phomopsis), the causal organisms of pod and stem blight, were planted in fields near Ames, IA, infection was measured on samples of 10 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 of each plot, of stems at the V4, R3t row infection on seedlings or on mature plants. Marked differences, however, occurred among cropping ofe h pt, of ste s at R3, an practices, with the most severe infection in the continuous-soybean field, less in the corn-soybean-R8 growth stages, of pods at R3, R7, and rotation field, and least in the continuous-corn field. Soil potassium content and plant lodging were R8, and of seeds at R7 and R8. At all eliminated as possible explanations for this disease pattern. Transmission of Phomopsis from growth stages except V4, respective plant viable artificially inoculated seeds to soybean seedlings was detected in a continuous-corn field, but parts were sampled from the lower, inoculum from that source could not be distinguished from that from other sources in a continuousmiddle, and upper sections of each plant soybean field. Transmission of Phomopsis from nonviable artificially inoculated seeds to adjacent and then bulked by plant. At V4, only a viable seedlings was not demonstrated. lower stem section was sampled. Within
