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Soybean Seed Decay: Sources of Inoculum and Nature of Infection. Konrad T. Kmetz, Former graduate research associate, Department of Plant Pathology, The Ohio State University, Columbus 43210, Present address of senior author: E. I. duPont de Nemours and Co., Wilmington, DE 19898; C. Wayne Ellett(2), and A. F. Schmitthenner(3). (2)Professor, Department of Plant Pathology, The Ohio State University, Columbus 43210; (3)Professor, Ohio Agricultural Research and Development Center, Wooster 44691. Phytopathology 69:798-801. Accepted for publication 29 September 1978. Copyright 1979 The American Phytopathological Society. DOI: 10.1094/Phyto-69-798.

Phomopsis-type pycnidia and alpha spores (fertile spores of the imperfect stage) were abundant on overwintered soybean straw and current-season plant debris. Of alpha spores placed on acidified potato-dextrose agar only 8% produced colonies of Diaporthe phaseolorum var. sojae, whereas 92% were of an undescribed Phomopsis sp. In contrast, perithecia of Diaporthe were found infrequently and only on overwintered debris; 13% of these produced Diaporthe phaseolorum var. caulivora colonies and 87% produced D. phaseolorum var. sojae colonies. The only secondary inocula detected were alpha spores on current-season debris. Maximum production of spores occurred during the pod-filling period. Alpha spores were detected on the surfaces of immature symptomless soybeans, primarily on the lower one-third of the plants, and were recovered from plants up to 2 m from an inoculum source. Phomopsis sp. and D. phaseolorum var. sojae mycelia introduced into soybean plants by a toothpick method caused local, latent infection of green cotyledons, hypocotyls, stems, petioles, and pods in both field and growth chamber tests. The pathogens spread in senescing and dead plants under moist, humid conditions.