Phomopis-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 Phomopis sp. In contrast, perithecia of Diaporthe were found infrequently and only on overwintered debris; 13% of these produced D. 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 1/3 of the plants, and were recovered from plants up to 2m 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.