SEED INFECTION OF SOYBEAN BY DIAPORTHE PHASEOLORUM AND ITS INFLUENCE ON HOST DEVELOPMENT

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Abstract
Diaporthe phaseolorum (Cke. & Ell.) Sacc., the causal agent of pod and stem blight of soybeans and generally regarded as a facultative parasite, has been found to be pathogenic during the seedling phase. The fungus caused severe losses of germinating seeds and young seedlings. In 1959 more than 30% of the soybean seed crop in southwestern Ontario was infected, and germination was severely reduced. D. phaseolorum was present in seed samples from this area in 11 of 13 years. Studies with a series of seed samples of varying levels of infection revealed that the fungus was still viable and pathogenic in the seed after storage for 2 years. However, during that time the levels of infection in most samples had decreased markedly whereas a concurrent increase in germinability indicated that infected seeds were killed during the germination phase. While several seed treatment fungicides were effective in increasing germination and emergence in heavily infected samples, none completely controlled the disease. At normal planting rates, yield was not affected by levels of seed infection up to 42% but the yield was significantly lower in a sample containing 73% infected seed. It is concluded that the disease may be important in the seed and seedling phases of host development.

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