A Soybean Genotype Resistant to Phomopsis Seed Decay1

Phomopsis seed decay (PSD) reduces soybean [Glyclne max(L.) Merr.] seed quality and seedling establishment. Experiments were conducted in Columbia, MO, and Isabela, PR, over a 3‐yr period to identify a source(s) of resistance to PSD in soybean. All genotypes evaluated were field grown under conditions favorable for infection by Phomopsisspp. Percentage infection was determined by counting colonies of Phomopsisspp. growing from surface‐sterilized seeds plated on acidified potato (Solanumspp.) dextrose agar. Plant introduction (PI) 417479 was free of Phomopsisspp. seed infection at Columbia in 1984 and Isabela in 1985, and had 3% infection at Columbia in 1985. A phenotypically indistinguishable genotype, PI 360841, had levels of infection that did not differ statistically from those observed for PI 417479 and may be a duplicate in the germplasm collection. In the same environments, the widely grown ‘Williams 82’ showed from 25 to 59% infection. The PI 417479 exhibited less infection than the reported P. sojaeLehman resistant genotypes PI 80837 and ‘Delmar’ in all but one test. Maturity dates of PI 417479 were near the average of all genotypes tested. The PI 417479 appears to have been exposed to average, or above average, disease pressure. Thus, the level of resistance to PSD measured cannot be attributed to maturation during periods of favorable environmental conditions. Also, antagonism of Phomopsisspp. seed infection by Cercospora kikuchii(T. Matsu. & Tomoyasu) Gardner did not appear to influence the conclusions relative to PI 417479. Detectable seed infection of PI 417479 by C. kikuchiiwas 2% or less in all experiments.