The inheritance of resistance to stem canker in soybeans

The fungus, Diaporthe phaseolorum (Cke. and Ell.) Sacc. war. caulivora Athow and Caldwell is the causal agent of the stem canker disease in soybeans [Glycine max (L.) Merr.]. Soybean yield losses up to 80& or more in areas of the southeastern United States during the early 1980s prompted the urgency of determining the inheritance of resistance in order to facilitate development of new varieties with resistance to the pathogen. The objectives of this study were to: (1) compare the relative virulence of two stem canker isolates from Mississippi and two from Tennessee, (2) determine the inheritance of resistance to stem canker among ‘Bay,’ ‘Tracy-M,’ ‘Bragg’ and ‘J77-339’ soybean varieties, and (3) determine if the gene(s) conditioning resistance is the same in the varieties Bay and Tracy-M.

All possible crosses, including reciprocals were made among the resistant parents, Bay and Tracy-M and the susceptible parents, Bragg and J77-339. During 1984 and 1985, the selfs, F₁, F₂, BC1 and BC2 plants of each cross were tested for their reaction to inoculation with the stem canker pathogen under greenhouse conditions. The Tennessee isolate, labeled FP3, was determined the most virulent from the greenhouse tests conducted with the four isolates. Thus, the FP3 isolate was the only isolate used for testing the selfs, F₁, F₂, BC1 and BC2 populations from each cross combination.

Resistance to stem canker in the four soybean varieties is likely conditioned in the following ways: (1) Bay has a single dominant gene (R₁R₁r₂r₂), Tracy-M has two dominant, genes (R₁R₁R₂R₂), (3) Bragg is segregating for the R₂ gene (r₁r₁R₂~~) and, (4) J77-339 is homozygous recessive for both genes (r₁r₁r₂r₂). The single dominant gene present in Bay appears to be the same as one of the dominant genes in Tracy-M. The high level of resistance in Bay and the low level of resistance in Bradd indicates that the R₁ gene confers more resistance than the R₂ gene.