Phenotypic and genetic patterns of resistance to the pathogen Phakopsora pachyrhizi in populations of Glycine canescens.

Phenotypic patterns of resistance to nine races of the pathogen Phakopsora pachyrhizi (soybean rust) in two natural populations of Glycine canescens were determined. In both populations there was considerable variability both within and between different host lines in their resistance or susceptibility to the nine different pathogen races. The genetic basis of these patterns of resistance was analyzed through an extensive series of crosses. In both host populations resistance was conditioned by single dominant genes with major phenotypic effects. One, two or three such genes were present in each host line. Using the principles of the gene-for-gene hypothesis, knowledge about the number of resistance genes present in each host line and by cross comparison of the phenotypic patterns of disease resistance detected in each line, estimates were made of the number of resistance genes or alleles present in each population of G. canescens. The two populations contained a minimum of 10 and 12 resistance genes. The relevance of these results to agriculture is discussed briefly.