Resistance of Glycine tomentella to soybean leaf rust Phakopsora pachyrhizi in relation to ploidy level and geographic distribution.

Accessions of five diploid and five tetraploid isozymically defined groups of Glycine tomentella collected from throughout the species range in Australasia were scored for resistance to three separate isolates of Phakopsora pachyrhizi, the causal agent of soybean leaf rust. Resistance levels were found to be high (>75%) in most of the groups. While resistance levels differed among groups, the overall levels in polyploids were similar to those in diploids. Geographical patterns of resistance and susceptibility to P. pachyrhizi indicate that two regions of susceptibility exist. The highest proportion of susceptible accessions occurs in the Kimberley Plateau region of Western Australia and the Northern Territory, while another region of susceptibility is found in the Townsville/Cairns region of Queensland. Results from genetic crosses between accessions within two forms of the tetraploids indicate that in the aneuploid form (2n = 78), resistance to P. pachyrhizi was under the control of a single dominant gene, whereas in a second group of tetraploids (2n=80), resistance was controlled by two or three gene loci.