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The changes in virulence phenotype frequencies of Puccinia coronata Cda. f.sp. avenae Eriks. since 1974 in the eastern (Ontario, Quebec, and the Maritimes) and prairie (Manitoba and Saskatchewan) regions of Canada were examined. In the eastern region, frequency of virulence to Pc39 increased after the release of a cultivar with this resistance gene. Virulence to Pc38 and Pc39 increased in the prairie region after cultivars with these two genes were released. The frequencies of virulence to Pc35 and Pc56 fluctuated between 9 and 53% in the eastern region, even though these genes are not known to be present in oat cultivars grown there. In the prairie region, the frequencies of virulence to Pc35 and Pc40 fluctuated between 21 and 60% from 1974 to 1990, and virulence to Pc46 increased from 7% in 1974 to 72% in 1985, even though these genes have not been used in commercial oat cultivars. The prairie population generally had higher levels of phenotypic diversity, as measured by the Shannon, Simpson, and Gleason indexes, than did the eastern population in almost all years examined. These results contrast with previous studies using an older set of differentials in which the eastern population had a higher level of phenotypic diversity than the prairie population. The Rogers index indicated that differences in frequencies of virulence phenotypes between the two populations has increased gradually since 1974. Key words: crown rust, resistance genes, selection, phenotypic diversity.
Chong et al. (Mon,) studied this question.