PHYSIOLOGIC RACES OF OAT CROWN RUST ISOLATED FROM AECIA ON BUCKTHORN AND THEIR RELATION TO THE RACIAL POPULATION ON OATS IN SOUTHEASTERN ONTARIO AND MANITOBA

Abstract

The physiologic race composition of isolates of oat crown rust, Puccinia coronata Corda f. sp. avenae Erikss., from aecia on buckthorn, Rhamnus cathartica L., was correlated with the races isolated from oats, Avena saliva L., in southwestern Ontario and Manitoba respectively. Among the races isolated from aecia in each region, predominant were virulent races which were isolated also from infected oats growing in the buckthorn areas. This suggests strongly that the alternate host is important in initiating crown rust epiphytotics.Races virulent on Landhafer and Santa Fe oats comprised 52% of the isolates from oats and 45% of the isolates from buckthorn in Manitoba. Isolates from oats and buckthorn in Ontario were also correlated, with the most prevalent races being those virulent on Victoria and Bond. They comprised 55% and 45% of the isolates from the principal and alternate hosts respectively.Eight races of crown rust recently discovered in Canada are described in terms of their reactions on the standard set of differential varieties. It is suggested that hybridization on the alternate host would adequately account for the origin of these new races.