Virulence dynamics of Puccinia graminis f. sp. avenae in Canada, 1921-1993

The virulence dynamics in the oat stem rust fungus. Puccinia graminis f. sp. avenae, in Canada from 1921 to the present are documented. The reidentification of isolates of P. g. avenae stored since 1953 was used as the basis to relate virulences in older and current populations. Compared to other cereal rust fungi in North America, virulence in P. g. avenae appears highly stable. Common pathotypes (races) of P. g. avenae have tended to dominate populations for 25 yr or longer--some races have been isolated for about 40 yr. In the prairie region, virulence to genes Pg9 and Pg13, currently important resistance sources, was relatively common (races NA3 and NA7 were the most frequently identified from stored isolates of races 1/5-C1 and 2-C2, respectively), then declined with the emergence and dominance of race 6AF/C10/NA27 in the 1960s. However, because races NA3 and NA7 are avirulent to cultivars carrying gene Pg2, and given the asexual nature of the prairie P. g. avenae population, the maintenance of Pg2 resistance in contemporary cultivars should reduce the threat to Pg13 resistance in this region. The frequency of virulence to Pg15 was very high across Canada but declined in the prairie region along with virulence to Pg9 and Pg13. Virulence to Pg16 has occurred in only one year, and virulence to gene Pga was found once in this study. Virulence to all of the Pg resistances has occurred at some time in the North American P. g. avenae populations, regardless of the exposure of the populations to these resistances