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 use d 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. av enae have tended to dominate populations for 25 yr or longer-some race s have been isolated for about 40 yr. In the prairie region, virulence to genes Pg9 and Pg13, currently important resistance sources, was re latively common (races NA3 and NA7 were the most frequently identified from stored isolates of races 1/5-C1 and 2-C2, respectively), then de clined with the emergence and dominance of race 6AF/C10/NA27 in the 19 60s. However, because races NA3 and NA7 are avirulent to cultivars car rying gene Pg2, and given the asexual nature of the prairie P. g. aven ae population, the maintenance of Pg2 resistance in contemporary culti vars should reduce the threat to Pg13 resistance in this region. The f requency 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 o ccurred at some time in the North American P. g. avenae populations, r egardless of the exposure of the populations to these resistances.