Physiologic specialization ofPuccinia graminison wheat, barley, and oat in Canada in 2000

Stem rust is a devastating disease of cereal crops that is currently controlled, in Canada, primarily through the use of resistant cultivars. Annual surveys are conducted throughout Canada to monitor rust severity and obtain rust collections that are used to determine the virulence spectrum of each pathogen population. This is necessary for the detection of novel races that may express high virulence to currently grown cereal cultivars and to identify changes in virulence in the pathogen population. No stem rust was observed on cultivated spring wheat in 2000. Stem rust severity was light (<5%) on barley and oat in Manitoba, eastern Saskatchewan, and Ontario in 2000 and stem rust was not detected in other regions. Stem rust samples were collected from Hordeum jubatum, Avena fatua, and trap-plot nurseries and commercial fields of wheat, barley, and oat in Manitoba, eastern Saskatchewan, and Ontario from June to September 2000. Eight races of Puccinia graminis f. sp. tritici were found in collections from wheat, 11 from cultivated barley, and 2 from wild barley, with RCRSK as the dominant race on all three hosts. However, a cluster of races (QCCJ_, RCCJ_, and RMCJ_) with virulence to cultivated barley composed 69.2% of the population in Manitoba and Saskatchewan and poses a threat to barley production in this region. Five races of P. graminis f. sp. avenae were found in collections from cultivated oat and six from wild oat. Races NA67 and NA29 were the most common on cultivated (52.2 and 30.4%, respectively) and wild (18.6 and 55.0%, respectively) oat, while NA76 increased in frequency (9.6%) in Manitoba and Saskatchewan in 2000. Races NA67 and NA76 are virulent to Pg2, Pg9, and Pg13, which are the resistance genes likely present in most currently grown Canadian oat cultivars, and threaten commercial oat production in this region.