Physiologic specialization ofPuccinia coronataf. sp.avenae, the cause of oat crown rust, in Canada from 1999 to 2001

Crown rust Puccinia coronata f. sp. avenae of oat (Avena sativa) was light in southern Manitoba in July in all 3 years during 1999–2001, but by mid-August in 1999 and 2001, heavy crown-rust infections (up to 90% severity) were found in late-planted fields of susceptible oat cultivars and in wild oat (Avena fatua) in Manitoba and eastern Saskatchewan. An important change in 2001 from previous years was the large increase in crown-rust severity (up to 90%) in late-planted fields of 'Triple Crown'. Crown-rust severity remained low on 'AC Assiniboia' during 1999–2001. Using 18 differential oat lines, the number of virulence phenotypes identified in the eastern prairies was 113 from 316 isolates in 1999, 111 from 260 isolates in 2000, and 112 from 217 isolates in 2001. From Ontario, 11 virulence phenotypes were identified from 25 isolates in 1999, 26 from 60 isolates in 2000, and 17 from 26 isolates in 2001. The Ontario and prairie rust populations were dominated by isolates having virulence to Pc38 and Pc39 combined. Virulence frequency to genes Pc68 and Pc96 ranged between 0.9% and 2.8% in the eastern prairies. Virulence to Pc48 increased from 3.8% in 1999 to 22.6% in 2001 in the eastern prairies and decreased from 28.0% in 1999 to 7.7% in 2001 in Ontario. No virulence was detected to Pc94 in Ontario or in the eastern prairies and to Pc58 in Ontario in all 3 years. Isolates with virulence to gene combinations Pc68, Pc96 and Pc48, Pc68 were identified for the first time in the eastern prairie region in 1999 and 2000, respectively.