Virulence of oat crown rust [Puccinia coronata f. sp avenae] in Canada during 2002-2006

Crown rust of oat (Avena sativa) was generally light (trace to 10% severities) in Manitoba and eastern Saskatchewan during 2002-2004 and 2006 and was severe in 2005. By 21 July 2005, fields of 'AC Assinboia' and 'Ronald' (with resistance genes Pc38, Pc39, and Pc68) near the buckthorn (Rhamnus cathartica) areas had 40%-80% crown rust severities. In mid-August, severities reaching 60%-100% were commonly observed in oat fields across Manitoba. Using 19 oat crown rust differentials, 59 races were identified in collections of isolates from will] oat (Avena fatua) in the eastern prairie region in 2002, 96 in 2003, 107 in 2004, 105 in 2005, and 76 in 2006. Forty-seven races (66.1% of the isolates) were virulent to Pc68 in 2006 compared with only two (1.2%) in 2002. Increased use of cultivars with Pc68, from 8% of the total area planted to oat in 1998 to >80% in 2006, resulted in a major shift in virulence to NO in the prairie rust population. The frequency of virulence to Pc48 increased from 6.5% in 2002 to 27.0% in 2003. It then decreased to 3.2% in 2006. Isolates with virulence to Pc38 and Pc39 remained prevalent, representing 57.4%-92.7% of the isolates from wild oat during 2002-2006. Virulence to Pc96 ranged from 0.8%-5.7% from 2003-2006. Virulence to Pc91, a gene in 'HiFi', was identified in 2002 and 2003 (one isolate each). Differences in frequencies of virulence to several Pc genes were significant between isolates from wild oat and cultivated oat in some years. An isolate (BRBG-94), virulent to genes Pc68 and Pc94 in the newly released cultivar 'Leggett', was obtained from a line with Pc94 in a rust nursery near Emerson, Manitoba, in 2006.