Traces of crown rust were found in wild oat and commercial oat fields in southern Manitoba in mid July. However, in areas near Morden and Carman where European buckthorn is present, crown rust infections were already heavy by this time, with severities ranging from 40% to 80% in susceptible oat plots and in wild oat. Frequent dew periods in late July and August were conducive for rust development. Crown rust severities of 40% to 90% were commonly observed in susceptible cultivars and cultivars that have resistance genes Pc38 and Pc39 (e.g. Dumont, Robert, Riel). Only trace levels of infections were found in the two newly released cultivars, AC Assiniboia and AC Medallion, planted in the uniform rust nurseries. These two cultivars have genes Pc38, Pc39, and Pc68 combined. The occurrence of these isolates is a major concern, as they would likely increase significantly after the new cultivars become widely grown.
Only 22 isolates were established from the Ontario collections in 1996, compared to 328 from Manitoba. Frequencies of these isolates virulent on oat lines with known Pc genes are shown in Table 1. As in recent years, virulence to Pc38 and Pc39 was high in Ontario and Manitoba. Virulence to Pc40, Pc51, and Pc59 was more frequent in Manitoba than in Ontario, whereas virulence to Pc48 and Pc56 was more frequent in Ontario than in Manitoba.
Based on a differential set that includes genes Pc40, 45, 46, 50, 38, 39, 48, 68, 51, 52, 58, 59, 54, 56, 62, and 64, 11 races were identified from Ontario and 129 races from Manitoba. The most common races found in Ontario were BQBB (27.3%) and BQBG (22.7%), whereas the most common races in Manitoba were BQBB (12.2%) and LQBB (7.3%).
Crown rust was less severe in 1997 than in 1996. Traces of crown rust infections were found in wild oat in the Red River Valley in mid July. Infections remained light on wild oat and in most commercial farm fields in the Red River Valley during the remainder of the growing season, except for areas (Carman and Portage la Prairie) where the alternate host is present. By early August, crown rust severities ranging from 60% to 100% were commonly found in susceptible oat plots and wild oat near Carman and Portage la Prairie.
Eighty-one and 255 isolates were established, respectively, from collections from Ontario and Manitoba in 1997. Frequencies of these isolates virulent on the 16 oat crown rust differentials are shown in Table 1. There was a slight decrease in virulence to Pc38 and Pc39 in Ontario and Manitoba in 1997, compared to the frequencies of virulence to these genes in the previous year. Frequencies of virulence to the other Pc genes were relatively similar to those detected in 1996.
Twenty-seven races were identified from Ontario in 1997, compared to118 races from Manitoba. As in 1996, BQBB (29.6%) and BQBG (12.3%) were the most common races in Ontario, and LQBB (5.5%) and BQBB (4.7%) were the most common races in Manitoba in 1997.
The newly derived gene, Pc94, from Avena strigosa, continues
to be highly effective to crown rust as it has since 1992. It was resistant
to all isolates from Canada in 1996 and 1997, and should prove to be valuable
for developing cultivars with complex resistance to crown rust.
|No. of isolates||22||328||81||255|