Incidence and virulence of Plasmopara halstedii on sunflower in western Canada during 1988-1991.
In Manitoba and Saskatchewan, the incidence of sunflower fields affected by downy mildew, caused by P. halstedii, increased from 6% in 1988 to >20% in 1990 and 1991. The incidence of diseased plants per field ranged from <1% in 1988 to 30% in 1990. Cultures of P. halstedii were obtained from field collections of zoosporangia during 1988-91 and were assessed for virulence on differential lines of sunflower. Before 1988, the dominant races had been 1 and 2. However, only races 3 and 4 were found in 5 cultures from each of 1988 and 1989. The reactions of the differential lines to most of 56 cultures obtained in 1990 and 1991 were not identical to reactions induced by the 7 known races of the pathogen, suggesting that the cultures contained either mixtures of races or new races. Reactions indicating the presence of races 3 and 4 were found in 35% to 45% of the cultures in 1990-91. Less commonly, reactions also indicated the presence of races 2, 6, 7 and 1. Resistance to race 2 occurs in many sunflower cultivars used in Manitoba and Saskatchewan and may have contributed to the change that has occurred in the race structure of P. halstedii in these provinces.