Virulence dynamics, phenotypic diversity, and virulence complexity in two populations of Puccinia triticina in Canada from 1987 to 1997

Populations of Puccinia triticina (causal agent of wheat leaf rust) in Ontario, Quebec, Manitoba, and Saskatchewan were analysed for frequency of isolates with virulence to leaf rust resistance genes Lr1, Lr2a, Lr2c, Lr3, Lr9, Lr16, Lr24, Lr26, Lr3ka, Lr11, Lr17, and Lr30 for the years 1987-1997. Winter wheats in the southern plains of the United States with resistance genes Lr24, Lr26, Lr11, Lr3ka, Lr17, and Lr16 selected virulent P. triticina phenotypes in the Manitoba and Saskatchewan population. The P. triticina population in Quebec and Ontario was comparatively stable, showing no directional trends in selection of virulence phenotypes. In the Manitoba and Saskatchewan population, diversity of virulence phenotypes changed relatively little despite the selection that occurred in this population. The average number of virulences per isolate in the Manitoba and Saskatchewan population increased from 1987 to 1994 during the period of selection for virulence to Lr24, Lr26, Lr3ka, and Lr11, and declined from 1995 to 1997 when selection for virulence to Lr16 and Lr17 occurred. The most complex virulence phenotypes were not necessarily selected in the Manitoba and Saskatchewan population because of how the resistance genes were deployed in the host population and the nonrandom distribution of virulences in the P. triticina population.