Virulence and Race Dynamics ofPuccinia reconditaf. sp.triticiin Canada during 1956-1987

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Abstract
The changes in virulence and Unified Numeration (UN) race percentages of Puccinia recondita f. sp. tritici since 1956 in three regions of Canada were examined. Virulence frequencies of Lr2c and Lr3 in eastern Canada and the Pacific region generally fluctuated between 20-100%, even though these genes are not known to be present in wheat grown in these regions. UN races in the eastern and Pacific regions fluctuated between low (0-20%) and intermediate (20-60%) levels, with no strong directional trends. In the prairie provinces, levels of virulence on lines having specific resistance genes increased in frequency after the corresponding resistance genes were released in the host population. In this region, changes in the race composition could be related to the changes in level of individual virulences and the use of resistance in the host population. Examples supporting and contradicting the concept of stabilizing selection are discussed with reference to wheat leaf rust populations in Canada.