Nonrandom distribution of virulence and phenotypic diversity in two populations of Puccinia recondita f. sp. tritici in Canada

The eastern and prairie populations of Puccinia recondita f. sp. tritici in Canada were examined for nonrandom distribution of virulence and phenotypic diversity. In the prairie region, where resistant cultivars have been grown and polymorphisms for unneeded virulences are low, virulence to Lr1 and Lr1a appeared to be positively associated from 1960 to 1974 and was negatively associated from 1975 to 1987. Positive and negative virulence associations were also detected among virulences to Lr1, Lr2a, Lr10, Lr16, Lr17, Lr18, and Lr24 in the prairie region. In the eastern region, where susceptible host cultivars are commonly grown and unneeded virulences are common, positive associations were found among virulences to Lr2c, LrB, Lr3ka, Lr11, and Lr18. Levels of phenotypic diversity between the two populations were compared with the Shannon index. Based on the infection types of the Unified Numeration differentials (Lrl, Lr2a, Lr2c, and Lr3), in the 3 yr examined, the eastern population had higher levels of phenotypic diversity than the prairie population. Most of the loss of diversity in both populations was caused by the departure of virulence frequencies from 0.5. The nonrandom distribution of virulence observed in both populations is characteristic of asexual cereal rust populations.