This study quantified the frequency of simple versus
complex races of Puccinia striiformis Westend. in mixtures
of wheat cultivars possessing different race-specific
resistance genes. A simple race of a pathogen can infect
only one component, and a complex race of the pathogen can
infect two or more components of an intraspecific plant
mixture. The treatments were designed so that the race that
was complex changed depending on the host mixture, thus
enabling us to observe the influence of pathogen complexity
in different host genetic backgrounds. Six cultivar
mixtures and one pure stand of winter wheat were inoculated
with three races of P. striiformis (CDL 27, CDL 29, and CDL
41) at two locations for two seasons. Potted plants of
three winter wheat cultivars (Paha, Tres, and Tyee) that
were each susceptible to one of the three races of the
pathogen were used to sample the pathogen during the field
epidemics. Disease incidence on the differential cultivars
was used to calculate the proportion of the three races in
each treatment. The specific cultivars included in the
mixtures influenced the frequencies of the three races.
Increasing the number of virulent races in a mixture reduced
the frequency of the complex race relative to the other two
races. When two of the races (races 29 and 41) were complex
on the same mixture, location had an effect on which of the
races was more frequent. When race 29 was the complex race
in the mixture, it was more frequent than when race 41 was
the complex race. The results suggest that environmental
interactions, genetic background of the pathogen race, host
composition, and interaction among pathogen races may be as
important in determining race frequencies in mixtures as is
stabilizing selection sensu Vanderplank (1968)
