EFFECT OF WHEAT CULTIVAR MIXTURES ON POPULATIONS OF PUCCINIA-STRIIFORMIS RACES

This study quantifies the frequency of simple and complex races (races that can infect two or more components) of Puccinia striiformis in mi xtures of wheat cultivars possessing different race-specific resistanc e genes. Treatments were designed so that the complex race changed dep ending on the host mixture, thus enabling us to observe the influence of pathogen complexity in different genetic backgrounds. Six cultivar mixtures and one pure stand of winter wheat were inoculated with three races of P. striiformis at two locations for two seasons. Potted plan ts of three winter wheat cultivars, each susceptible to one of the thr ee races of the pathogen, were used to sample the pathogen during the field epidemics. Disease incidence on the differential cultivars was u sed to calculate the proportion of the three races in each treatment. The specific cultivars included in the mixtures influenced the frequen cies of the three races. Increasing the number of virulent races in a mixture reduced the frequency of the complex race relative to the othe r two races. The results suggest that genetic background of the pathog en race, host composition, and interaction among pathogen races may be as important as cost of virulence in determining race frequencies in mixtures.