STRIPE RUST, YIELD, AND PLANT COMPETITION IN WHEAT CULTIVAR MIXTURES

Mixtures of two, three, four, or five winter wheat (Triticum aestivum) cultivars and their component pure stands were either exposed to or protected from two stripe rust (Puccinia striiformis) races at three and two locations in 1986 1987 and 1987-1988, respectively. Disease severity and yield components per cultivar were determined for a subset of the mixtures and also for the pure stands. Disease severity in the mixtures compared to the mean of the components was reduced between 13 and 97%. Changes in disease severity could be separated into two effects. First, selection changed frequencies of the cultivars in the mixtures by up to 35% at harvest compared to the planted frequencies. Reductions in overall disease severity in mixtures due to selection for the more resistant cultivar were as high as 42%, and increases in overall disease severity due to selection for the more susceptible cultivar were as high as 11% over the mean disease severity in the pure stands. Second, disease severity on individual cultivars was reduced below that observed in pure stands because of the epidemiological effect of host diversity. Mixtures yielded between 0 and 5% more than the mean of the pure stands in the absence of disease. In the presence of disease, mixing increased yield between 8 and 13%. Location and year influenced the effects of disease on plant performance indirectly by affecting the competitive abilities of the cultivars.