WHEAT LEAF RUST SEVERITY AS AFFECTED BY PLANT-DENSITY AND SPECIES PROPORTION IN SIMPLE COMMUNITIES OF WHEAT AND WILD OATS

While it is generally accepted that dense stands of plants exacerbate epidemics caused by foliar pathogens, there is little experimental evi dence to support this view. We grew model plant communities consisting of wheat and wild oats at different densities and proportions and exp osed these communities to Puccinia recondita to induce wheat leaf rust . Wild oats was included because it is a common competitor of wheat an d may act as a barrier to the dispersal of P. recondita spores among w heat plants. Disease severity was estimated as percentage of wheat fla g leaves covered by rust lesions. Seeding density rarely had a signifi cant influence on rust severity, probably because of compensation due to increased tillering at low seeding densities. In contrast, increasi ng the proportion of wheat in mixtures with wild oats consistently inc reased wheat leaf rust severity. Regression parameters describing whea t leaf rust severity as a function of wheat seeding density did not di ffer significantly between pure wheat stands and wheat-wild oat mixtur es and, thus, failed to support an effect of wild oats on wheat leaf r ust other than through its competitive impact on wheat tiller density.