Rust resistance in Salix induced by inoculations with avirulent and virulent isolates of Melampsora larici-epitea

Six Salix clones and four Melampsora larici-epitea isolates were used in two leaf-disc inoculation experiments to determine whether disease severity was affected by the presence of both virulent and avirulent rust genotypes. In the first experiment, an equal amount of urediniospores of a virulent isolate and an avirulent isolate was applied simultaneously using four levels of spore suspension. In the second, the willows were inoculated with one isolate first and then, after 3 days, with another using two spore concentrations. In the first experiment, overall rust spore production was reduced by 48.6% in inoculations with mixed inocula compared with that in the inoculations with single virulent isolates. In the second experiment, 20 of the 36 combinations involving pre-inoculations with avirulent isolates showed significant reduction in spore production. The suppressive effects on rust sporulation were more obvious at the higher inoculum densities and on the clones S. x calodendron and S. x mollissima 'Q83'.