Effects of dual infections of Puccinia hordei and Erysiphe graminis on barley, cv. Zephyr

SUMMARY In replicated field trials in 1971 and 1972, plots of Zephyr barley were obtained with (1) powdery mildew, (2) brown rust, (3) both diseases or (4) neither disease, by using appropriate systemic fungicides. Mildew was fairly severe (up to 40% infection of individual leaves) and significantly reduced yield and grain size in both years. Rust was not severe (< 5 % on any leaf) in either year and it reduced yield and grain size only in 1972. In these conditions the mean percentage losses (relative to treatment 4) in plots with both diseases were approximately equivalent to the sum of those from powdery mildew and brown rust alone. There was no significant mildew × rust interaction in respect of yield or senescence of leaves, but comparisons of the infection rate of Erysiphe graminis on leaves with and without rust in similar situations within the 1972 trial suggested that rust affected the development of mildew on the same leaf. Experiments in the glasshouse using the first leaf of seedlings, not chemically treated, confirmed that there could be marked interactions between the pathogens on a single leaf. Pre‐inoculation with one pathogen generally reduced the development of the other.