The effect of fungicide dose on the composition of laboratory populations of barley powdery mildew

The effect of the use of different doses of the fungicide fenpropimorph on populations of barley powdery mildew Blumeria (Erysiphe) graminis f. sp. ho rdei was investigated in a laboratory selection experiment. A sample from t he Danish aerial population of powdery mildew was split into populations, a nd these were kept separately for 31 generations on susceptible barley seed lings treated with fungicide at two concentrations, as well as on a control . Samples from these populations were tested for their resistance to fenpro pimorph and their virulence spectra. There was a large amount of environmen tal variation in the ED50 values used to measure fungicide resistance. In b oth treated populations, the average level of fungicide resistance increase d, this increase being faster and greater in the population treated with th e high dose. The diversity of pathotypes of the treated populations decreas ed, with the decline being more rapid in the population treated with the hi gh dose, where one pathotype dominated the population after 31 generations. This pathotype was apparently not the fittest in the population treated wi th the low dose. This implies that knowledge of ED50 is not sufficient to p redict pathotype evolution under different fungicide treatments. The domina nt pathotype in the high-dose treatment may not have been clonal, as there was evidence of two levels of fungicide resistance. The large environmental variation observed in estimated ED50 values for resistance towards fenprop imorph may help to explain why this resistance has evolved at a slower rate than resistance towards other fungicides.