The effect of fungal density on fungicide dose-response curves in barley powdery mildew (Erysiphe graminis f.sp hordei)

The effect of inoculation density on fungicide dose-response curves and the estimated ED50 value (the fungicide concentration needed to halve the infe cted leaf area relative to the infected area of leaves that had not been tr eated with a fungicide) were investigated theoretically, and predictions we re tested experimentally using powdery mildew of barley (Erysiphe graminis [syn. Blumeria graminis] f.sp. hordei). A host leaf was assumed to consist of a number of independent compartments, in each of which only a single fun gal spore could germinate successfully. The number of fungal spores landing in a compartment was assumed to be Poisson-distributed. The spores were as sumed to vary in their sensitivity toward a fungicide, and the sensitivity of the spore population was assumed to be normally distributed around a mea n sensitivity. Under these assumptions, the ED50 value was shown to be a po sitively biased estimate of the mean sensitivity in the fungal population, and the bias increased with inoculum density. Consequently, the estimated E D50 value is expected to vary between experiments and laboratories if the i noculation density varies over a considerable range. The correlation betwee n inoculum density and estimated ED50 value is expected to be strongest whe n the assayed fungal population consists of several genotypes differing in sensitivity. This expectation was tested by estimating the ED50 values for one barley powdery mildew isolate at different inoculation densities for th ree different fungicides. These ED50 values were positively correlated stro ngly with the inoculum density for fenpropimorph and more weakly for triadi menol, whereas no correlation was observed for propiconazole.