IV. Variation in fungicide tolerance: Variation in the response of plant pathogens to fungicides

Abstract

ABSTRACTThe response of a fungal population to a fungicide is normally highly variable. Individual lethal doses usually form a normal distribution when plotted against log dose. The shape of this distribution varies with chemical and organism. Spore germination, haustorial formation, etc. vary independently. Variation in disease control is the product of the variation at each stage. Variation is both genotypic and phenotypic.The successful use of a fungicide should, theoretically, result in a shift in the dose‐response of the survivors. The shift ought to be greatest where the original spread of the lethal dose was widest.The effect of ethirimol on barley powdery mildew supports these theoretical conclusions. Good control of mildew following the use of ethirimol resulted in residual populations which were less sensitive to the fungicide than those in nearby untreated fields. Within treated fields the most sensitive members of the population were absent and the overall variation decreased. The progressive shift in sensitivity which might have been expected did not occur. Successive sampling of the same fields revealed rapid changes in sensitivity, but in either direction. The proportion of less sensitive individuals declined toward the end of the season and much of the original variability returned.