Independent action of fungicides and its consequences for strategies to retard the evolution of fungicide resistance

Abstract

Several authors have discussed the evolution of fungicide resistance in a pathogenic fungus which contains a distinct resistant genotype to one fungicide (A), but not to another (B). Each author predicts when alternating use of the two fungicides will retard the evolution of resistance to A more than use of a mixture, but their conclusions differ. This is partly because they use in different ways the concept that the fungicides act independently in the mixture. A parameter ψ is introduced that quantifies the extent to which component B modifies the evolution of resistance to A. If admixture of B has no effect, ψ = 1 and A and B are independent with respect to the development of resistance. ψ is defined as the ratio of the change in frequency of the resistant genotype when a mixture of A and B is applied for a given time, to the change when A is applied alone. It is useful for interpreting the results of experiments and the predictions of mathematical models. Small values of ψ indicate mixtures that greatly retard the evolution of resistance. These are more likely to occur if fungicides, with the other properties assumed, act at the same stage of the life cycle of the fungus. Values <0·5 indicate that mixtures will be more favourable than alternation in the absence of cross resistance.