Abstract
A continuous-time analytical model of selection for fungicide resistance among distinct clones of a pathogen is used. Clones are assumed to be either resistant or sensitive to two different fungicides. Parameters are introduced that quantify the effect of using a mixture of pesticides on the rate at which dual resistance to both components is selected. The rate at which phenotypes resistant to both fungicides will be selected depends on how the activities of the two fungicides combine in a mixture. If the activities have additive effects on fungal population growth rate, mixtures will select for dual resistance approximately twice as fast as will sequential use of the two components. Such additive effects correspond to multiplicative effects on survival or fecundity of groups of organisms exposed to pesticide (a common definition of ‘independent action’). The increased rate of selection for joint resistance when using a mixture can be minimized by using components that are not synergistic, or by reducing doses so that effectiveness of control is no greater than with only one component. However, even under these circumstances the rate of selection of dual resistance by mixtures will be no lower than that for sequential use; any advantage in using mixtures must depend on some other mechanism, such as extreme rarity of the dual-resistant phenotype, or breakdown of linkage disequilibrium during sexual recombination.
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