Modelling Resistance Management Benefits of Incomplete Cross‐Resistance Within a Fungicide Mode of Action Group

ABSTRACT

Mixing fungicides with different modes of action (MoA) is a well‐established resistance management tactic which relies on the general absence of cross‐resistance between MoAs. Mixtures of fungicides with the same MoA and complete cross‐resistance are not expected to contribute to resistance management. However, in some cases cross‐resistance is only partial within a MoA, for example azole fungicides in the demethylation inhibitor (DMI) group. We developed and used a mathematical population genetic model to explore possible resistance management benefits of mixtures of fungicides with the same MoA with incomplete cross‐resistance. An important example in agriculture is the deployment of DMIs against the wheat pathogen Zymoseptoria tritici . We modelled selection for known Z. tritici CYP51 haplotypes following application of solo azoles or two, three or four‐way mixtures of epoxiconazole, prothioconazole, prochloraz and tebuconazole. The resulting rate of resistance development was measured as the rate of change in the pathogen population growth rate, : this indicates how fast the level of disease severity will increase as resistance develops. We focused on within‐MoA programmes to investigate resistance management benefits of within‐MoA active substance diversity; in practice azoles should be used in combination with other MoA wherever possible. Our results show that mixtures of fungicides with incomplete cross‐resistance within a MoA can contribute to resistance management, achieving greater stability of disease control. Increasing the number of azole active substances available to mix in various combinations reduced on average, but with diminishing returns. A generalised linear model fitted to the population genetic model results showed that increased with the average level of disease control and with the maximum variance in haplotype sensitivity to an individual azole component. Azole combinations with low or negative cross‐resistance maximised resistance management benefits, but any degree of incomplete cross‐resistance was useful.