Abstract/Summary

Evolution of resistance to fungicides threatens control of plant pathogens. Resistance management tactics such as application of fungicides with different modes of action (MoA) in mixture or alternation can slow selection for resistance, but it is unclear which strategies work against resistance evolving concurrently to two or more MoA. In this thesis, mathematical epidemiological models of polycyclic fungal foliar diseases were used to investigate potential resistance management tactics. Use of integrated pest management (IPM) can reduce the fungicide intensity required for disease control, but resistance management benefits of cultural control methods are rarely quantified. I developed a model to estimate the resistance management benefits of a phytosanitary cultural control measure, the ‘soybean-free period’, used in Brazil to delay infection of soybean crops by Phakopsora pachyrhizi (Asian soybean rust). I considered interactions with use of fungicide mixtures and varying dose, application rates and timings. Mixing two MoAs may require splitting the total dose of each MoA across more applications, increasing exposure time. Using a model of Zymoseptoria tritici (septoria tritici blotch), I showed that dose splitting of a solo MoA increases selection, but the effect varies with fungicide properties and the type and magnitude of resistance. I then compared alternation with ‘splitting and mixing’ as tactics against concurrent evolution of resistance to two MoA, modelling a sexually reproducing Z. tritici population. The best strategy varied with fungicide and resistant strain properties. Incomplete cross-resistance between active substances with the same MoA could be utilised for resistance management. I used a novel modelling approach to investigate the resistance management benefits of within-MoA mixtures with incomplete cross-resistance. Resistance management benefits were greatest when the level of cross-resistance between active substances was low or negative; the rate of selection was also dependent on the variation in the fitness of pathogen strains against individual mixture components.