Dose splitting increases selection for both target-site and non-target-site fungicide resistance – a modelling analysis

Fungicide resistance management principles recommend that farmers avoid splitting the total dose applied of a fungicidal mode of action (MoA) across multiple applications per season (‘dose splitting’). However, dose splitting may sometimes be needed to make another proven resistance management tactic - application in mixture with a different MoA - practically achievable, especially in cases where there are limited MoAs available for disease control. Variable effects of dose splitting on selection for resistance have been observed in field experiments, and its effect on selection for partial resistance in fungal pathogens is not well studied. An improved understanding of whether the effect of dose splitting depends on fungicide properties and type of fungicide resistance is required. We developed a compartmental epidemiological model of septoria leaf blotch (STB) (Zymoseptoria tritici) to investigate the effect of dose splitting on selection for both complete and partial target-site and non-target-site resistance. To measure solely the effects of dose splitting, we restricted the analysis to solo fungicide application (solo use is not recommended in practice). Our results show variable effects of dose splitting: in general, it increased selection for both target-site and non-target-site resistance. Within the range of dose response parameters expected for commercial fungicides, dose splitting increased selection most for partial resistance mechanisms that result in a reduction in fungicide efficacy at low fungicide concentrations but not at high concentrations. We predict that dose splitting of a succinate dehydrogenase inhibitor (SDHI) fungicide (solo) will increase selection for target-site and non-target-site resistance by between 20-35%.