Delaying Infection Through Phytosanitary Soybean-Free Periods Contributes to Fungicide Resistance Management in Phakopsora pachyrhizi: A Modelling Analysis

Fungicide resistance threatens control of Asian soybean rust (Phakopsora pachyrhizi) in Brazilian soybean crops; deployment of sound resistance management tactics is crucial to prolong the effective life of new fungicides. A key integrated pest management (IPM) strategy in Brazil is the delay of P. pachyrhizi inoculum influx through soybean-free periods, mandated through restrictions on sowing dates. We developed an epidemiological model of fungicide resistance evolution in P. pachyrhizi to explore the impact of delayed inoculum influx on selection for resistance to a succinate dehydrogenase inhibitor (SDHI) fungicide and to compare the relative benefits of alternative mixture partners. We fitted model parameters describing the efficacy of fungicides using disease severity data from field trials and validated our approach using a separate dataset from trials of solo and mixture products. Our results suggest that mixture with a multisite-acting fungicide such as mancozeb or chlorothalonil could slow the rate of selection for resistant strains and more than double the number of years for which effective disease control can be maintained. We show how cultural control measures to delay inoculum arrival contribute to resistance management. Delaying infection timing relative to crop emergence reduced selection for fungicide-resistant strains and increased fungicide effective life through a reduction in the length of time of pathogen exposure to the fungicide and decreased disease pressure. The fungicide resistance management benefits of IPM strategies will be highest in cases where these keep to a minimum the level of fungicide treatment required to maintain effective disease control.