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The effect of the dose rate of a fungicide on the emergence of resistance.
P. HOBBELEN (1), N. D. Paveley (2), F. van den Bosch (1). (1) Rothamsted Research, Harpenden, United Kingdom; (2) ADAS UK Ltd., High Mowthorpe, Duggleby, Malton, United Kingdom

The evolution of fungicide resistance can be divided into an emergence phase and a selection phase. In the emergence phase, the resistant strain arises through mutation and its spread in the pathogen population depends on stochastic processes. Fungicide applications will decrease the size of the pathogen population and therefore the rate at which resistant mutants arise, but mutants will experience less competition for healthy host tissue due to the smaller pathogen population. To the best of our knowledge, all published experimental and modelling work on fungicide resistance concerns the selection phase. This study aims to determine the effect of the dose rate of a high-risk fungicide on the emergence time of a resistant pathogen strain. We use <i>Mycosphaerella graminicola</i> on winter wheat as model for a fungal pathogen on a cereal crop. The simulation model describes the seasonal dynamics of the canopy and the seasonal dynamics of the sensitive and resistant pathogen population. We consider the resistant strain to have emerged, when its survival during 100 consecutive growing seasons in the absence of new mutations is 95% or higher. The emergence time is then defined as the number of consecutive growing seasons since the start of a treatment until the resistant strain emerges. The results suggest that the mean emergence time of a resistant strain initially decreases and then stabilises with increasing dose-rate of a high-risk fungicide.<p><p>Keywords: Fungus, Cereals-Grains, Wheat

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