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Delaying Selection for Fungicide Insensitivity by Mixing Fungicides at a Low and High Risk of Resistance Development: A Modeling Analysis

October 2011 , Volume 101 , Number  10
Pages  1,224 - 1,233

P. H. F. Hobbelen, N. D. Paveley, and F. van den Bosch

First and third authors: Rothamsted Research, Harpenden, Hertfordshire, AL5 2JQ, United Kingdom; and second author: ADAS UK Ltd., High Mowthorpe, Duggleby, Malton, North Yorkshire, YO17 8BP, United Kingdom.

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Accepted for publication 6 June 2011.

This study used mathematical modeling to predict whether mixtures of a high-resistance-risk and a low-risk fungicide delay selection for resistance against the high-risk fungicide. We used the winter wheat and Mycosphaerella graminicola host–pathogen system as an example, with a quinone outside inhibitor fungicide as the high-risk and chlorothalonil as the low-risk fungicide. The usefulness of the mixing strategy was measured as the “effective life”: the number of seasons that the disease-induced reduction of the integral of canopy green area index during the yield forming period could be kept <5%. We determined effective lives for strategies in which the dose rate (i) was constant for both the low-risk and high-risk fungicides, (ii) was constant for the low-risk fungicide but could increase for the high-risk fungicide, and (iii) was adjusted for both fungicides but their ratio in the mixture was fixed. The effective life was highest when applying the full label-recommended dose of the low-risk fungicide and adjusting the dose of the high-risk fungicide each season to the level required to maintain effective control. This strategy resulted in a predicted effective life of ≤12 years compared with 3 to 4 years when using the high risk fungicide alone.

© 2011 The American Phytopathological Society