The effect of dose and mobility on the strength of selection for DMI fungicide resistance in inoculated field experiments

Plots of spring wheat cv. Baldus were inoculated at GS 13 with four Mycosph aerella graminicola isolates, two relatively susceptible and two relatively resistant to DMI fungicides. Changes in the ratio of relatively susceptibl e to resistant types following fungicide or water sprays were measured. Thr ee fungicides were compared: flutriafol, which is very mobile within leaves , fluquinconazole, which is less so, and prochloraz, which is almost immobi le. All are inhibitors of sterol demethylation. In 1996, fungicide-treated plots were sprayed once with half the recommended dose at GS 39-47. In 1997 , three doses were used: one-quarter and one-eighth of the recommended dose and a dual application of two one-eighth recommended doses, a week apart. Isolates were classified using a discriminating dose assay and the ratio of relatively susceptible to relatively resistant isolates in each field plot before and after fungicide application calculated. In both years, the numb ers of relatively susceptible and relatively resistant isolates were equal just before fungicide application. All fungicides caused significant select ion towards resistance, but the strength of selection varied with fungicide , dose and position in the crop canopy. Fluquinconazole selected most stron gly and gave the best control of disease. Interactions between fungicide an d dose were not significant. Selection was equally strong all along leaves sprayed with prochloraz, but increased smoothly from base to tip of leaves sprayed with fluquinconazole or flutriafol. Averaged over fungicides, reduc ing the dose of a single fungicide application from one-quarter to one-eigh th slightly reduced selection towards resistance on both leaf layers. The d ual one-eighth dose caused twice the change of the single one-eighth dose o n the flag leaf, but was similar to a single spray on leaf 2.