The new powdery mildew fungicide quinoxyfen belongs to the novel quinoline class of chemistry. Although its biochemical mode of action is unknown, quinoxyfen does not act in the same way as other cereal fungicides. It is a systemic protectant which inhibits the early stages of mildew infection on a wide range of crops, and provides season-long protection from a single earlyseason spray applied around GS 31.
The base-line sensitivity proรผle of quinoxyfen was deรผned for barley powdery mildew (Erysiphe graminis f.sp. hordei) from over 340 รผeld isolates collected from diรพerent parts of the UK from 1991 onwards. Sensitivities ranged from \0โข0001 ] 0โข16 mg litre~1 with a mean of 0โข003 mg litre~1. Current work is extending the base-line sensitivity studies to wheat powdery mildew (E. graminis f.sp. tritici), and includes isolates from European trials, but so far this new data set has shown no diรพerences from barley powdery mildew. Quinoxyfen-resistant mutants were generated in the laboratory, and some similar resistant strains were obtained from treated รผeld crops. These laboratory and รผeld strains were always defective, in some way, for sporulation and, curiously, all required the presence of quinoxyfen for survival in culture. Attempts to generate resistant mutants that sporulated normally were unsuccessful.
These studies suggested that the resistance risk for quinoxyfen is low. The recommended anti-resistance strategy accompanying introduction of quinoxyfen avoids seed treatments and late-season applications. Instead, a single early (GS 31) treatment using either pre-formulated mixtures or alternating with a fungicide with diรพerent mode of action is recommended. This strategy will be supported by continued monitoring of wheat and barley powdery mildew.