Laboratory studies on benzimidazole resistance in Septoria tritici1

A technique is described for testing isolates of Septoria tritici from winter wheat for resistance to benzimidazole fungicides. Secondary spores from 23 isolates were tested on Czapek Dox V‐8 agar amended with benomyl at 1–10 μg ml. Twenty‐one isolates were recovered from eight crops in England in 1984 and two (PBI isolates) were obtained in 1973. Thirteen isolates, including both PBI isolates, were sensitive to benomyl at 1 μg ml−1 and nine were resistant at 10 μg ml−1. The remaining isolate had a low proportion (1:3.7 x 104) of resistant spores. The minimum inhibitory concentration for sensitive isolates was 0.2–0.4 μg ml 1 benomyl and for resistant isolates was more than 1000 μg ml 1. Benomyl‐resistant isolates were cross‐resistant to carbendazim, thiabendazole and thiophanate‐methyl, but not to 12 other fungicides with different modes of action. The implications of these findings are discussed.