Sensitivity monitoring of powdery mildew pathogens to cyflufenamid and the evaluation of resistance risk

Sensitivity monitoring studies for a novel fungicide, cyflufenamid, (Z)-N-[alpha-(cyclopropylmethoxyimino)-2,3-difluoro-6-(trifiuoromethyl)benzyl]-2-phenylacetamide, were performed on various pathogens causing powdery mildew. The mean EC50 value for Blumeria graminis f. sp. tritici was 0.029ppm by pot assay and that for Sphaerotheca cucurbitae was 0.0019ppm by leaf disk assay in Japan. The mean ECsub(50) values for B. graminis f. sp. tritici were between 0.0022ppm and 0.0111 ppm and those for B. graminis f. sp. hordei were between 0.0249 ppm and 0.0457 ppm in 2000 to 2004 by leaf segment assay in Europe. The ECsub(50) values of each strain in these pathogens were distributed within a narrow range, and no classes less sensitive to cyflufenamid were found. No significant change in the sensitivity of B. graminis f. sp. tritici to cyflufenamid was observed throughout selection pressure tests in the greenhouse and field. Cross-resistance between cyflufenamid and other commercial fungicides was not observed in S. cacurbitae.