Characterizing resistance risk of Erysiphe graminis f.sp. tritici to strobilurins

Abstract Strobilurins have been commercially used since 1996 for the control of Erysiphe graminis on cereals in Europe. Sensitivity monitoring of airborne and field samples of the pathogen detected only sensitive populations from 1996 to 1997, but in 1998 resistant wheat isolates (resistance factors of EC50s>500) were found in airborne samples from three different regions in North Germany. These isolates already occurred at up to 90% frequencies suggesting that resistant individuals may have high levels of fitness, and that they could have previously escaped detection with conventional methods which measure EC50s of bulk samples or which sample limited numbers of single-colony isolates per region. An alternative method measuring frequencies of resistant colonies in bulk samples at a discriminatory dose of 3 ppm trifloxystrobin relative to that at 0 ppm, showed that resistance to stobilurins can occur at frequencies which do not affect the mean EC50s of these samples. In 1999, high levels of resistance were still concentrated in parts of Germany but became detectable in other regions of the country, and in other countries including France, Belgium, UK and Denmark. Losses in performance were detected in areas with high frequencies of resistance. In a competition experiment between resistant and sensitive isolates over three generations, resistance was stable in the absence of selection; the resistant isolate appeared equal in fitness to the sensitive isolate on untreated plant material; and was rapidly selected for on strobilurin-treated material. Selection studies on bulk isolates confirmed this finding but also indicated the existence of a range of fitness levels. There were high levels of cross-resistance between strobilurins but not with demethylation inhibitors, morpholines, anilinopyrimidines and quinoxyfen. This assessment of resistance risk suggests that use recommendations of strobilurins for wheat powdery mildew control must include elements for strict resistance management.