Mycosphaerella graminicola strains with different levels of QoI resistance have similar frequencies for the G143 A mutation

Analysis of QoI sensitivity of Mycosphaerella graminicola strains isolated in Belgium was performed in PD broth in microtitre plates. The frequency distribution of EC50 values for azoxystrobin revealed at least three populations, a QoI sensitive population with EC50 values ranging from 0.01 to 0.64 mg L-1 azoxystrobin (median 0.04) , a QoI resistant one with EC50 values ranging from 1.28 to 20.48 mg L-1 azoxystrobin (median 5.12) and a highly QoI resistant one with EC50 values exceeding to 20.48 mg L-1. The relative proportion of the two QoI resistant populations varied between 2003 and 2005. The quantification of the frequency of G 143 A mutation was assessed by real-time polymerase chain reaction (PCR). Strains with EC50 values ranging from 1.28 to >31.62 mg L-1 azoxystrobin had the same mutation frequency. Moreover, a greenhouse assay on strains from those three populations showed that the difference of EC50 values between the two resistant populations was not correlated with a loss of azoxystrobin efficacy.