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First detection of resistance to QoI fungicides in Mycosphaerella graminicola on winter wheat in Belgium.

A total of 740 Mycosphaerella graminicola strains were isolated between 2000 and 2002 from winter wheat F1 or F2 leaves showing Septoria leaf blotch lesions (SLB) collected mainly at the soft dough stage in fungicide trials, analysing at 12 locations in Belgium the possibilities and risks associated with the use of epoxiconazole and azoxystrobin at various doses, mixtures and application dates. Fungicide sensitivity tests were performed in microtitre plates on potato dextrose broth amended with various concentrations of azoxystrobin. A wide range of sensitivity to azoxystrobin was observed, with EC50 values ranging for 735 strains between 0.002 to 0.7 microg/ml, the highest frequency gradually shifting from EC50 classes 0.01 and 0.02 microg/ml azoxystrobin in 2000 to EC50 classes 0.02 and 0.04 microg/ml in 2002. No clear selection effect of particular fungicide use strategies was observed. Among the 382 strains isolated in 2002, five originating from 2 locations, showed azoxystrobin EC50 values >1 microg/ml. On medium amended with 100 microg/ml salicylhydroxamic acid (SHAM), 58% of the 2002 strains were strongly inhibited, which affected adequate azoxystrobin ED50 determination. This suggests widespread occurrence of M. graminicola strains relying in vitro on the alternative respiration pathway. In the presence of SHAM, strains 339 and 880 showed azoxystrobin EC50 values of 3 and >30 microg/ml, respectively. This high level of resistance to a QoI fungicide was confirmed by analysing mycelium growth inhibition on PDA. Cross-resistance to trifloxystrobin and kresoxim-methyl was demonstrated. Greenhouse assays on wheat plants revealed that control of QoI resistant strains by azoxystrobin is decreased, compared to control of sensitive ones. This highlights the risk of resistance to QoI fungicides also in M. graminicola populations, although up to now no decrease in field performance was noticed. It is recommended to delay build up of QoI resistance by an integrated approach, combining optimised fungicide use with the choice of SLB resistant cultivars and the application of farming practices promoting stubbles break down and so the reduction of the teleomorph stage.