Fungicide sensitivity in Swedish isolates of Phaeosphaeria nodorum

This is the first report of variability in sensitivity of Phaeosphaeria nodorum to the fungicide azoxystrobin, and also reports on sensitivity to propiconazole, prothioconazole and cyprodinil. An in vitro sensitivity test of 42 isolates from five Swedish winter wheat fields, collected in 2003-2005, was performed on malt extract agar amended with six concentrations of each fungicide. Four isolates collected during the early 1990s, before azoxystrobin had been commercially used in agriculture, were used as references. Fragments of DNA from 231 isolates, including the reference isolates, were sequenced for the genes of cytochrome b and CYPS1 in search for amino acid substitutions known to cause loss of sensitivity to strobilurins and triazoles, respectively. The majority of the P. nodorum isolates possessed the amino acid substitution G143A, associated with loss of sensitivity in fungi to strobilurins, except in one field where only half of the isolates had the substitution. The EC50 values varied between 0.66 mg L-1 to estimations far above 1000 mg L-1, with an estimated median value of 366 mg L-1. The EC, values of the reference isolates ranged from 0.02 to 80.72 mg L-1. The P. nodorum population is still sensitive to propiconazole, prothioconazole and cyprodinil, even though some isolates varied in sensitivity to triazoles. Part of the CYP51 gene, a possible target for triazole sensitivity, was sequenced but no nonsynonymous substitutions were found.