Rapid decline in the sensitivity to pyraclostrobin fungicide in Mycosphaerella pinodes

Mycosphaerella blight, caused by Mycosphaerella pinodes (Berk. & Blox.) Vestergr., is a destructive disease of field pea on the Canadian prairies that occurs in almost every field in most years. Genetic resistance is not available, so severity is managed primarily with application of foliar fungicides. Strobilurin fungicides have been used for management of mycosphaerella blight since their introduction to western Canada in 2003. Sensitivity of baseline isolates to pyraclostrobin (a widely used strobilurin) has been assessed previously, with EC50 values for mycelial growth ranging from 0.03 to 0.3 mg L-1 and a discriminatory dose of 5 mg L-1. Of the 324 isolates collected in Canada and the northern USA in 2010 and 2011, 8% (all from Alberta or Saskatchewan) were insensitive to pyraclostrobin using that discriminatory concentration. In isolates of M. pinodes collected in 2013 2016 from sites across the field pea production region of Saskatchewan, 72% (46 of 64) were insensitive. This observation, taken together with concurrent studies that demonstrated a high degree of cross-sensitivity between pyraclostrobin and azoxystrobin, indicated that effective management of mycosphaerella blight with solo applications of these two widely used fungicides is likely no longer possible in Saskatchewan.