Sensitivity of Mycosphaerella pinodes to Pyraclostrobin Fungicide.

Mycosphaerella blight, caused by Mycosphaerella pinodes, is a destructive disease of field pea that is managed using foliar fungicides. Strobilurin fungicides have been used in western Canada for disease management since 2003. To assess the baseline sensitivities of M. pinodes isolates to the strobilurin fungicide pyraclostrobin, the effective concentration to reduce mycelial growth by 50% (EC) was determined for 70 isolates collected prior to 2003 from Alberta, Saskatchewan, North Dakota, and Washington State. Each of these isolates was sensitive to pyraclostrobin, with EC values ranging from 0.03 to 0.29 mg liter. The pyraclostrobin concentrations required to reduce conidia germination by 50% was lower, ranging from 0.008 to 0.041 mg liter. In all, 324 isolates collected in 2010 and 2011 were tested for high levels of insensitivity by examining mycelial growth using a discriminatory dose of 5 mg liter. Nineteen isolates were highly insensitive to pyraclostrobin, with EC values of 80 to 216 mg liter. Conidia of these isolates germinated when exposed to a discriminatory dose of 0.1 mg liter. Insensitive isolates infected field pea plants treated with pyraclostrobin but sensitive isolates did not. The identification of insensitive isolates indicates that insensitivity may be emerging in the pathogen population.