Baseline Sensitivity of Australian Phoma ligulicola Isolates from Pyrethrum to Azoxystrobin and Difenoconazole

Ray blight caused by Phoma ligulicola is an important disease of pyrethrum in Australia, and successful management relies upon the fungicides, azoxystrobin and difenoconazole. Azoxystrobin and difenoconazole were introduced into pyrethrum production in 2001. The sensitivity of P. ligulicola isolates collected in 2003 to azoxystrobin (n = 56) and difenoconazole (n = 61) was tested. Testing for sensitivity to azoxystrobin and difenoconazole used a conidial germination and mycelial growth assay respectively. For each fungicide, the effective dose required to reduce mycelial growth or conidial germination by 50% (EC₅₀) was determined by probit analysis. The EC₅₀ values ranged from 0.007 to 0.193 μg/ml for azoxystrobin and 0.04 to 13.8 μg/ml for difenoconazole. No evidence was found for cross‐resistance between azoxystrobin and difenoconazole in this baseline population. This information serves as important baseline data for tracking future changes in sensitivities of P. ligulicola to these fungicides.