Distribution of Baseline Sensitivities to Azoxystrobin Among Isolates of Plasmopara viticola

An excised leaf disc assay was developed for determining the sensitivity of isolates of Plasmopara viticola (causal agent of grapevine downy mildew) to the strobilurin fungicide azoxystrobin. Five repeated assays with the same five single-sporangiophore isolates showed that the technique yielded reproducible results; that is, coefficients of variation ranged from 4.0 to 20% (mean 12%) for effective doses for 50% control (ED₅₀ values) based on disease incidence and from 4.4 to 14% (mean 8.1%) for ED₅₀ values based on disease severity. Then, the assay was applied to 81 single-sporangiophore isolates of P. viticola collected from 10 geographically distinct vineyards in western New York, providing a baseline distribution of sensitivities within this population. For disease incidence, individual ED₅₀ values ranged from 0.05 to 0.94 μg/ml (mean 0.40 μg/ml), whereas for disease severity they ranged from 0.04 to 0.78 μg/ml (mean 0.24 μg/ml). When 61 of these isolates were similarly tested at a single discriminatory dose of 0.50 μg/ml, azoxystrobin provided 61% control of disease incidence versus 41 and 1.1% control for trifloxystrobin and kresoximmethyl, respectively; for disease severity (colony diameter), azoxystrobin provided 80% control versus 57 and 1.1% control for trifloxystrobin and kresoximmethyl, respectively. These results provide information that can be utilized in future monitoring of P. viticola resistance to azoxystrobin and indicate differences in the intrinsic activities of the three strobilurin fungicides against this pathogen.