A method for measuring the level of sensitivity of Plasmopara viticola populations to cymoxanil

A leaf-disk bioassay was developed for monitoring the level of sensitivity of Plasmopara viticola populations to the fungicide cymoxanil. Grapevine leaf disks were treated with various concentrations of fungicide and inoculated one day later with a suspension of P. viticola sporangia. Fungal inhibition was strongest when the leaf disks had their upper side in contact with the fungicide solution and were sprayed on the other side with the same solution. The dose response was influenced by the level of disease on the untreated leaf disks and the date of evaluation which required standardization. Test-to-test variation was small but differences existed between laboratories where the assay was performed. There was good agreement between the results obtained in the leaf-disk assay and those of a whole-plant assay.