Haustorial development test to characterize metalaxyl resistance and genetic variability in Plasmopara viticola1

A new method is described for assessing haustorium formation in Plasmopara viticola on metalaxyl-treated grape leaves. Using this method, three types of sensitivity of P. viticola to metalaxyl could be distinguished and the precise ratio of sensitive, ‘reduced sensitive’ and resistant zoospores of selected isolates could be determined.