Inheritance of resistance to carboxylic acid amide (CAA) fungicides in Plasmopara viticola

Mandipropamid is a new mandelic acid amide fungicide expressing high activity against foliar infecting oomycetes, including the grapevine downy mildew, Plasmopara viticola. Because cross-resistance with the valinamide fungicides iprovalicarb and benthiavalicarb and the cinnamic acid amide fungicides dimethomorph and flumorph was postulated, all five compounds are classified as carboxylic acid amide (CAA) fungicides. To support this classification, cross-resistance among these compounds with field isolates and the segregation of resistance in F-1 and F-2 progeny of P. viticola were evaluated. A bimodal distribution of sensitivity in field isolates and cross-resistance among all CAAs for the vast majority of isolates were detected. Crosses between sensitive (s) and CAA-resistant (r) isolates of opposite mating types, P1 and P2, yielded abundant oospores. All F-1-progeny isolates were sensitive to CAAs (s: r segregation 1: 0), whereas in F-2 progeny segregation of about 9: 1 (s: r) was observed suggesting that resistance to CAA fungicides is controlled by two recessive nuclear genes. Mating type segregated in a ratio P1: P2 of c. 2: 1 in F p(1) and 1: 1 in F-2 progeny. In the same crosses, resistance to the phenylamide fungicide mefenoxam segregated in a ratio of c. 1: 3: 2 (sensitive: intermediate: resistant), reflecting the monogenic, semidominant nature of resistance. The risk of resistance in P. viticola was classified as high for phenylamide and moderate for CAA fungicides. This is the first report on the inheritance of phenotypic traits in P. viticola.