Inhibition of the pre- and postinfection processes of Plasmopara viticola on Vitis vinifera leaves by one protectant and four systemic fungicides

One protectant (mancozeb) and four systemic (mefenoxam, fosetyl-Al, azoxystrobin and dimethomorph) fungicides provided varying levels of activity against Plasmopara viticola at different stages during the pre-and postinfection processes of the pathogen on grapevine (Vitis vinifera) leaves. Azoxystrobin and fosetyl-Al produced the strongest inhibition, with inhibitory effects (compared with the control) evident at all stages of the asexual phase of the life cycle assessed. Mancozeb had the strongest protectant activity and mefenoxam was the most inhibitive as a curative [applied at 4 days after inoculation (dai)] and as an antisporulant (applied at 4 dai and 7 dai). Zoospore motility was the most sensitive of the stages to fungicides in in vitro tests, while the production of sporangia following treatments at 4 dai was the most sensitive during the in vivo phases. The performance of fosetyl-Al and azoxystrobin on the parasitic and reproductive phases of the pathogen was outstanding, especially in the treatment involving application at 7 dai. Fosetyl-Al and azoxystrobin reduced viability of sporangia, with < 20% of sporangia germinating at all rates for these two fungicides. Both fungicides significantly decreased zoospore release at all rates tested. Fosetyl-Al provided the most inhibitive effect on sporangial germination, with < 1% of sporangia germinating at 2.8 g a. i./L. When azoxystrobin was applied at 7 dai, less than 40% of the leaf surface area showed sporulation. For both fosetyl-Al and azoxystrobin, the total number of sporangia produced was significantly reduced by >93% for the two higher rates of these fungicides. We believe our study to be the first to identify that the specific germination and infection phases of P. viticola, such as encysted zoospores, zoospore germination and germ tube growth, are differentially responsive to the major classes of fungicides available for use against this pathogen. As a relatively inexpensive and non-toxic chemical, fosetyl-Al, a phosphonate, has significant potential to become an effective management tool against grapevine downy mildew.