ACTIVITY OF DIFFERENT PLANT PROTECTION PRODUCTS AGAINST CHASMOTHECIA OF GRAPEVINE POWDERY MILDEW

Erysiphe necator, the casual agent of grapevine powdery mildew, is one of the most harmful pathogens in viticulture; its control accounts for most of the fungicides applied to grapes. At the end of summer, powdery mildew colonies on grapevine leaves form chasmothecia (fruiting bodies) that survive the winter. In the following spring, the chasmothecia discharge ascospores that cause primary infections, which can then trigger epidemics. To determine whether treatments against chasmothecia reduce the primary inoculum and improve disease control in the next season, the efficacies of eight fungicides, one biofungicide, and one insecticide (with collateral activity against powdery mildew) were compared in three strategies with applications in: (i) summer/fall against developing chasmothecia (in a greenhouse and in a vineyard); (ii) winter against mature chasmothecia; and (iii) spring to control ascosporic infections. Applications to developing chasmothecia in the greenhouse reduced the maturation of fruiting bodies by 64% (spiroxamine) to 41% (A. quisqualis) compared to the untreated control, and reduced ascospore viability by 75%. Applications to developing chasmothecia in the vineyard prevented ascocarp formation in the fall and reduced disease intensity by 50% at early berry development, particularly if primary inoculum was abundant. Winter applications were less effective while spring applications reduced disease by >90% or by 80% (sulphur). In conclusion, late-season fungicide application directed toward the developing primary inoculum reduced the need for intensive fungicide application in the following spring in high-risk vineyards. Nonetheless, early season, preventative powdery mildew control is recommended in vineyards where early, severe disease outbreak is expected.