Effect of different plant protection products on the sexual stage of grapevine powdery mildew

In many grape-growing areas, the primary inoculum of Erysiphe necator, the casual agent of grapevine powdery mildew, consists of ascospores produced in chasmothecia; these ascocarps form in late summer on the affected host tissue and are dispersed by rain splashes to the bark of vines, where they overwinter. Ascospores are repeatedly released from chasmothecia in spring and cause primary infections on basal leaves of the developing shoots. These infections trigger powdery mildew epidemics, which are then driven by the asexual infection cycles. Despite the usual approach to powdery mildew control that consists of repeated applications of fungicides from budburst to berries pea-size, sanitation, i.e. the process that reduces or eliminates the initial inoculum from which the disease epidemic starts, would be highly suggested against powdery mildew. Three sanitation strategies: (i) summer/fall treatments against the developing chasmothecia; (ii) winter treatments against mature chasmothecia; and (iii) spring treatments to control ascosporic infections were compared. Eight fungicides, one biofungicide, and one insecticide (with collateral activity against powdery mildew) were compared for the ability of reducing production of chasmothecia (in greenhouse experiments) and the efficacy in suppressing powdery mildew in early season (in vineyards). Chemicals were also applied directly to chasmothecia at three different times: leaf fall, dormant buds, and bud break in order to evaluate their efficacy in reducing ascospore viability. Experiments in the greenhouse demonstrated the efficacy of the products in reducing the production of fruiting bodies by 64% (spiroxamine) to 41% (the hyperparasite Ampelomyces quisqualis) compared to the untreated control, and of chemicals in reducing ascospore viability by 75% on average. Summer/fall applications in the vineyard reduced disease intensity on average by 50% at early berry development in the following spring, particularly in the vineyards where the primary inoculum was abundant. Winter applications against mature chasmothecia onto the vine bark were less effective while the control of ascosporic infections enabled a reduction of disease by >90% or by 80% (sulphur). Viability of ascospores within chasmothecia was reduced by some tested products applied at complete leaf fall and dormant buds, but not at bud break. The most effective fungicides were metrafenone (with a maximum reduction of ascospores viability of 55%), mepltyldinocap (45%), and spiroxamine (29%). In conclusion, sanitation treatments, such as late-season application of fungicides once after harvest targeted to developing chasmothecia or just dispersed ones, as well as two applications of A. quisqualis (before and after harvest), reduced the primary inoculum, delayed the epidemic development of powdery mildew, and therefore reduced the need for intensive, early-season fungicide applications in the following year. Nonetheless, in vineyards where early, severe disease outbreak is expected, early-season, control of ascosporic infections is recommended.