Department of Plant Pathology

Powdery mildew is an economically-important pathogen of grapes worldwide. This report details the findings of our annual powdery mildew fungicide trials on grapevine cultivar Chardonnay (Vitis vinifera). The trials were conducted at Herzog Ranch, near Courtland, California in 2009. Treatments were placed in five adjacent trials in the vineyard. Spraying commenced in mid April, shortly after a heavy rainfall event that likely promoted the release of powdery mildew (Uncinula necator) ascospores from overwintering chasmothecia. Spraying was completed in mid July and treatments were evaluated for disease incidence and severity on 21 July 2009, at about the beginning of veraison.

Trial I consisted of IR-4-funded biofungicide research that focused on two products: caprylic acid (a novel control agent in current development) and Actinovate (a registered product containing the bacterium Streptomyces lydicus WYEC106). Trials II-V included various fungicide products (either currently registered or in various stages of product development) including strobilurins, demethylase inhibitors, oils, and other materials. Spray frequencies varied from weekly applications to 21 day intervals.

Temperatures were mild during much of the 2009 growing season, providing optimal conditions for the asexual reproduction and dispersal of powdery mildew. Risk index data was high from about mid-May through the rest of the evaluation period and the pathogen quickly colonized leaves. Disease pressure was higher than in similar trials conducted in 2007 or 2008. By mid-June, heavy to severe mildew was evident on untreated clusters.

Treatment performances varied widely. Biological fungicides generally reduced disease severity, but were more effective when used in a combination program with synthetic materials. As a stand-alone product, caprylic acid was most effective when applied weekly at a concentration of 0.3% (v/v). At a concentration of 0.5% it consistently showed phytotoxic effects on berries. A few organic materials performed well, lowering disease severity to <10%: paraffinic oil + Foliar Supreme (sulfur + nutrient cocktail) and two types of fertilizers alternated with trifloxystrobin. Among synthetic products, difenoconzaole + cyprodinil, flutriafol, fluopyram, fluopyram + tebuconazole, quinoxyfen + alcohol ethoxylate (an adjuvant), various GWN4617 treatments, picoxystrobin, penthiopyrad, boscalid + pyraclostrobin alternated with triflumizole, and metrafenone gave good to excellent control of disease severity on fruit. These trials easily separate highly effective materials from those which give only moderate control of powdery mildew, however long-term assessment of treatment efficacy is best evaluated with experiments conducted at multiple locations, across several growing seasons, and on multiple grape cultivars.