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 2010. Treatments were placed in four adjacent trials in the vineyard. Spraying commenced in mid April, amidst significant rainfall events that likely promoted the release of powdery mildew (Erysiphe necator) ascospores from overwintering chasmothecia. Powdery mildew pressure began slowly with cool temperatures early on, but quickly built to very high disease pressure levels as temperatures warmed. Spraying was completed on July 23 and treatments were evaluated for disease incidence and severity.

Trial I consisted of soft chemistry products, including biologicals, sulfurs, nutrient applications, oils, and other materials. Spray frequencies varied from weekly applications to 21 day intervals. Many applications were based on the Gubler-Thomas Risk Index, with application intervals based on the index.

Temperatures were mild during much of the 2010 growing season, providing optimal conditions for the asexual reproduction and dispersal of powdery mildew. Overall disease pressure was higher than in similar trials conducted in 2007, 2008, and 2009. By late June, heavy to severe mildew coverage was evident on untreated clusters. Progression of the disease during spring was faster towards the eastern end of the research area. However, by the time of disease evaluation, disease severity in untreated plots in all four trials reached 95-100%.