Options to Control Powdery Mildew Evaluated

(Wines&Vines) - Grapegrowers, vineyard managers, consultants, pest control advisors (PCAs) and ag chemical suppliers had the opportunity to inspect the results of 90 different fungicide spray treatments to control powdery mildew Aug. 7 as part of a University of California Cooperative Extension (UCCE) vineyard field trial conducted by the University of California, Davis, Department of Plant Pathology.

 UC Davis plant pathologist Dr. Douglas Gubler, an expert on grape fungal pathogens and diseases and a frequent lecturer at industry meetings, oversees the annual research trial that has been conducted in the same Chardonnay vineyard for 28 years in cooperation with grower John Baranek of Herzog Ranch in the Clarksburg American Viticultural Area.

As Gubler explained, this vineyard near the Sacramento-San Joaquin River Delta is an excellent test site, with powdery mildew (Erysiphe necator) disease pressure each season. “This site has high mildew pressure because the weather is generally cool, and it’s next to a body of water (Snodgrass Slough) that provides cooling from the wind blowing across the water and into the vine canopies,” Gubler said. The 2013 season had moderate pressure with powdery mildew (PM) showing up in early to mid-May, a little later than in 2012.

 “When the disease shows up by April 15, it’s usually a severe pressure year,”

Gubler said. The 90 treatments tested in 2013 included synthetic, biological and organic fungicides, in different combinations, and with varying application rates and timing. Applications were made beginning in early April, and final treatments were applied the week of July 15. Leaf removal was done in the cluster zone in late May and early June to open the canopy to air movement and better expose clusters for spray coverage. Leaf removal assists with control, because PM spores are thin-walled and negatively affected by UV light.