Understanding how the powdery mildew fungus behaves in various environmental conditions is the first step to effectively controlling the disease. Dr. Doug Gubler, professor of plant pathology at the University of California—Davis, has studied powdery mildew since the early 1980s. Over the last three decades, Dr. Gubler and his team have organized a wide range of data on the disease into a powdery mildew model designed to help vineyard owners get better control.
“Knowing what the pathogen is doing and how it’s reacting to the environment allows growers to make good management decisions,” says Gubler. “The model we’ve developed helps growers visualize how the fungus is behaving at any given time.”
To date, roughly a quarter million acres in California use the model to control powdery mildew. According to Gubler, most of the growers using the model have saved two to three fungicide applications a year because the model helps growers time applications more accurately for best results.
Recently, Gubler has discovered that many growers are using the model to decide what product to use at a specific time in the powdery mildew life cycle. Under high disease pressure, growers will use DMIs, strobilurins or phenoxyphen. Under low-medium disease pressure, growers will use materials such as sulfur, oils and biological and soft chemistry products.
“Viticure has always looked pretty good in our trials,” says Gubler. “Over the years that we’ve worked with Viticure, we’ve had good success with it.”
As a systemic DMI fungicide with different chemistry than the triazole chemistry found in most other DMIs, Viticure has been more active against powdery mildew than other DMI products in trials. In addition to controlling powdery mildew, Viticure is also labeled for suppression of black rot and Botrytis bunch rot.
In general, Gubler recommends kicking off a powdery mildew spray schedule with two to three applications of a 5 lb/A micronized flowable sulfur or a 2% JMS stylet oil at bud break. As broad-spectrum, multiple-site-of-action materials, sulfur or oil applications resulted in 95% disease control at disease onset in Gubler’s trials.
“We’ve been recommending this treatment since 1986 and California growers have done a good job incorporating these treatments into their powdery mildew control programs,” says Gubler.
After initial sulfur or oil applications, Gubler stresses the importance of switching up the types of chemistries used in the vineyard to delay or manage fungicide resistance. “Never use more than one application of a fungicide without switching to a different class of chemistry,” says Gubler. “There are enough good products out there right now that switching products doesn’t decrease the amount of control.”
In addition to resistance management, Gubler also reminds growers about the importance of getting good spray coverage in the vineyard—especially when using a nonvolatile or nonsystemic product. In those instances, getting good control is greatly dependent on getting close to 100% coverage. Systemic or volatile products have chemical properties that allow them to move in the local tissue after application, resulting in more complete coverage and control even in incomplete spray coverage situations.