The potential for powdery mildew infection is high this year because of high pressure during the last growing season. Depending on how stringent growers have been with their control programs, there could be high spore loads just waiting for the right temperature, according to Larry Bettiga, viticulture farm advisor for Monterey County Cooperative Extension.
Berries are most susceptible now.
“Now is a very critical time for infection,” says Bettiga. “It is the point in time when berries are starting development and are most susceptible to infection so it really is the time when people should put emphasis and thought into what they will use to treat powdery mildew.”
“If you have been using sulfur and plan to supplement with a synthetic material, now is the time to apply it and ensure good coverage. These early berry development treatments are the one’s that will keep the clusters clean for the rest of the season,” he adds.
Plan synthetic applications wisely.
There are many products on the market that provide powdery mildew control and many that also provide botrytis control or suppression. The key is to plan a spray program to use specific materials and modes of action when they will be the most beneficial.
Bettiga believes a rotation starting with a DMI (or sterol inhibitor) followed by a combination product that offers added botrytis protection and then followed by another DMI or other mode of action will be effective.
Doug Gubler, PhD, plant pathology professor at University of California, Davis, has also done significant research on powdery mildew control materials. Gubler outlines the products that are most useful in IPM programs online. Sterol inhibitors top the list followed by combination products. Both Bettiga’s and Gubler’s research show Viticure® as a more efficacious rotational partner in the DMI or sterol inhibitor spot compared to Rally and Elite®.
Prevent outbreaks all season.
Bettiga reminds growers to start early to prevent any outbreaks. Then pay close attention to the pressure level throughout the season and adjust spray intervals and rates within the labeled parameters as pressure increases or decreases. The UC Davis Grape Powdery Mildew Risk Assessment Index) is a useful tool to determine pressure.