The race to keep powdery mildew in check is off with a bang as much of the state chalked up at least some rainfall over the past few days. If current forecasted warmer temperatures truly follow on the heels of this latest rain event, the powdery mildew risk index will likely come into play in many areas.

“The rainfall is perfectly timed for ascospore release, i.e. after bud break,” says Dr. Doug Gubler, UC Davis Plant Pathologist. “Depending on temperatures over the next several days, the disease will be visible as .5-1 cm colonies growing on the lower leaf surfaces of basal leaves. Oftentimes there is a slight yellowing visible on the upper leaf surface in association with the lower leaf lesions. Should temperatures stay warm - 68-75 degrees - we would expect to see disease onset in 7-10 days following the rainfall. Should the temperatures be cool, disease onset may not be visible for many days.”

Don’t be fooled into complacency.

“The past two years we documented ascospore release and infection with early rains, but cool temperatures prevented lesion formation after infection,” Gubler says. “However, once temperatures increased, the pathogen grew rapidly and disease increased suddenly giving the impression that disease occurred overnight. In reality, disease didn’t ‘just’ suddenly appear, but it had been there for some weeks before.”

This early stage of disease can be controlled by using JMS Stylet-Oil as an eradicant, he says. “It works well because coverage can be next to 100 percent early in the season, and the oil kills the colonies on contact.” Gubler said at this early stage of grapevine growth, it is better economically to use oil than to use more expensive fungicides

Coastal, delta and southern San Joaquin Valley are more likely to be affected by ascospore infection.