It’s been a mixed bag of weather for growers throughout the state this spring. An early arctic blast followed by relatively mild temperatures set the stage for disease in early spring.

Then it turned hot, only to cool off precipitously again.

“Powdery mildew has not yet shown up in the north coast and those who are using the Gubler-Thomas Risk Index should have saved at least three to four fungicide applications this year so far,” says Doug Gubler, UC Davis Extension Plant Pathologist. “The index has read 0-10 for the past five weeks and is just starting to increase this week.

Monitoring in our fungicide trials has yielded no mildew, thus the correlation is as it should be. The southern part of the state has had a high index for a couple of weeks and growers should be on a tight fungicide application schedule and be watching the index.”

Sulfur applications have been going out on a 7-10 day schedule in grapes in Kern County, according to Vern Crawford, Wilbur-Ellis PCA in Shafter. “The powdery mildew index has been running in the 80s for the past three weeks,” he says. “We’re not taking any chances. If it gets worse, we’ll start with a premier dusting sulfur that has a fungicide in it.”

Staying ahead of the game is the key when it comes to powdery mildew, but the situation should even out in the next few weeks. “As temperatures increase in the Valley, the index should begin to fluctuate, thus allowing growers to start stretching spray intervals,” Gubler says. “Remember, an index of 0-30 equals absolutely no growth of the pathogen. An index of 40-50 equals a 15-day reproductive cycle, and an index of 60-100 equals a reproductive rate of only five days.”