Jim Alfieri, a PCA with Wilbur-Ellis, based at Manteca, Calif., has been helping wine grape growers manage pests and control diseases for over 40 years. In early April, he rated the threat posed by the powdery mildew fungus in his area of the state as about average.
This fungus likes the recent warm, moist weather, says Alfieri, who works with growers in the northern San Joaquin Valley.
“So far, it’s not a terrible year, for powdery mildew,” he says
The key to controlling the disease is prevention.
“You want to keep the vineyard clean from the start of season. Normally, if you can make it through to the first of June and don’t find any mildew, you’ve probably done a good job of spraying. Otherwise, you’ll be behind the eight ball until harvest. If you have mildew on June 1 you will be fighting it all season until the grapes are at 12° Brix.”
His advice: Treat early and regularly. And, don’t be afraid to use your fungicides.
“No matter what material you have, it won’t do you any good inside the barn,” Alfieri says. “It has to be on the vine. Some growers hold off applications because it looks like rain or it’s a little windy. You need to do as much as you can before that happens. An April shower can pop up and wash off any sulfur dust you may have applied. But copper and sulfur, oil and some fungicides are pretty tenacious. They will hang on the vines even if you do get a rain. That’s why my growers spray early.”
Alfieri’s powdery mildew treatment program, which he’s been using for nearly two decades, is timed to growth of the vines. Also, to help reduce development of resistance in the disease-causing fungus, he changes frequently among products with different chemistries.
“You want to keep that new vine growth covered with some type of fungicides so that the powdery mildew never gets going,” he explains. He began his treatment program in early April with the first of two sprays with a copper and sulfur combination – the first when the canes reach about 4 to 6 inches length, and the second about 10 to 14 days later, when the canes will be about 12 inches long. Another 10 to 14 days after that treatment, Alfieri will dust the vines with sulfur or make a foliar application of micronized sulfur, depending on the weather. As the season goes on Alfieri will use fungicides to continue his mildew control program to keep the fruit clean to harvest.
In the last week of March, when the vines were about 2 to 3 inches long, he treated the vineyards for phomopsis, applying a strobilurin product.