“Last year the powdery mildew threat in our area was huge,” says Fresno County raisin grower Monte Schutz, who grows organic and conventional Thompson Seedless grapes near Caruthers. “But, the problem wasn’t all that bad for us because we have a regular spray program and we didn’t deviate it from it.”
The program, which Schutz has been following for more than two decades, calls for treating vineyards with sulfur every 7 to 10 days, or a sterol inhibitor every 14 to 18 days, depending on pressure from the disease. “If the pressure eases up I’ll extend the interval between applications, he says. “The only time I’ve gotten into trouble with powdery mildew is when I tried to stretch the interval beyond the recommendations.”
Last year, because of the added threat, he treated vines on the tighter intervals.
“You can control powdery mildew by staying on top of it,” Schutz says. “If you sit around and wait until you see the disease before spraying, it’s too late — it can blow up on you.”
He starts powdery mildew control once he sees 2 to 3 inches of new shoot growth, and combines the first two sprays of micronized sulfur with powdered copper to control phomopsis. After that he’ll continue with the liquid sulfur treatment or switch to sulfur dust, treating every 7 to 10 days.
His sole control for powdery mildew in his organic grapes is sulfur, which he dusts at the rate of about 12 pounds per acre. For his conventional grapes, Schutz will increase the sulfur application rate to 15 pounds per acre.
In May or June, the canopy begins to close, making it more difficult to get good coverage with the sulfur, so he switches to a sterol inhibitor for better control of the disease. He stops fungicide treatments at veraison, except for a light treatment, if needed, to control any powdery mildew on the stems.