Sonoma County did not escape the powdery mildew onslaught, but as he’s done for a number of years, veteran vineyard manager Duff Bevill was able to keep disease under control by following a strict season-long fungicide spray schedule based on the Powdery Mildew Risk Assessment Index (RAI) control model developed by Gubler.
Bevill Vineyard Management LLC, Healdsburg, Calif., oversees the care of wine grape vineyards, ranging in size from 5 to 200 acres, throughout the county. The RAI assesses the risk of disease development by relating it to air temperature and moisture, and indicates how often fungicide applications are needed protect the vines.
The higher the risk indicated by the model, the shorter the interval between sprays needed to control the disease and vice versa.
Controlling powdery mildew takes priority over all of Bevill’s other field work
“Once we start spraying, we stay on a tight schedule throughout the season, shortening or stretching spray intervals as needed to match the disease pressure,” he says. “Controlling powdery mildew is our top responsibility — everything else we do in the vineyard is secondary. If our crews are suckering and it’s time to spray, we take our people out and spray and wait the appropriate number of days before we go back into the fields. We don’t want mildew, so we’re pretty tough on following the spray schedule exactly as needed.”