- The drought just about eliminated any powdery mildew disease pressure this year for Sonoma County wine grape grower John Kiger.
Until the 100-degree weather that moved into his area of northern Sonoma County at the end of June, John Kiger’s wine grape vineyard was looking promising, with a larger than average crop. However, the high temperatures are raising his concerns about heat damage.
Established 14 years ago by John and his wife, Deb, Kiger Family Vineyard features hillside blocks of Syrah, Grenache and Cabernet Sauvignon vines on a 20-acre parcel in the Sonoma Valley.
Their season started early, and the grapes have maintained a fast pace. “We’ve had a warm spring and early summer, and the crop is about one to two weeks ahead of the last three or four years,” John says “I’m thinking veraison will start in late July.” For the past three years, veraison has started in early to mid-August.
With no significant rainfall since December, his first full irrigation was in the second week of May. That’s three week sooner than his previous earliest start. In some years, he hasn’t made his first irrigation until early July. He continues to irrigate through harvest.
The shallow rocky soils in his vineyard blocks, which sit on land sloping an average of about 25 degrees, complicate the task of keeping adequate moisture in the root zones. “The dry year has made it tough to keep the vines growing, because the soils can dry out so quickly,” John says.
To make sure his vines stay hydrated, he applies shallow irrigations at least three times a week, based on evapotranspiration rates. This also minimizes drainage away from the vines. Drawing water from his 400-foot deep wells, he operates his drip system using programmable electronic controllers. Permanent cover crops, including native grasses, as well as clovers, brome and fescue grasses, also help conserve soil moisture.
Except for some powdery mildew on a few small Grenache berries, the drought has just about eliminated any fungal disease pressure this year. The treatments are based on the University of California Powdery Mildew Index.
“By using this model, we can get by with two to four fewer sprays each season than most growers who rely strictly on calendar-based spray programs. They may spray every 14 days,” John says.
However, the dry weather has reduced canopy size, exposing more fruit to damaging sun. With fewer laterals to provide shade, John has altered his leaf thinning. Instead of pulling most laterals in the fruit zone, he’s thinned very few leaves in his Syrah and Cabernet blocks. Because the Grenache is more prone to powdery mildew, he’s pulled in the fruit zone but only on the morning side of the vines to improve spray coverage of the tight clusters.
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