Mites hitting vines in Napa; rain deficit prompting conservation measures

In spite of the relatively dry winter the wine grape crop looks “strong”, according to Steve Matthiason, president of Premier Viticultural Services in Napa. “All indications are this could be a good crop,” he says. “From bud dissection data that clients have shared with us to what I’ve seen in the field with clusters and shoots, it looks very positive.”

The real bugaboo at this early point in the season is the shortage of winter rains that are bringing on early mite infestations and creating concern over adequate supplies as the season progresses.

“We’re about six weeks behind average at this point as it pertains to moisture,” Matthiason says. “We’re already taking out part of the permanent cover crops and cultivating to conserve moisture and make sure the vines don’t get too stressed.”

While the mostly dry winter has produced very little disease, mites are another story, according to Matthiason. “I haven’t seen any Phomopsis, botrytis or powdery mildew,” he says. “However, we’ve had a lot of mite pressure already. We’re using oil for our first powdery mildew and mite spray to help reduce sulfur applications.”

In the Paso Robles area, mites have not yet been detected, according to Lowell Zelinski of Precision Ag. “That’s a little surprising considering it’s been warm and dry,” he says. “However, a lot of our vines are just at bud break except for Syrah and Chardonnay which have 12-inch shoots.”

Mildew sprays have already started on some varietals. Growers are also beginning to irrigate, according to Zelinski, although that water may not be necessary at this point. “It’s difficult to tell unless you have a soil moisture monitoring system in place,” he says. “A lot of our rainfall events have been light so how much of that moisture ends up in the root zone and is ‘effective’ moisture is anyone’s guess unless it’s measured with a probe, or if nothing else, at least a shovel. However, I still think some of this early irrigation is unnecessary.”

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