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Sprayers on standby as new grape season develops

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Managing grape diseases during late dormancy, bud break a high priority.

As activity levels in vineyards begin their ramp-up to another season, it represents a new year with new challenges and new opportunities — for growers and pests alike.

Managing grape diseases during late dormancy and bud break should be high on the To Do list, according to Gabriel Torres, Viticulture Farm Advisor for the University of California Cooperative Extension service in Kings and Tulare Counties.

“With the new growth season comes a repeat of concerns among growers on how they will be affected by diseases,” he says. “If rain is not expected and farm operations permit, pruning should be delayed until close to bud break in order to reduce the risk of trunk disease infections. Our experimental results have shown that pruning too early can result in at least two chemical sprays needed to reduce trunk disease incidents, while more timely pruning can reduce spraying to a single event.”

Once dormancy is a thing of the past and bud break arrives, that’s a signal that temperatures have warmed up enough to spur plant growth. Those same conditions also are optimal for powdery mildew and botrytis proliferation.

“Controlling both those diseases at early stages is critical to reduce the inoculum for the remainder of the season,” Torres says. “If wet conditions are expected after bud break, especially in the San Joaquin Valley, extra measures may be necessary to control Phomopsis cane infections.”

Powdery mildew, botrytis

When it comes to powdery mildew and botrytis control, soft chemistries can be used during the initial phases of bud break to blossom, including sulfur for powdery mildew and stylet oil for both diseases. The farm advisor warns, “Remember that stylet oil, while it is an excellent eradicant, can cause plant tissue burning in temperatures above 90 degrees or if sulfur is used two weeks before or after the oil application.

“If growers are considering the use of strobilurines (Qols) during the season, we recommend that prior to the first fungicide spray, a field sample be sent to check resistance factors. Local viticulture advisors can provide swabs for sample collection. If a resistant population is discovered, growers should avoid the use of any FRAC 11 fungicide during the season on that sampled field.”

Since the aforementioned diseases are weather mediated, knowing when to intervene is a critical part of the process.

“If rain is expected after pruning, we suggest delaying pruning if possible or spraying a protective fungicide after any pruning. If six hours or more of continuous temperatures between 70-85 degrees are forecast, growers should be prepared to spray for both botrytis and powdery mildew. If rain chances remain low, sulfur application can be done every two weeks, more often (7-10 days) if rain probability increases.”

Rhonda Smith is Torres’ counterpart in Sonoma County and spoke earlier this year at their annual Grape Day on the issue of grape downy mildew, how to recognize it and how to control it.

“This disease was noticed by a few growers last year (for most of them, a first-ever observation) they need to get more familiar with because weather conditions lead to sporulation and infection. Grape downy mildew in Western states is less common and less severe than in regions with in-season rainfall, high humidity, and warmer temperatures,” she said.

But climate conditions around the country are changing and being prepared is never a bad way to go. “Management programs where GDM occurs include application of preventative materials or if the disease is already in the vineyard, curative products are called for. Monitoring is the key along with familiarity of the pathogens life cycle and weather conditions that impact it.”

For more news on pests, disease management and other issues affecting vineyards, subscribe to the bi-monthly newsletter The Grape Line.

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