Grape Disease Risks 7/26 – 8/2/2023

If you missed the VVA, here are my handouts. Please consider joining the downy mildew model project. We don’t need a lot of people, but I would like to have enough feedback to improve our outputs and models.

Here’s a snapshot from the last week, showing the number of days with the risk of each disease: Ph = Phomopsis, PW = Powdery mildew, BR = Black rot, and DM = Downy mildew. At this point, Phomopsis and black rot risk probably won’t be as important as earlier in the season. You can ignore Phomopsis and black rot since Phomopsis should be done with their spore production, and clusters are resistant to black rot infection. This information was made available through the Wine Board-funded Sentinel Vineyard project.

The disease risk trend for the state was similar to the past five weeks. The central VA had another wet week, which resulted in many days with downy mildew risk event(s). In two weeks, the range of rains reached the east of the Blue Ridge Mountains (finally!). Other locations showed days with downy mildew risk, while small numbers of black rot risk days indicate warm and humid nights and short rain events.

We are through the critical period (i.e., ~6 weeks from bloom) for black rot and powdery mildew cluster infection. If your vineyards are clean at this point, you may be able to extend the spray interval from “7 to 10 days” to “10 to 14 days”, depending on the weather and other factors (i.e., Botrytis spray).

Please ensure you won’t violate the 66-day PHI of Mancozeb products. Also, some other products have a relatively long PHI (e.g., Ranman’s PHI is 30 days). Make sure to read the label or refer to our PMG. Also, you may need to protect your vines against powdery mildew because they can infect leaves and shoots. Please check my slide presentation for veraison/pre-harvest disease management considerations.

The image above concisely summarizes the past seven days, aiding your understanding of recent events. However, the best action is proactively safeguarding your vines. For more comprehensive information, please visit https://newa.cornell.edu/, where you can access detailed daily weather data and disease and insect pest model results, including forecasted risks. We have covered the annual fee to ensure free access for growers in Virginia to the NEWA platform.

Utilizing this information, you can adjust your spray intervals accordingly. If you notice several days with disease risks, you may need to shorten your spray interval. Conversely, if you missed a material for downy mildew or black rot, you might consider using materials with kick-back activity. On the other hand, if the number of risks has been limited this year, you could extend your spray interval.

Remember that your spray decision should not solely rely on weather conditions. It is crucial to consider factors such as the cultivar and the disease’s history in your vineyard.

List of fungicides with a short PHI

It is hard to believe we are discussing harvest, but it is coming! Please see below for the list of fungicides with a relatively short PHI (less than or equal to 7 days).

VCE Virtual Vineyard Meeting will be held 8/10 (next Thursday)

The virtual vineyard meeting will be held next Thursday (8/10) from 1 PM. We will discuss pre-harvest considerations. If you have not registered, please use the link below.

Registration link to our monthly virtual viticulture meeting series – Virginia Grape Disease Updates (grapepathology.org)

Email list

This blog has a subscription system; however, it has been hit or miss in the past year or so. I have spent quite a bit of time talking to the email and plugin providers in the past five months. I made another change in the system, and I hope this will work for more than a few weeks! If you receive more than one emails, that is most likely because the system is in a transition from the old plugin to the new one.

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