Grape Disease Risks 6/21 – 6/28/2023

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 risk probably won’t be as important as earlier in the season. I am still listing for my interest, but you can ignore Phomopsis since they are probably done with their spore production. This information was made available through the Wine Board-funded Sentinel Vineyard project.

The storm came over the weekend and resulted in quite a bit of infection risk events. I hope you had a chance to apply fungicides with kick-back activity for downy mildew and black rot earlier this week. The storm also brought strong wind and at some locations, hail. If you have seen damage to clusters (cuts, dings, bruises) or leaves (holes), it may be due to the storm.

Some are done, but due to the long flowering period many of us experienced, other cultivars are still in the critical period of cluster infections for downy mildew, powdery mildew, black rot, and Botrytis; thus, please protect your vines. We can relax six weeks after bloom, but until then, please keep your guard up. Also, please make sure that you won’t violate the 66-day PHI of mancozeb products.

The image above is just a summary of the past seven days. These risk events help you understand what happened. However, preparing your sprays to protect your vines would be best. Please visit to obtain more detailed information. You can check daily weather data and disease and insect pest model results, including forecasted risks. We paid the annual fee so that growers in Virginia could freely access NEWA.

One way to use this information is to adjust your spray intervals. If you see many days with disease risk(s), you may need to shorten your spray interval, or if you missed a material for downy mildew or black rot, you might wish to spray material(s) with kick-back activity. Or you may want to extend your spray interval, if you have not seen many risks as we have been experiencing this year. 

It is essential to understand that the spray decision must be based not only on the weather condition but also on the cultivar and history of the disease(s) in your vineyard. Here’s a slide set from the previous virtual viticulture meeting which covers post-bloom disease management.

One last item! If you see an outbreak of black rot despite the spray (especially DMI fungicides such as Rally and tebuconazole, FRAC group 3), please contact either me or Dr. Anton Baudoin who can test for fungicide resistance.

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