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Powdery Mildew in Dry and Wet Environments

We obviously don’t have to worry about a dry environment at this time in Georgia, but as a learning point, I will share this photo with you (expand it to see the symptoms).  This shows how severe powdery mildew can be in a relatively dry environment (Lodi, CA). Stephanie Bolton, a previous Ph.D. student who now works there, sent this photo to me this afternoon. In this case, sprays for mildew management were started late. Stems show the dark brownish-purple discoloration, while clusters are covered in white, powdery spores. Leaf symptoms are not as apparent in this photo, but I bet the tops of the leaves are showing powdery symptoms as well.  Powdery mildew is the one grape fungal disease that does just fine when it is dry, generally better in fact. It does not require free moisture to infect. Just FYI for the future, keep this in mind; I think most of you know this, but it does not hurt to review.  Remember, we are now seeing powdery mildew in Georgia as well; just because it is raining, it does not mean we will not observe powdery mildew.  Though this is the one grape-infecting fungus that does not require free moisture to infect, wet springs increase mildew, as the cleistothecia (survival structures) sporulate better with free moisture.  From my observations, I would say that we always have sufficient and damaging powdery mildew on unsprayed or poorly sprayed vines in Georgia, without regard to our environment.  Add the potential for fungicide resistance development, and this is a very dangerous fungus in all environments.

This mildew was photographed this week by Stephanie Bolton in Lodi, CA. Dry conditions are prevalent, but powdery mildew does well under dry conditions.

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About Phil Brannen

Phil Brannen is a Professor in the Plant Pathology Department at the University of Georgia. He attended the University of Georgia for his undergraduate degree in Plant Protection and Pest Management, where he also received an M.S. in Plant Pathology, followed by a Ph.D. in Plant Pathology from Auburn University. He has extensive experience with disease management programs in numerous cropping systems. He serves as the extension fruit pathologist for Georgia – conducting research and technology transfer for multiple fruit commodities. His efforts are directed towards developing IPM practices to solve disease issues and technology transfer of disease-management methods to commercial fruit producers. He also teaches the graduate level Field Pathology Course, the History of Plant Diseases and their Impact on Human Societies Course, team-teaches the IPM Course, coordinates the Viticulture and Enology in the Mediterranean Region Course (Cortona, Italy), and guest lectures in numerous other courses throughout the year.