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Powdery Mildew Management Questions — Answers from an International Workshop on Powdery Mildew

I recently had questions at a grower meeting about several aspects of powdery mildew management.  I am currently attending an international meeting on powdery mildew, so it is very helpful to have my opinions supported by true experts in the field of powdery mildew management.  Relative powdery mildew, the following comments may help you in your management decisions, especially for next year.  These comments mainly apply to vinifera grapes, so keep than in mind.

(1) Sulfur is effective for management of powdery mildew, so perhaps we need to rethink how we rate it for control.  Given, DMIs and strobilurins also work well (possibly better) if resistance has not developed.  However, we need to incorporate sulfur in our spray programs, as it does not develop resistance.  Relative temperature, application of sulfur at temperatures below 85 will not likely result in phytotoxicity, even if temperatures climb above 95 after the application.  With that said, there might be some damage till plants adapt to higher temperatures; the first really hot day may have damage, whereas future days may not. Use weather predictions to consider when this early damage could occur, and use another material just prior to the first really hot day(s).  The key to damage prevention is that the application timing needs to be at a cooler temperature. If you are still seeing sulfur damage if you try this method, it might mean that the environment is problematic (e.g. clear hot day versus periodic clouds), or it could be the cultivar is at issue.  Without regard, I feel pretty comfortable that we can use sulfur in this manner.  We have used it in multiple trials at the Blairsville station, and I have not observed significant sulfur damage on Merlot or Chardonnay in the research vineyard over multiple years of testing.  Bottom line — try it, but use common sense.  If you see significant burn, stop and reevaluate.  Microthiol disperse does appear to be one of the favorite sulfurs, possibly due to handling and spraying characteristics.

(2) Milstop (potassium bicarbonate) is NOT very efficacious for powdery mildew.  According to some, there is indication that it may take off the waxy covering of the leaves and berries, but I don’t have good information to support that premise.  If true, it would be another good reason to avoid use though, as the waxy layer is important for many reasons.

(3) Oxidate (hydrogen dioxide) is also of limited or no value for powdery mildew management.  Like Milstop, it is considered to be a “flash in the pan” product with no protectant or kickback activity.

(4) When using multiple applications of DMI fungicides for powdery mildew management, consider rotation of different DMI chemistries (products).  This might seem counterintuitive, in that DMI resistance can often result in cross-resistance.  However, the cross-resistance is not perfectly complete across all DMI fungicides for powdery mildew. If resistance has not occurred yet, using different DMI materials in sequence over time (never more than the maximum of DMI applications which would occur on one label) can have value.   This might look something like this: Rally (DMI), Tebuzole (DMI), Abound (Strobilurin/QoI), Procure (DMI), Mettle (DMI). These could be further separated by sulfur applications, but the key here is the rotation of materials, as opposed to using four applications of one material, e.g. Rally, Rally, Abound, Rally, Rally in sequence.

(5) Luna Experience (fluopyram + tebuconazole) is an excellent, highly efficacious material for powdery mildew control.  Therefore, one does not want to use it too often due to the possibility of resistance development.  However, targeting a couple of back-to-back applications at the inflorescence, bloom, berry set timeframe can provide excellent management of mildew.  If one application is specifically targeted to fall on berry set, this is the best time for a “bang for your buck” timing for powdery mildew management — particularly effective.

I hope these comments will help you to better manage powdery mildew in your vineyards.

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Phil Brannen

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.