Sarah Campbell, a graduate student in the Plant Pathology Department at the University of Georgia, has been reviewing our situation with downy mildew resistance to three fungicide classes utilized in Georgia. The maps in the document below show the counties which she has surveyed and whether resistance has been found in those counties. Of critical importance, resistance to the strobilurin (quinone outside inhibiting) or Fungicide Resistance Action Committee (FRAC) class 11 is widespread, and these fungicides (Abound, Pristine, Sovran, Flint), when utilized for control of other diseases, should always be mixed with mancozeb (early season) or Captan products to increase or provide downy mildew control. Ridomil (mefenoxam) is still active, as is Revus. Neither mancozeb nor Captan products are known to develop resistance, so use these products for downy mildew management as the labels allow; make sure to observe the long preharvest interval on mancozeb or products containing mancozeb. For other classes, I recommend that you try to limit yourself to one application per season — when possible. This will require producers to purchase multiple chemicals that will be utilized only once per season, but alternation of chemical classes is critical to maintaining these fungicides for years to come. I have added Gavel to the list (see slide four in the link), as I have had a few questions about this fungicide; I am not personally familiar with Gavel (no research in Georgia to date), but there is a consensus of information that indicates it is an effective downy mildew material, as well as providing another class for early-season management. Again, see the link below for an updated list of downy mildew fungicides. Also, please let Sarah know if you find downy mildew in your vineyards (sarah.campbell@uga.edu), as she is surveying again this year for resistant isolates.
