Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland; keverts@umd.edu
Downy mildew on cucurbits has been a problem on Delmarva beginning in early July for the last few years. Good fungicides for management are available. However, last year in my trials, one of these fungicides, Presidio, was not as effective as expected. Looking ahead to your spray program, be careful not to rely on one fungicide class. It is difficult to know which fungicides will be effective here, because our population does not overwinter and is reintroduced from the South each year. Therefore use excellent resistance management practices to avoid allowing the pathogen to develop resistance and to improve the efficacy of your fungicide management program.
Fungicide Resistance management guidelines by crop are available online http://mdvegdisease.umd.edu/Disease%20Management/Fungicide.cfm and hard copies are available in Delaware at the county Extension offices.