Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland; keverts@umd.edu and Andy Wyenandt, Assistant Extension Specialist in Vegetable Pathology, Rutgers University; wyenandt@aesop.rutgers.edu
Downy mildew outbreaks are confined to Florida, southern Georgia and Texas at this time. We hope that downy mildew will not be transported here for several months. However, now is a good time to learn a little more about this disease and the tools available for its management.
One of the biggest questions each summer is when to start spraying for cucurbit downy mildew control. The downy mildew pathogen does not overwinter here, so rotation is not effective in managing this disease. In a normal year, the disease has to work its way north from cucurbit crops grown in the South, and there need to be weather patterns favorable to the disease which spread it to our region. The best approach for determining when to spray for downy mildew will be to stay aware of weather patterns and to scout on a regular basis. As cucurbit crops begin to develop a canopy make sure to scout on a regular basis, especially if wet weather has been around for a few days. Use resistant cultivars, if possible, apply broad spectrum fungicides in early season and scout for downy mildew. When downy mildew appears in our region, apply targeted fungicides.
The downy mildew pathotype that is of greatest concern in the mid-Atlantic is virulent (causes disease) on cucumber. Within that cucumber pathotype are strains that have overcome the resistance that was bred into the processing crop and that are insensitive to several fungicides, such as Ridomil. This strain (or strains) is the one that has caused large yield losses since 2004.
Cucurbit downy mildew on cucumber leaves
In addition to differences in the ability of pathotypes to cause disease on different cucurbits (for example: pumpkin, cucumber and watermelon) they also differ in their sensitivity to fungicides. For example the strain of downy mildew that caused the epidemics on processing cucumber recently is not controlled with Ridomil Gold. Likewise, chemical trials have indicated that the new fungicide Revus is more effective on downy mildew on pumpkin than on downy mildew on cucumber.
A longer and more complete article on downy mildew on cucurbits is online at http://mdvegdisease.umd.edu/Disease%20Management/index.cfm. It includes information on fungicides. To track the progress of Downy mildew in the eastern US and to keep up with reports of downy mildew from other states please visit North Carolina State University’s Cucurbit Downy Mildew Forecasting Center at: http://www.ces.ncsu.edu/depts/pp/cucurbit.