Cucurbit Downy Mildew Alert

6/3/2021
Cucurbit downy mildew has been verified on cucumbers and cantaloupe in New Jersey and Pennsylvania. Because of prevailing west-to-east weather patterns, and the projected path of tropical storm Elsa, there is a chance that this disease will continue its spread toward us.
The Cucurbit Downy Mildew Forecasting website is predicting much of New England and Maine to be at risk of downy mildew spread.
Screen-capture from https://cdm.ipmpipe.org/forecasting/, July 8, 2021
Cucurbit downy mildew can be very destructive if conditions are favorable to it. Like cucurbit powdery mildew, this disease does not overwinter here, and its severity in any given season is often dependent upon when it arrives, if at all.
The first noticed symptoms of cucurbit downy mildew are often angular yellow leaf spots, which are angular because they are constrained by leaf veins. Corresponding dark lesions, and sometimes spores, are seen on the bottom of the leaf as the disease develops. This website from Cornell links to many photos for comparison and has lots of information.
The best “control” is to choose resistant varieties when available. Some strains of the disease have overcome resistance in older cucumber varieties, but new varieties have been developed, and NY 264 and Bristol have been promising in trials in Long Island and at the University of Massachusetts.
Materials for controlling fungi, and fungi-like diseases like downy mildew, are most effective when applied preemptively, so watching for the spread of the disease is important. Scouting should start in earnest when the disease has been reported nearby.
For commercial farm control options, you can consider a copper product and/or a bio-fungicide like Actinovate, Double Nickel, etc. Zonix should also have some effect against zoospores produced by the disease. However, cucurbit downy mildew can produce another spore type, and there’s no easy way to know which type of spore might be blown to your field. More information about organic fungicides for cucurbit downy mildew can be found here.
Gardeners willing to help track the annual spread of cucurbit downy mildew can find more information here, as well as links to more informational resources.

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