From Meg McGrath, Long Island Horticultural Research & Extension Center, mtm3@cornell.edu:
Downy mildew was found affecting cucumber in a commercial crop in Saratoga Co., New York, yesterday plus reported on cucumber in northern New Jersey, eastern Pennsylvania and Delaware since last update. No symptoms seen yet in western NY.
Cloudy conditions today (which protect spores from lethal sunlight UV) followed by rain (which brings spores from air currents down onto wet leaves conducive for infection) starting tonight plus wind moving from known occurrences toward NY means there is high risk of successful pathogen spread today for Long Island (first high risk forecast this year for that area) and east-central and southeast NY. Low risk for cucumbers in western, west-central, and northeast NY. However, note that the low risk forecast for northeast NY was generated before downy mildew was known to be developing in Saratoga Co.
Growers should be weekly checking their cucumber and cantaloupe crops for symptoms and checking the forecast website for risk of pathogen spread to their crops plus reports of new occurrences. The cucurbit downy mildew occurrence map has information about cucurbit crop types observed affected which appears when the cursor is placed over a colored county. There has only been one report so far of downy mildew on cantaloupe in the northeast (southern NJ). These are the only crops at risk presently. There are host-specific genotypes of this pathogen.
Applying a protectant fungicide (chlorothalonil or mancozeb) to cucumber is warranted where risk is forecast, especially when high risk. Treating cantaloupe is also worth considering. Continue checking leaves for symptoms to know when downy mildew has started to develop in a crop and thus time to apply targeted fungicides for this disease. See cucurbit downy mildew webpage for information about management in conventional and organic crops.
Please report when seen to your extension specialist or at the forecast website. Knowledge of where downy mildew is occurring is critical to the success of forecasting.