This disease can occur anywhere throughout the eastern US, even in a garden with just one cucumber plant and no past occurrences. This is because the pathogen spreads via wind-dispersed spores that can be moved long distances and be deposited by chance anywhere. Cucumber is most commonly affected. Occurrence of downy mildew on melon, squash, pumpkin and other cucurbit types depends on what strain(s) of the pathogen are present.
Identifying
Leaf spots typically have an angular shape due to the pathogen being unable to move through larger veins as it invades leaf tissue (see images below). This is particularly distinctive on cucumber. Spots initially appear water-soaked, turn yellow, and then brown as the tissue dies. It is important to look at the underside of leaves for the pathogen’s fuzzy growth and spores because seeing this confirms the spots are downy mildew.
View images of symptoms on other cucurbit plant types:
Reporting
All reports are very valuable because knowing where downy mildew is occurring is needed for a forecasting program that growers use to determine when they need to protect their crops by applying fungicides. Your report will be especially important if the occurrence in your garden is among the first cases in an area, as was the case for the cucumber photographed below. Gardeners can also use the forecasting program to determine when there is a risk of downy mildew occurring in their garden. Monitoring work at LIHREC started in 2006.
Managing
Growing resistant varieties is the most effective practice for gardeners. NY264 and DMR 401 are green slicer types with high level of resistance. Fungicides available to gardeners are not as effective as those growers can use.
Below: images of an affected cucumber plant taken 13 days after a tropical storm on 4 Aug provided favorable conditions for pathogen spread and infection. There was a high risk forecast that day. No symptoms were seen before the storm.
Below: water-soaked symptoms on leaves of the cucumber plant. These were taken 18 Aug which was 2 days after a day of rain.
