Gardens and CDM

Downy mildew in garden cucumber and other cucurbit plants

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.

Resources to help with identifying downy mildew:

Reporting

All reports are very valuable because knowing where downy mildew is occurring is needed for the forecasting program at this website 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.

Managing

Growing resistant varieties is the most effective practice for gardeners.  Unfortunately, resistance has only been bred into cucumbers, and many resistant varieties, due to their low level of resistance, will develop enough symptoms of downy mildew on leaves to reduce quantity of fruit produced and quality (shape) of those fruit. NY264 and DMR 401 are green slicer cucumber types with high level of resistance.

Starting to grow cucumbers as early as possible in spring maximizes the growing period before the pathogen’s spores are moved in wind on to the plants. Note that cucurbit plants are sensitive to cold temperatures (below 50 F). Plants started in pots can be brought inside over cold nights until it is warm enough to transplant them in the garden. Downy mildew typically starts to develop in Florida during March, then progressively later in other states as the pathogen spreads, with first occurrences during July in northern states.

Fungicides available to gardeners are not as effective as those growers can use. None have curative activity thus applications need to be made before infection. Helping growers determine when to apply fungicide is the main goal of this website.

Below: images of an affected cucumber plant taken 13 days after a tropical storm on 4 Aug 2020 provided favorable conditions for pathogen spread and infection. There was a high-risk forecast that day. No symptoms were seen before the storm.