Downy mildew in garden cucumber and other cucurbit plants
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:
Cucurbit Downy Mildew: Master Gardener Diagnostic Webinar
Originally recorded for the Penn State Master Gardener program, this webinar is applicable for all Master Gardeners who are interested in strengthening our ability to monitor for cucurbit (and basil) downy mildew across the east coast. During this webinar, you will learn about:
- The importance of downy mildew for both home gardeners and commercial growers;
- How to recognize signs and symptoms;
- Current cucurbit downy mildew to monitor and forecasting efforts;
- Disease management strategies for the home gardener;
- How you can help us in our monitoring effort by either scouting your own garden and reporting occurrences or by planting a few seeds of two cucumber varieties and an acorn squash (seed provided only for the 2021 season) and monitoring those plants.
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.
