Cucumber downy mildew update

Michigan State University manages spore detection network and welcomes plant samples.

Spore trap in a young cucumber field
Spore trap in a young cucumber field. Photo by Mary Hausbeck, MSU.

Downy mildew on cucumber or any other cucurbit has not been confirmed yet in 2019 in Michigan or in adjacent states of the Great Lakes region. Our molecular analysis of the spore traps has recently confirmed cucumber downy mildew spores in the air via our molecular testing a couple of times, but this has not happened frequently or to the level that might be expected if downy mildew was well established in our production areas. Go to our Downy Mildew News page for updates.

Hop downy mildew spores are being consistently found in our spore traps with relatively high numbers, in some cases. These hop downy mildew spores do not pose any risk to our cucumber industry. In previous years, we would not have known whether these spores were cucumber or hop downy mildew as they look identical under the microscope. It’s only through our molecular tools that we are able to know if what we are seeing via the microscope is cucumber or hop downy mildew.

Downy mildew has been a yearly problem for Michigan growers since 2005. The reason my lab works so hard on spore trapping is that the cucumber downy mildew pathogen does not overwinter in Michigan fields. The downy mildew spores we capture in the spore traps cannot overwinter or even survive a hard frost. The good news is that each year we begin with a clean slate for our field production. The bad news is we don’t know when the downy mildew will arrive each year, so we can’t know when the highly susceptible crops, including cucumbers and melons, should be protected.

The cucumber downy mildew spores move via air currents and must blow into our growing regions. Fungicides are costly and applying them before they are needed is not sustainable. However, applying fungicides when the disease is already well established in the field is not recommended because it can be too late to protect the crop and can contribute to the downy mildew pathogen developing resistance to our most important fungicides.

The recent humid and wet weather is favorable for downy mildew although the extreme heat isn’t an advantage to the pathogen. Cucumber downy mildew will likely arrive in Michigan relatively soon, and growers may want to consider using a broad-spectrum fungicide program from now until we consistently detect a cucumber downy mildew spore load or verify a plant sample with downy mildew. Examples of cost-effective fungicides that could be used until we find cucumber downy mildew spores or verify a downy mildew disease sample in the state include chlorothalonil, mancozeb, Gavel or Zing!

Downy mildew look-a-likes this year have been primarily angular leaf spot. The angular leaf spot samples we’ve collected have had the water-soaking symptoms that is often what is looked for with downy mildew. This is why we work with growers, scouts, consultants and MSU Extension educators to make sure we get these samples immediately and make a diagnosis. Click here for instructions on how to submit a sample.

This material is based upon work that is supported by the National Institute of Food and Agriculture, U.S. Department of Agriculture, under award number 2016-68004-24931, and by Pickle Packers International Inc.

Did you find this article useful?