Cucurbit downy mildew in the garden is here

Homeowners should look for downy mildew symptoms on cucumbers.

Recently, there have been reports of downy mildew on cucumbers in commercial fields in three Michigan counties including Allegan, Monroe and Lenawee (see Michigan State University Extension article “Downy mildew spores detected in Michigan vegetable fields.”) This downy mildew is not the same one that causes downy mildew symptoms on impatiens. The name of the downy mildew pathogen on cucumbers is Pseudoperonospora cubensis and is able to only infect a variety of cucurbits including cucumbers, squash, pumpkins, watermelon, cantaloupe, zucchinis, gourds and honeydew melons. Downy mildew re-emerged as a problem on cucumbers in Michigan in August 2005 when the disease spread across the eastern region of the United States and has recurred every year since then.

Downy mildew causes yellow lesions that may be visible on the top surface of infected leaves. However, the telltale sign of downy mildew is the gray-to-black fuzz on the underside of the leaf giving a somewhat “dirty” or “velvet” appearance. This fuzz may be most evident in the morning.

Downy mildew on cucumber
Top side of cucumber leaf showing yellowed lesions that can
become brown and necrotic and are bound by the leaf veins.
Photo credit: Mary Hausbeck, MSU

Underside of cucumber leaf showing downy mildew
Underside of a cucumber leaf showing dark, fuzzy downy mildew
growth bound by the leaf veins. Photo credit: Mary Hausbeck, MSU

Downy mildew up close
Underside of a cucumber leaf (magnified five times), showing
small, dark spore masses. Photo credit: Mary Hausbeck, MSU

Recognizing downy mildew on cucurbits

  • Yellowing/browning on top surface of leaves that may be bound by veins.
  • Velvety or fuzzy dark spore growth on the underside of leaves.

This pathogen can cause catastrophic losses in a brief period of time when conditions are wet. Ps. cubensis cannot live long without a host plant and cannot survive Michigan winters. Downy mildew can spread from field to field on air currents via tiny, microscopic spores. Cool (about 60 to 80 degrees Fahrenheit), wet and cloudy conditions create an ideal environment for downy mildew. When the conditions are favorable, unprotected foliage can become completely blighted within 14 days from the time the downy mildew spores land on the plant’s leaves. Hot and dry weather will slow disease progress.

To detect and monitor cucurbit downy mildew in Michigan, the airborne spores of the pathogen are sampled using spore samplers placed in Michigan’s major growing regions during the spring and summer. These spore traps continuously sample the air and collect spores by imbedding them on a tape covered in a film of sticky material that is removed and taken to the laboratory for identification and counting. A compound microscope is used to identify and count Ps. cubensis spores that are present on the tapes.

There are few management practices that can be used to control downy mildew in the home garden. Before the downy mildew outbreak of 2005, most cucumber cultivars were resistant and would not become diseased. However, since 2005, no cucumber cultivar has been identified that can completely resist downy mildew. A management strategy for gardeners should focus on using preventive measures to reduce the chances that cucurbit downy mildew becomes established. Gardeners should avoid watering at times when moisture will remain on the foliage for extended periods of time, such as in the evenings. If downy mildew becomes established in or near a garden, fungicides containing the active ingredient chlorothalonil will provide some protection against the disease.

How to control cucurbit downy mildew

  • Plant cucurbit crops early in the season.
  • Water when foliage can dry rapidly.
  • Avoid watering during the evening.
  • Fungicides containing the active ingredient chlorothalonil can be used to prevent downy mildew and slow the progression of the disease once it has become established.

For more information about this disease, go to www.veggies.msu.edu and look under “Cucurbit Downy Mildew News.” If you need help identifying cucurbit downy mildew, click on the link “How to submit samples” for instructions on how to submit diseased samples to MSU Diagnostic Services.

Note: Remember the pesticide label is the legal document on pesticide use. Read the label and follow all instructions closely. The use of a pesticide in a manner not consistent with the label can lead to the injury of crops, humans, animals and the environment, and can also lead to civil or criminal fines or condemnation of the crop. Pesticides are good management tools for the control of pests on crops, but only when they are used in a safe, effective and prudent manner according to the label.

Funding provided by: Project GREEEN, Michigan AgBioResearch; and the Pickle and Pepper Research Committee for MSU, Pickle Packers International Inc.

Dr. Hausbeck’s work is funded in part by MSU‘s AgBioResearch.

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