Downy mildew spores detected in Michigan vegetable fields

Cucumber and melon growers should spray now for downy mildew.

Downy mildew was recently confirmed on cucumbers in Ohio. Downy mildew has been a management priority for Michigan cucumber growers each year since 2005 and recent spore counts from Monroe County indicate that all cucumber and melon growers should initiate their preventive sprays immediately.

Although all cucurbit crops such as pumpkins, summer squash, zucchini, gourds, cucumbers (Photos 1-2), cantaloupe (Photo 3), watermelon and winter squash may develop downy mildew symptoms, cucumbers and melons are usually most susceptible in the Michigan growing region. Downy mildew is caused by the water mold Pseudoperonospora cubensis and produces yellow and brown angular lesions on the top of cucumber leaves bounded by the leaf vein (Photo 1). Under the leaf, dark, fuzzy spore masses and water soaking occurs in the locations where the lesions are present (Photo 2).

Downy mildew
Photo 1. Angular lesions caused by downy mildew on cucumber.
Photo credit: Mary Hausbeck, MSU

Downy mildew
Photo 2. Underside of cucumber leaf showing water soaking.
Photo credit: Mary Hausbeck, MSU

Downy mildew
Photo 3. Downy mildew lesions on a cantaloupe leaf.
Photo credit: Mary Hausbeck, MSU

Downy mildew is capable of devastating fields within 10 days of initial infection, given favorable conditions. Michigan’s weather conditions have been very favorable this year (2013) for the development of downy mildew. Because the pathogen is wind dispersed, it is able to travel long distances and therefore frequent scouting for the pathogen and preventive sprays are necessary to avoid costly losses in yield.

It is likely that this pathogen has become established in Michigan, but has not yet been detected. If you are unsure whether your cucurbit crop is infected with downy mildew, see how to submit a sample (commercial growers) or a digital photo (home gardeners) for diagnosis. You can also view a map of confirmed downy mildew reports in Michigan.

Our research over the last seven years has highlighted the effectiveness of fungicides available to commercial growers that limit downy mildew (Table 1). For Michigan State University Extension’s spray recommendations for other vine crops, including squash, pumpkin, melon, etc., see Spray Recommendations for Downy Mildew.

Table 1. Spray recommendations for downy mildew on cucumber based on MSU research

Applied before disease (7-day intervals)

Applied after disease (5-day intervals)

  • Gavel 75WG (5 day PHI)
  • Presidio 4FL (2 day PHI)
  • Previcur Flex 6SC (2 day PHI)
  • Ranman 3.6SC (0 day PHI)
  • Tanos 50WG (3 day PHI)
  • Zampro 4.4SC (0 day PHI)
  • Presidio 4FL (2 day PHI)
  • Previcur Flex 6SC (2 day PHI)
  • Ranman 3.6SC (0 day PHI)

Alternate products and mix each with either:

  • Dithane (mancozeb) 3 lbs or
  • Bravo (chlorothalonil) 2 pt

Alternate products and mix each with either:

  • Dithane (mancozeb) 3 lbs or
  • Bravo (chlorothalonil) 2 pt

Downy mildew spore trapping

The pathogen causing downy mildew does not overwinter in Michigan. Instead, downy mildew arrives and spreads across the state via microscopic spores that allow it to infect healthy plants and then produce more spores. For each year since 2006, we have monitored the potential threat of downy mildew infection for Michigan production areas using spore traps. A spore trap operates by continuously sampling the air and embedding any spores in the air column onto a petroleum jelly-covered tape located inside the trap. This tape is transferred from the field to the lab once each week. The tape is cut and marked so that it can be viewed under a microscope and the spores are counted.

In 2013, two spore traps were placed in Monroe and Saginaw counties representing key points of entry to the state or high production areas for cucumbers. Historically, data from the spore traps show an influx of spores prior to the detection of downy mildew field symptoms being found within that county. See the daily spore trap totals for the two sites.

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

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