Downy mildew on cucumber confirmed in Michigan

Check your cucumbers now for downy mildew before it is too late.

Downy mildew was confirmed on pickling cucumbers in Tuscola County on Friday, July 6. The infection appeared to be fairly recent. Downy mildew has been a management priority for Michigan cucumber growers each year since 2005. 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).

cucumber leaf
Photo 1. Angular lesions caused by downy mildew on cucumber.

underside of leaf
Photo 2. Underside of leaf showing water soaking.

Downy mildew is capable of devastating fields within 10 days of initial infection, given favorable conditions. Although conditions are hot and dry, the overhead irrigation will provide the moisture needed for the disease to develop. 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 very likely that this pathogen has also become established in additional counties on the east side of the state, but has not yet been detected. If you are unsure whether your cucurbit crop is infected with downy mildew, read information on how to submit a sample (commercial growers) or a digital photo (home gardeners) for diagnosis. A map of confirmed downy mildew reports in Michigan is also available.

Our research over the last seven years has highlighted the effectiveness of fungicides available to commercial growers that limit downy mildew (Table 1). See the spray recommendations for other vine crops (squash, pumpkin, melon, etc.).

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)
  • Presidio 4FL (2-day PHI)


  • Previcur Flex 6SC (2-day PHI)
  • Previcur Flex 6SC (2-day PHI)


  • Ranman 3.6SC (0-day PHI)
  • Ranman 3.6SC (0-day PHI)



  • Tanos* 50WG (3-dayPHI)



Alternate products and mix each with either:

  • Dithane (mancozeb) 3 lb
  • Bravo (chlorothalonil) 2 pt


Alternate products and mix each with either:

  • Dithane (mancozeb) 3 lb
  • Bravo (chlorothalonil) 2 pt

*NOTE: Tanos did not provide adequate control of downy mildew when used alone on cucumbers during a 2011 Michigan field trial.

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 2012, two spore traps have been 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.