June 20, 2015. What’s the Cucumber/Melon Downy Mildew Situation?

Last year downy mildew appeared on cucumbers considerably later than in previous years. The table shows the dates and locations of first Ohio reports in 2014 vs. 2013. Even in 2013, downy mildew appearance was later in some counties than previously. The first report of downy mildew on cucumbers generally occurred around the 4th of July in the northern third of Ohio.

Cucurbit First Report 2013 First Report 2014
Cucumber July 3 Aug 15 (Wayne); Aug 25 (Huron)
Cantaloupe August 2 Sep 8 (Clark)
Pumpkin August 19 Sep 3 (Ross)
Watermelon August 22 ?
Squash         ? Sep 12 (Guernsey)

We don’t know why downy mildew appeared later than “usual”. Weather conditions were generally favorable in both 2013 and 2014 in early summer. However, it is possible that better control of early sources of inoculum in the Great Lakes Region may be slowing spread of the disease. There have been no reports of cucurbit downy mildew north of North Carolina to date. However, given the gravity of a downy mildew outbreak in cucumbers, and to a slightly less extent, melons, cucumber and melon growers in the northern third of Ohio and especially in the lake counties should start the following this week:

  1. SCOUT cucumber and melon fields for symptoms. Early symptoms on cucumbers are yellowish angular lesions on the tops of the leaves. Lesions on melon leaves are less angular than on cucumbers. A downy mold growth with tiny dark purple/black specks may be seen on the underside of the leaves.
Cuc DM-5compressed

Downy mildew on cucumber


Downy mildew on melon

Cucumber DM sporulating1.JPG

Downy mildew sporulation on underside of leaf

  1. SEND downy mildew-suspected samples to the OSU Vegetable Pathology Lab (http://www.oardc.ohio-state.edu/sallymiller/t08_pageview3/Diagnostics_Services.htm; miller.769@osu.edu or 330-263-3678) for confirmation. There is no charge for diagnosis of vegetable diseases from Ohio growers. Samples may also be dropped off at OSU-OARDC facilities: Wooster Campus (234 Selby Hall); the North Central Agricultural Research Station in Fremont; the Western Research Station in Urbana; and the Muck Crops Experiment Station in Celeryville.
  2. FOLLOW me on Twitter @OhioVeggieDoc for reports of downy mildew in Ohio; tweets will direct you to more information and recommendations on this site. You can also see forecasts and sign up for alerts on downy mildew throughout the US on the Cucurbit Downy Mildew IPM PIPE website http://cdm.ipmpipe.org/ (don’t use Chrome for your web browser for this site).
  3. TREAT cucumbers and melons preventatively with fungicides.  Given the recent rainy weather, you should be applying a protectant fungicide to prevent anthracnose, gummy stem blight and other diseases. Add the downy mildew fungicides once the disease starts moving in the Great Lakes Region. See the Midwest Vegetable Production Guide for more details.

Fungicide application:

General protection: Apply Bravo, Manzate, Dithane or other broad-spectrum protectant fungicide on a 7-10 day schedule to prevent a number of diseases. 

Protection before downy mildew appears but is “on the move” in the Great Lakes Region: Apply one of the following fungicides on a 7-10 day schedule, tank mixed with Bravo, Manzate or Dithane: Presidio, Ranman, Previcur Flex, Tanos, Curzate, Gavel (Gavel already contains mancozeb), Zampro or Omega (melons only).  Alternate products.  The application interval can be lengthened under dry conditions.  Use the shorter interval under cool, moist conditions.

Management after disease appears: Apply one of the following fungicides on a 5-7 day schedule, tank-mixed with Bravo or Dithane: Presidio, Ranman, Previcur Flex, Zampro or Tanos.  Alternate products.  The application interval can be lengthened under dry conditions.  Use the shorter interval under cool, moist conditions.  See product labels for fungicide rates.

Always tank mix targeted fungicides with a protectant fungicide and alternate targeted fungicides with different modes of action (see FRAC Codes below). Note that the fungicides recommended above have different pre-harvest intervals (PHI).  Keep this in mind when fungicides are applied after harvesting begins. Note also that some fungicides have plant-back restrictions that may affect decisions regarding crop rotations.

Product PHI (days) FRAC Code Comments
Chlorothalanil e.g. Bravo Weather Stik 0 M5 Protectant; tank mix with targeted fungicides below
Mancozeb e.g. Dithane or Manzate 5 M3 Protectant; tank mix with targeted fungicides below
Ranman 0 21 No reports of reduced efficacy
Previcur Flex 2 28 Reduced activity suspected in some Ohio counties
Tanos 3 11 + 27 Up to 2 days curative activity but low residual (3-5 days)
Gavel 5 22 Contains mancozeb
Presidio 2 43 Likely resistance in some CDM populations
Curzate 3 27 Up to 2 days curative activity but low residual (3-5 days)
Zampro 0 40 + 45 No Ohio data; moderate efficacy in Eastern US
Omega 500F 30 29 Use only for Cucurbit Vegetables Subgroup 9A – includes muskmelons/cantaloupes/honeydew; use early for protection: note 30 day PHI


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