Cucumber Downy Mildew – Medina County OH

Tape mount from the underside of a cucumber leaf with downy mildew. Characteristic branched sporangiophores (center) and oval, brown sporangia. Micrograph by Francesca Rotondo.

Downy mildew was confirmed today on cucumbers in Medina County – the field is in the Homerville area and symptoms were just beginning to show.  The pathogen that causes cucurbit downy mildew, Pseudoperonospora cubensis, was sporulating well on the underside of leaves (see photo).

This is the second confirmation of downy mildew on cucumbers in Ohio this year – the first was on August 11 in Huron County.  Please see my August 11 post in this blog for management recommendations.

We still have not confirmed downy mildew on squash or pumpkins, but we have received quite a few lookalikes, most of which were bacterial spot or angular leaf spot (also a bacterial disease). Bacterial diseases will not be controlled using any of the fungicides recommended for downy mildew, with the exception of copper-based products. However, these are only partially effective against downy mildew and bacterial diseases. If you are not sure about your diagnosis, send samples to the OSU Vegetable Pathology Lab – diagnoses are free for OH growers.

Organic Options for Cucurbit Powdery Mildew Management

Powdery mildew is a scourge of summer for squash, pumpkins, and other cucurbits.  Organic growers should always start with varieties with some degree of resistance to powdery mildew – seed catalogues often call partial resistance “tolerance”.  Although resistance will generally not be complete, efforts to manage powdery mildew with organic-acceptable products will be more productive if growers start with a variety that can put up a fight on its own than one that is highly susceptible.

Dr. Meg McGrath, Cornell University, has summarized field research results throughout the US for organic-approved products tested against various diseases of vegetables and herbs.  Her summary for zucchini powdery mildew research in NY includes the following:

Best results are obtained when these products are used preventatively or at the very first signs of powdery mildew, usually in mid-July in Ohio.  If you wait until powdery mildew has progressed to the stage you see in the photo above, it will probably be too late to get it under control.

First 2018 Report of Cucurbit Downy Mildew in Ohio

Cucumber downy mildew

This has been a very unusual year for cucurbit downy mildew. The disease usually appears on cucumbers like clockwork on or around July 4 in one of the northern Ohio counties, but this year we found it only yesterday, August 10, for the first time in Huron County – with just two mature lesions in one of our cucumber research plots on the OSU OARDC Muck Crops Experiment Station in Celeryville. We have been expecting it due to reports in MI, IN, PA, and Kentucky during the last few weeks.

Bacterial spot of pumpkin

We have been receiving many samples of cucurbits suspected of downy mildew during the past month, including cucumbers, squash and pumpkins, but nearly all of these had bacterial spot or angular leaf spot.  While we expect that these bacterial diseases will continue to be a problem, growers and scouts should be on the lookout for downy mildew in all cucurbit types.  Symptoms caused by bacterial diseases, Alternaria and sometimes anthracnose can look like downy mildew.  If you are unsure, send a sample to the OSU Vegetable Pathology Lab in Wooster for confirmation. There is no fee for Ohio residents. You can also text or email photos – please be sure the images are sharp, as close up as possible, and include both the upper an lower side of the leaf – to me at 330-466-5249 or  We can’t always diagnose from photos but they can be a good place to start.

Most growers have been protecting cucurbits for the last few weeks with a protectant fungicide such as chlorothalanil (Bravo, Echo, Equus, Initiate versions), which will also help manage anthracnose and Alternaria leaf spot.  At this late date and with confirmed cases in Ohio and our surrounding states, growers should consider including additional fungicides in their spray programs.  The chart below shows our 2017 bioassay results for fungicide efficacy against downy mildew. Always rotate fungicides with different modes of action and follow label instructions. Remember that Orondis Opti applications are restricted to 1/3 of the total fungicide applications. Under highly conducive environmental conditions, apply fungicides on a 5-7 day schedule.  When the risk is lower due to hot, dry, sunny weather, or downy mildew has not been reported in the area, the schedule may be stretched to 7-10 days.

Information on fungicides for vegetables, including Fungicide Resistance Action Committee (FRAC) code and greenhouse use can be found in a table beginning on page 79 of the 2018 Midwest Vegetable Production Guide for Commercial Growers. Pre-harvest intervals are shown for each crop/fungicide combination throughout the guide.

Options for organic cucurbit production are limited.  Many organic-approved products include cucurbit downy mildew on their labels, but most are not very effective or ineffective.  A copper-based fungicide such as Champ usually is the most effective in research trials, but generally control is not complete. These products must be applied preventatively, before the downy mildew pathogen infects the plants. Cultivars with some resistance to downy mildew should be used. Dr. Meg McGrath of Cornell University has summarized recent research throughout the US on organic-approved products for control of downy mildew and other diseases of vegetable and herb crops.