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 for the first time only yesterday, August 10, 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.
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 firstname.lastname@example.org. 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.