Cucurbit downy mildew is marching up the U.S. eastern seaboard from Florida to central Pennsylvania, but has not been reported in Ohio, Michigan or Ontario as of July 12. This is later than normal for Ohio – last year our first cucumber downy mildew report was on June 28. Excessive heat followed by sunny days have likely contributed to the delay. However, cucurbit growers should be vigilant and scout their fields regularly for downy mildew. These crops should be protected now with an effective protectant fungicide such as chlorothalanil (Bravo, Echo, Equus, Initiate versions), which will also help manage anthracnose and Alternaria leaf spot – we have seen an unusually large amount of Alternaria leaf spots in various crops this summer. When we start experiencing cooler, rainier weather with high humidity and overcast skies, the downy mildew risk will increase. We evaluated numerous fungicides for efficacy against cucumber downy mildew in 2017 (see chart below). Ranman 400SC, Orondis Opti, Omega 500F and Gavel 75 DF performed best in these tests. Bravo Weather Stik 6F, Zampro 52SSC, Tanos 50DF, Presidio 4SC and Curzate 60DF were intermediate in efficacy and can be used as rotational partners in a fungicide program, particularly under low to moderate disease pressure. The poorly-performing fungicides are not recommended for downy mildew management. 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. Cucumber and cantaloupe downy mildew risk is much higher in northern than in central and southern Ohio at this time.
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.