cdm.ipmpipe.org July 29, 2019.
Downy mildew was confirmed today in cucumbers in an unsprayed MSU research plot in Benton Harbor, MI. In our experience, when downy mildew is identified in Michigan, it is probably already or soon to be seen in Ohio, and vice versa. Cucurbit growers should have been applying protectant fungicides such as chlorothalanil or mancozeb , but should now consider transitioning to downy mildew fungicides. Moderate temperatures, humidity, overcast skies and rain showers expected in much of Ohio in the next few days are conducive to downy mildew spore movement, deposition and infection. MSU’s recommendations for effective fungicides against downy mildew are shown here:
- Ranman + chlorothalonil or mancozeb
- Orondis Opti (chlorothalonil is part of the premix)
- Elumin + chlorothalonil or mancozeb
- Zampro + chlorothalonil or mancozeb
Alternate products on a 7-10 day schedule. Follow the label regarding limitations on number and timing of applications. If you have already applied Orondis Ultra or Orondis Gold for Phytophthora blight management you may have reached the limit on Orondis applications. 
Cucurbit crops must be protected from downy mildew in advance – applying fungicides after the disease is well-established is not effective.