Following recent reports this week of downy mildew on cucurbits in Michigan, central Pennsylvania, Wisconsin and Indiana, we have our first confirmed report of downy mildew on cucumbers in northern Wayne County. This is very late for this area – we usually see downy mildew on cucumbers in early July in northern OH. Many growers have been spraying preventatively due to the seriousness of downy mildew on cucumbers and other cucurbits. All of the reports this week from MI, WI and PA were from cucumber, although the report from southwestern Indiana was from watermelon.
Cucurbit growers who have not transitioned from applying only protectant fungicides such as chlorothalanil or mancozeb to downy mildew fungicides should now do so. The environmental conditions – cooler temperatures, high humidity, overcast skies and rain showers- expected in much of Ohio during this part of the season are conducive to downy mildew. Effective fungicides against downy mildew are Ranman, Elumin, Orondis Opti and Zampro. Tank mix these with chlorothalanil or mancozeb, with the exception of Orondis Opti, which includes chlorothalanil inn the pre-mix.
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 and yield losses are likely.