Downy mildew pathogen sporulation on underside of a cucumber leaf.
On July 5, downy mildew was found on cucumbers in Huron County, one week after the first 2017 Ohio report (Wayne County) of the disease. In this case, disease symptoms had just become visible in a low spot in the field, but the sporulation was very strong, indicating a good potential for the pathogen to spread. The disease was also confirmed in Ontario on June 27 and in Michigan on June 29. Given the favorable weather conditions of the last week, with high humidity, overcast skies and rainstorms moving through the state, growers in the northern third of Ohio should assume that the risk of downy mildew in cucumbers and melons is very high. See the Cucurbit Downy Mildew ipmPIPE website for a detailed forecast; you can also find the map of outbreaks on this site. As indicated in my post on June 28, growers need to scout fields carefully, especially in low or shady spots that may hold moisture longer than other parts of the field. The only way to manage this disease at this point is through application of effective fungicides – and recommendations are provided in the June 28 post.
There were reports of downy mildew on cucumbers in Erie County in western NY and Columbia County in central PA, but so far there have been no reports of downy mildew on pumpkins or squash in the US Midwest or Northeast.
Reminder: I report outbreaks of cucurbit downy mildew and other veggie diseases as soon as we know about/confirm them on Twitter: @OhioVeggieDoc.