Cucurbit downy mildew was confirmed in Western Michigan on July 25, 2018. See http://www.canr.msu.edu/news/downy-mildew-found-in-michigan-cucumbers-in-2018 for more details about the current infection.
The 2018 Cucumber Downy Mildew Control Strategy has a list of the most effective downy mildew control products registered for use in Ontario. It also includes information on spray scheduling and timing.
When scouting for downy mildew, the first sign are often water-soaked lesions on the upper and lower leaf surface. These lesions are most obvious in the early morning hours.