These are growing in my small backyard greenhouse.

HI, Daniel

This could well be downy mildew.  The best ways to prevent downy mildew in the cooler months (when it thrives) is to  improve air circulation and irrigate early in the day.  Make sure not to reuse the same soil to replant anything in the crucifer family. 

 It sometimes hard to catch downy mildew early enough for fungicides to be helpful.

Here is a link from Cornell on fungicides available for downy mildew: https://www.vegetables.cornell.edu/pest-management/disease-factsheets/downy-mildew-of-cucurbits/current-management-guidelines-based-on-research-results/#:~:text=Chlorothalonil%20and%20mancozeb%20are%20the,fungicides%20for%20this%20is%20available.