This fast growing fungus affects Brassicas, cucumbers, melons, peas and more. It may appear at any time of the season, but is most often a problem in humid areas, especially in cool (below 75 degrees F) wet weather (the spores need water to germinate and grow). It enters the plant through wounds and natural openings and first appears on older leaves, as white, yellow or brownish spots on the upper surfaces and downy grayish mold on the corresponding undersides (this eventually releases more spores). These spots eventually turn darker in color and the leaf dies. Because of the whitish patches of mildew it is sometimes confused with powdery mildew or gray mold. Remember that Downy Mildew appears down on the underside of the leaf, while powdery mildew is on top of the leaf too.
Control Downy Mildew by improving air circulation and keeping the leaves dry (if you must use overhead sprinklers then water early in the morning or evening, so plants don’t stay wet all night). Spores overwinter on crop debris, so clean up the beds in fall. Also rotate plants and remove any infected plants promptly. The spores can travel long distances on the wind, especially in moist air. Some varieties are resistant to downy mildew.
Image: Virginia Tech Learning Resources Center, Virginia Polytechnic Institute and State University, Bugwood.org