Cultural control, fungicides and biological pesticides can control downy mildew. For example, a simple cultural control is to water the soil directly and avoid overhead watering. While downy mildew can spread quickly, good gardening practices can keep it in check and often prevent infection.
Cultural controls can prevent downy mildew from occurring or limit its damage if it develops. In vegetable and herb gardens, certain plant varieties resist downy mildew, including cucumber, basil, and melon hybrids and heirlooms. For example, the Southern Exposure Seed Exchange recommends growing the Suyo long cucumber because field trials showed its natural resistance to downy mildew. Also, make sure that plants have good air circulation, and look for signs of infection when temperatures begin to cool.
If plants have downy mildew, limit its spread by keeping the area clear. It thrives in cool, moist air and can survive winters on garden debris. In autumn, clear away dead plant material from the ground around affected plants. Destroy the debris by burning if burning is permitted in your area, or dispose of the debris in the trash. For especially hard-hit infections, you may want to use an organic or botanical fungicide. Spray the affected plants and the surrounding soil with a mixture of 4 teaspoons per gallon of water.