Downy Mildew

Irregular brown or yellow spots on leaves are an early symptom of this fungal disease, which can infect many vegetable crops. Check the undersides of affected leaves; presence of a hairy white or purple mold confirms the diagnosis. Severely infected leaves may die, and the disease can eventually spread to stems, flowers, and fruit.

The fungus overwinters on plant debris. Downy mildew is a host-specific disease. Thus, if your cucumber plants become infected, that doesn't necessarily mean your onions will succumb.

Prevention and Control

• Grow mildew-resistant varieties such as Fanfare cucumber and Allstar melon. Late-season crops tend to be more vulnerable to downy mildew.
• Plant disease-free seeds and plants.
• Space plants widely to promote good air circulation. This will help leaf surfaces stay dry, which inhibits infection.
• Keep foliage as dry as possible by using drip irrigation or soaker hoses rather than overhead watering.
• Control weeds, such as wild mustard, that can harbor the disease.