A common sight in home gardens, quick-growing zucchinis (Cucurbita pepo) produce large leaves that almost completely shade the ground and the developing squash. Often, the leaves develop a white discoloration on their upper surface. Although the white spots may be natural, they could also signify a fungal infestation that could damage the plants or weaken your crop if you don't treat it immediately.
Natural White Spots
Zucchini leaves have natural white spots that develop on the leaf surface. Each leaf is covered in small hairs, which may give the white spot a powdery look. Rubbing the leaf with your thumb won't remove the white if it's from natural coloration. Natural white blotches have no negative effects on the health or productivity of your zucchini plants. Wear gloves when rubbing the leaves, because the hairs on the foliage and stems can irritate your skin.
White spots that rub off are usually caused by powdery mildew. Some zucchini varieties are more prone to this fungus than others are. The growth usually appears as a white or gray powdery growth on the upper leaf surfaces. The mildew prevents the affected leaf from absorbing sunlight, which weakens it until it turns yellow and dies. Although a minor infestation doesn't usually affect the plant, severe powdery mildew growth can kill enough leaves to affect the plant so it grows slowly, fails to fruit or dies off completely.
Planting mildew-resistant zucchini varieties, such as “Ambassador” or “Wildcat,” prevents almost all instances of powdery mildew in the home garden. Providing the plants with a full six hours or more of daily sunlight further prevents mildew growth, even if you aren't growing resistant varieties. Keep the foliage dry when watering or water early in the morning so the leaves dry quickly. Avoid overcrowding zucchini. Poor air circulation around the plants makes them more susceptible to powdery mildew.
Only a fungicide can combat powdery mildew after it infects the plant. Premixed horticultural oil sprays provide the best option for the home gardener. These sprays contain neem or jojoba oil, which kills the mildew while preventing future infestations. You can also sprinkle wettable sulfur on the plants before the first symptoms appear if mildew has posed a problem in the past. If you use sulfur as a preventive, you must wait two weeks before making an oil application. Only use oil or sulfur when temperatures are below 90 degrees Fahrenheit. To treat the plants, cover the leave surface with the oil or sulfur treatment and then reapply at seven to 10 day intervals so all new growth is also treated. Wear long sleeves, eye protection and a mask when applying any fungicides and store them where children cannot access them.