White Powder on the Leaves of a Cucumber

Identification

• Powdery mildew is a white fungal growth that appears as mycellium, a whitish fuzzy or hairlike covering on the leaves and stems of cucumbers. It usually starts on the lower leaves. It can spread quickly between 50 and 90 degrees Fahrenheit, but prefers temperatures between 68 and 80 degrees. It favors areas with high humidity and plants with dense growth without adequate air circulation and light. Periods of heavy dew also encourage its spread.

Damage

• Leaves of the cucumber plant begin to wither and die when infected with powdery mildew. The resulting damage affects total yield and size of individual fruits. Powdery mildew affects the length of the harvesting season as well. Powdery mildew can inhibit the plant's ability to make food for itself via photosynthesis with its coating of mycellium. Further damage occurs because of its ability to spread easily to other plants by wind-blown spores and the splashing of leaves by rain and irrigation.

Control

• Control powdery mildew with well-timed fungicide applications. A combination of protectants and eradicants will yield the best results. Apply preventative sprays every seven to 10 days to help aid in the cucumbers' resistance to powdery mildew. These include a wide range of commercially available fungicides with many containing sulfur. Do not apply sulfur when temperatures are above 90 degrees Fahrenheit to avoid burning the leaves. Apply horticultural and biological oils after the disease has appeared to aid in control, but do not apply oils until two weeks have elapsed because a sulfur application or this can injure the plant as well.

Prevention

• The most effective prevention begins with the selection and purchase of disease-resistant varieties of cucumbers such as "Dasher II", "Eureka" and "Green Dragon Burpless." Proper site selection also aids in the prevention of powdery mildew. Select a site that receives full sun where your cucumbers can receive good air circulation. Do not over fertilize because this can cause abundant foliage growth that favors the spread of the disease. When possible, select slow-release fertilizers that deliver nutrients to the plant over the course of the season instead of all at once.