Cucumbers belong to the cucurbit family, a family that also includes squash, melon, pumpkin and watermelon. All of these garden plants are susceptible to a damaging fungal disease called powdery mildew, which turns the leaves of the plant white with spores before eventually killing them. Powdery mildew can be difficult to treat if it isn't caught early. Even if it is treated, once the spores are present they can persist in surrounding plants and weeds, causing further outbreaks in the garden.
Powdery mildew is easily confused with another fungal disease called downy mildew. On cucumber, the culprit is likely powdery mildew, which appears in hot, dry weather when days are extremely warm and nights are cool. Downy mildew is common during cool, wet weather. Powdery mildew on cucumber is caused by two species of fungus, Erysiphe cichoraceanum and Sphaerotheca fuliginea. S. fuliginea is the more aggressive of the two species, according to Cornell University, and tends to progress rapidly in warm weather.
Cucumbers develop on twining vines that, ideally, lift the fruit of the plant off the ground where they are shaded by the large leaves of the plant. Powdery mildew attacks the leaves that shade the fruit, causing sunburn of the fruit in some cases and deformity of the fruit in others. A plant that is completely defoliated cannot photosynthesize, which will eventually kill the plant.
Fungicides are the only method of control on cucumber plants, but in order to be effective, cucumber plants need to be treated as soon as symptoms appear. According to North Dakota State University, fungicide sprays containing one of a few active ingredients, including benomyl, chlorothalonil and dinocap, are effective. The sprays should be reapplied every seven to 10 days after the disease develops.
According to Cornell University, powdery mildew is less common on cucumber plants because there has been successful development of disease-resistant varieties. When selecting your transplants, always select disease-resistant varieties. In addition to the variety you plant, make sure the plants are spaced adequately and that there is plenty of room for air and light movement throughout the plant. Plant cucumber plants in full sun. According to Cornell University, rain and free moisture do not favor development of the fungus, so consider overhead watering in the morning, leaving plants plenty of time to dry out during the day.