Cucumber plants are a vining vegetable plants that have large, flat and lobed leaves. As with most plants, their leaves are tender and susceptible to infection, and often they are the first signifiers of problems. A white or greyish-looking powdery substance on the leaves indicates the presence of powdery mildew, which can spread to the other parts of the plant if left untreated.
Powdery mildew first appears as circular white spots on the tops of the cucumber plant's leaves. The infection will begin as small spots and spread to cover the entire leaf. Over time, the powdery mildew also will infect the stems and fruit. If left untreated, powdery mildew will kill the plant.
Powdery mildew prefers wet conditions, although it is not necessary for water to be present on the cucumber plant for it to develop. Powdery mildew requires only humid conditions for it to take root. Because of this, close, crowded plants with little air circulation create the best conditions for powdery mildew to develop. Younger plants are more susceptible to the infection.
Chemical and organic sprays can be used to treat powdery mildew. Neem oil is the most common natural spray while potassium bicarbonate is a frequently used chemical spray. Chemical sprays should be used only if there is a very severe infection on the cucumber plant.
Altering the growing conditions of cucumber plants is a very effective way of controlling and preventing powdery mildew. Grow cucumber plants in well-draining soil that's in a sunny location. Properly space the cucumber plants to ensure good airflow between them. Alternatively, choose a cultivar that is resistant to powdery mildew.