Two species of fungi can cause powdery mildew on cucurbits: Sphaerotheca fuliginea and Erysiphe cichoracearum. S. fuliginea is the more common. E. cichoracearum can, however, be common in areas with cool springs and summers. All cucurbits are susceptible to powdery mildew, but the disease is less common on watermelon than on other cucurbits. Powdery mildew first appears as pale yellow spots on stems, petioles, and leaves. These spots enlarge as the white, fluffy mycelium grows over plant surfaces and produces spores, which give the lesions a powdery appearance. Affected leaves become dull, chlorotic, and may wilt in the afternoon heat; eventually they become brown and papery. Plants may die.
Life cycle
All powdery mildew fungi require living plant tissue to grow. Most powdery mildew fungi grow as thin layers of mycelium on the surface of the affected plant part. Spores, which are the primary means of dispersal, make up the bulk of the powdery growth and are produced in chains that can be seen with a hand lens. Spores are carried by the wind to new hosts. All powdery mildew species can germinate and infect in the absence of water. Moderate temperatures and shady conditions are generally the most favorable for development. Spores are sensitive to extreme heat and direct sunlight.
Solutions
Control weeds and follow good sanitation practices. Avoid overhead sprinklers. A few resistant varieties are available. Applications of sulfur may be needed. Good coverage of undersides of leaves are required. Note that some cucurbit varieties are sensitive to sulfur.
For more information, see the Powdery Mildew Pest Note.
