Powdery mildew of Cucurbits

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 show some degree of wilting in the afternoon heat; eventually they become brown and papery.
The first sign of powdery mildew begins with small white lesions on both the top and underside of the leaves. Over the course of several days, the fungus spreads, and leaves quickly wilt, turn brown, and die.
Powdery mildew affects all plants in the cucurbit family — which includes pumpkins, gourds, cucumbers, melons, summer and winter squashes, and watermelon — so if you discover powdery mildew on pumpkins, keep an eye out on other plants in the same family.