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Melon Disease - Powdery Mildew

Fact sheet 2009, Powdery Mildew in Melons Reference: UC IPM: SYMPTOMS 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. COMMENTS ON THE DISEASE Powdery mildew of cucurbits may occur at any time in coastal production areas but is more common in fall in the San Joaquin Valley, Sacramento Valley, and desert production areas. The disease may be caused by one of two species of mildew fungi, although E. cichoracearum is rare. Several races of each fungus also exist. The pathogens generally overwinter on weeds and their spores can be carried long distances by air currents. Infection is favored by water on the plant surface or high humidity (50 to 90%), and disease development is favored by vigorous plant growth and moderate temperatures.

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