Powdery Mildew

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.

Powdery mildew of cucurbits may occur at any time in coastal and desert production areas but is more common in fall in the San Joaquin Valley and Sacramento Valley. 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 high humidity (50 to 90%), and disease development is favored by vigorous plant growth and moderate temperatures.

Plant resistant varieties, follow good sanitation practices, and control weeds. Start monitoring for powdery mildew during the vegetative growth stage and continue through fruit development. Carefully monitor even those fields with powdery mildew resistant varieties, because there is evidence that plant resistance-breaking races are present in California. Strains resistant to strobilurins (group 11) have already been found throughout the state. If multiple fungicide applications are needed to control powdery mildew, alternate materials with different modes of action especially if using fungicides with medium to high resistance potential (azoxystrobin [Quadris], myclobutanil [Rally], pyraclostrobin [Cabrio], pyraclostrobin/boscalid [Pristine], trifloxystrobin [Flint,] and triflumizole [Procure]). Apply a treatment when disease symptoms first occur and repeat if symptoms reappear.