Powdery Mildew on Squash and Other Cucurbits

Powdery mildew is a common disease of cucurbits under field and greenhouse conditions in most areas of the world. Yields are reduced because of a decrease in the size or number of fruit, or a shortened harvest period. Premature senescence of infected leaves can result in lower market quality because fruit become sunburned or ripen prematurely or incompletely.

Symptoms are white, powdery fungal growth developing on both leaf surfaces, petioles, and stems. Symptoms usually develop first on crown leaves, on shaded lower leaves, and on leaf undersurfaces. Yellow spots may form on upper leaf surfaces opposite powdery mildew colonies. Older plants are affected first. Infected leaves usually wither and die. Plants may senesce prematurely.

Control - Fungicides should be applied every 7-10 days beginning early in disease development following detection through an IPM scouting program. Inspect plants weekly beginning in July and after fruit initiation (when plants become more susceptible). Examine upper and under surfaces of five older leaves at ten separate sites or until symptoms are found. Initiate a weekly spray program when symptoms are found.

Genetic resistance is used extensively as a control measure in cucumber and melon, and is being incorporated into other cucurbit crops. Yield potential should be considered when selecting varieties because some resistant summer squash varieties produce less fruit than susceptible varieties that have not been treated with fungicides. Resistant varieties of pumpkin and winter squash are under development. Successive cucurbit plantings should be physically separated because older plants can serve as a source of conidia.

Information for this article was taken from the fact sheet, "Powdery Mildew of Cucurbits" Cooperative Extension, New York State, Cornell University.