On Crops: Pumpkins, squash, cucumber, melons
Warm to temperate climates worldwide
White deposits develop on the top sides of leaves that make them look like they have been dusted with flour. As this fungal disease advances, leaves become grayish-green and become dry to the touch. Eventually these leaves turn brown and fall off. Powdery mildew is most likely to infect older plants that are beginning to decline after producing a crop. Warm temperatures between 70 and 80F (21 to 27C) favor cucurbit powdery mildew, but rainy weather is not required to trigger an outbreak.
Powdery mildew fungi clog up leaf pores and block light to photosynthetic cells, so the plants are weakened in their ability to use light as an energy source. New growth stops, old leaves fall off, and the plants struggle to stay alive. Pumpkins or winter squash produced by mildewed plants may lack flavor, and quickly lose quality in storage.
Use resistant varieties when they are available. Thin plants to proper spacing so each leaf gets good exposure to sun and fresh air. Plant fast-growing varieties of summer squash to sidestep this disease. Starting in early summer, spray plants every 10 days with a mixture of one part milk (any kind) to four parts water.
Pick off individual leaves that show powdery mildew, or pull up plants and compost them if they are no longer producing a good crop.