Host Plants:
On Crops: Beans and some clovers and flowers
Where Found:
Warm to temperate climates worldwide
Description:
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 ghostly white, shrivel and fall off. Powdery mildew is most likely to infect older plants that are beginning to decline after producing a crop. Warm temperatures between 21 and 27F (70 to 80F) favor powdery mildew, but rainy weather is not required to trigger an outbreak.
Damage:
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.
Preventing Problems:
Do not overfertilise beans, which can invite problems with this disease. Thin plants to proper spacing so each leaf gets good exposure to sun and fresh air. Plant fast-growing French beans two or three times, three weeks apart, and pull up old plants as soon as production slows. Compost old bean plants and fallen bean leaves. Use resistant varieties in areas where beans have persistent problems with powdery mildew.
Managing Outbreaks:
Pick off individual leaves that show powdery mildew, or pull up plants and compost them to keep the disease from spreading.
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