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.
Do not overfertilize 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 bush snap 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.
Pick off individual leaves that show powdery mildew, or pull up plants and compost them to keep the disease from spreading.