This plant disease is caused by a fungus (Uromyces phaseoli) that affects only bean plants. It is most common on mature plants, and most damaging to pole beans and lima beans. Scarlet runner beans are sometimes mildly affected. Each rust spot develops thousands of spores that are spread by wind and splashing water. The disease develops rapidly during periods of cool nights and warm days. High humidity from rain, dew, or watering also encourages rust. Heavy vine growth that shades the ground and prevents air circulation produces ideal conditions for the disease. At the end of the summer when the nights are longer, the fungus produces another type of spore. This thick-walled black spore spends the winter on infected bean plant debris.
Apply a fungicide containing chlorothalonil to beans at the first sign of the disease. Weekly applications may be necessary if conditions favorable to the disease continue. Avoid overhead irrigation. Water in the morning rather than evening to allow wet foliage to dry quickly. Thin seedlings and space plants far enough apart to allow air to circulate freely. In the fall, remove and destroy all infected plants to prevent the fungus from surviving and reinfecting in the spring. Do not plant beans in the same area more often than every third year unless you plant resistant varieties (see Beans Resistant to Rust).
Plant Care for Beans