Answer: Those are classic symptoms of a common bacterial disease called halo blight. This disease thrives in high humidity with temperatures in the low 70s. It's mostly a problem east of the Rocky Mountains and is spread primarily by infected seed or plant debris left on the soil. Infection usually begins with small, brown, angular, water-soaked spots on the lower leaves. The spots later expand and the leaves turn yellow, sometimes producing the characteristic halos around the spots. The disease can eventually defoliate the plant and infect the bean pods.
If you notice the disease on young plants, pull them up, destroy them and replant. Consider growing less-susceptible varieties such as Blue Lake. If the plants are already producing beans when the disease strikes, harvest what you can, then destroy the plants as soon as production begins to decline. In the future, try the following preventive steps. Buy disease-free seed from a reputable company. Plant beans in the same ground only once every three years. Choose a spot where they'll get morning sun so foliage dries quickly. Use a wider spacing between plants for air circulation. And don't work in the bean patch when it's wet.
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