Unless you see evidence of spider mites, my vote is for bean rust, a fungal disease. This is often spread by water splashing up on the plants from below (when there's no mulch covering the soil), or from spores that have drifted in the wind and have found optimal conditions in which to grow. Avoid working with the plants or picking beans when they're damp with dew or wet after watering. That would spread the disease from plant to plant. You can remove the infected leaves now to also prevent spread.

Hope that helps,
Sharon