Don't know for sure where you are located, but that looks like bean rust and/or bacterial blight. The best way to manage is to plant resistant/tolerant varieties. Unfortunately, most commercial beans don't have very much.

http://www.extension.umn.edu/garden/yard-garden/vegetables/edible-bean-disease-and-disorder-identification/

This post was edited by fusion_power on Sat, Nov 22, 14 at 21:57