Common Blight, Halo Blight, and Bacterial Brown Spot are the blights commonly seen in the MN/DAK area.  Leaf and pod lesions, defoliation, and shrunken discolored seed are the visible effects of the bacterial blights.

Bacterial blights are spread by rain splash and driving winds.  Blights can over winter on the bean trash and can be seed borne.  Plant quality "Western Grown Seed" or "Certified Seed" that has been field tested and lab tested to keep field blight levels low.

Notice that the left side of the row is ok and the right side is defoliated from the Bacterial Brown Spot.

Avoid cultivation when the bean foliage is wet. Copper sprays are the only thing labeled for blight control. Spraying copper, every 7 to 10 days may not prove to be cost effective in the MN/DAK area.

This could be mistaken for rust but it's not.  You can rub off rust.  Please check your fields.