Posted: 17/05/2013 at 07:13
Probably chocolate spot - it's a fungus which enjoys wet overcrowded conditions - another year plant your beans a bit further apart.
It's a nuisance and may reduce the yield but hopefully you should still get a crop and the beans will be perfectly edible.
If your bean plants are flopping about a bit, try staking them with canes to allow air to flow between the plants - this will help reduce the spread.
And don't compost the bean plants - dry and burn them if you can or otherwise dispose of them.