The broad beans have developed chocolate spot.

I hadn’t encountered of this before although it would appear that it is a common fungal disease that broad beans are susceptible to. It initially looks similar to broad bean rust so you may want to be clear in your diagnosis as there are differences in the impact of each.

The RHS site has a great explanation of both diseases http://www.rhs.org.uk/advice/profile?pid=89

But briefly:

• Damp and/or wet conditions seem to exacerbate the problem
• Chocolate spot can kill the whole plant – rust apparently won’t
• Once you have either there seems to be limited control or elimination – except pulling up all the plants at the end of the season and destroying them
• Don’t use the seed from plants that have been affected by either disease

However, as far as I can see, neither disease stops you eating the beans from an affected plant but I leave that bit up to you.