Bean Pest

There a few too many possibilities for the trouble with your beans. One is spider mites, which leave this tell tale pattern of tiny dots and yellowing leaves. The other is a cultural condition, too much water, too little drainage. In addition, there a number of fungal disease that are transmitted to beans via small insects that suck on the leaf. It isn’t the insect that is big problem, it is the disease they vector. If the plant is seriously in decline, remove it an don’t put it in the compost. If it tends to ‘get better’ with time, remove affected leaves, and continue to grow the bean. Its always hard to tell whats happening if you can’t see the actual pest. Here’s some information on bean pests from University of California: http://www.ipm.ucdavis.edu/PMG/GARDEN/VEGES/beans.html
Let us know if we can help some more. Send a good close-up if you find something. Or take a sample to a local nursery for id and control measures.