Certified GKH Gardening Expert
This can be caused by several things. If they have been producing for awhile, they may have just come to the end of their run. It could also be a water issue. How is the drainage? The soil may not be draining well enough, resulting in yellowing. This can also be due to too much water. The addition of organic matter can often improve drainage problems in soil. Too much nitrogen is another possibility.
Excessive moisture is an ideal environment for the development of fungus as well. While fungicides are available, timing of the disease is crucial in order to save the plant. Root rot can cause bean leaves to turn yellow. To check for root rot, carefully pull out one of your bean vines and check the roots. Healthy roots should be white.