Answer: I would check the leaves closely to see if they have aphids or spider mites, both of which can defoliate a plant. Spider mites are so tiny that their most visible sign is mottling and yellowing of leaves and webs on the undersides of leaves and where the leaves attach to the stems. Aphids are easier to see. They are greenish to brownish pear-shaped insects that cause curling leaves and weakened growth. Both insects also attack flowers.
Another possible cause of your beans decline is insufficient water. Consistent watering is important when the beans are forming. Mulching the soil around the plants will help conserve water. The plants could have a nutrient deficiency. In general, yellowing foliage indicates a need for nitrogen and purplish leaves indicate a need for phosphorus. If other plants are also not growing well, it would be worth having your soil tested. Good luck.
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