Answer: It sounds like your beans contracted a fungal rot known as watery soft rot. I'm surprised the crop rotation didn't work -- the recommended rotation time to avoid this problem is 3-4 years. The organisms that cause the problem require moisture to grow, so your dry summer would seem to prevent the problem. But if dew was heavy and plants were spaced close together, the organism could have taken hold easily. If this problem recurs, cull the affected plants as soon as you see a problem. Stay out of the bean patch when the foliage is wet, as you can easily spread the disease. I wish you better luck with this year's crop.
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