Now I prefer Runners to French, and my dear Misses vikki verki so we are all very happy here with two long rows of climbing long pods. The French climbers are nearly round like an oval pencil shape Mr B. the seeds inside like baked beans, not purple kidney beans.
Mr G, dig up a plant and inspect the roots. Normally a deluge, constant deluges of rain will only be a nuisance to the setting of the flowers. But it will wash out ground nutriants and trace elements. Runners can occasionally have their roots attached by a root grub, that bores into them like a small white maggot. Check for red ants also, they can eat away merrily. Wind rock, rotting the roots is normally only a problem in very exposed locations and when the ground is quite cold - normally associated with asparagus - not runner beans !
Yellowing ! - Have you got a shortage of nitrogen and potasium and phosphate nutriants in the trench ?
Well rotted horse muck and garden wood ashes are best. Failing that, all your garden and kitchen compost and composted lawn mowings is satisfactory.
I don't see this as wet rot, unless you're siting the bean trench in the location an old garden pond full of old slime and algae !
Most odd that all plants are affected by this blite ? Regetably it's probably too late for a cure. The season is very late this year.
I have the dreaded blackfly - when have't I had the curse of these tiny black gluey things ? Comes in on a strong souwesterly. The ladybirds only seem to like white or small greenfly ! bit like Goose really, he don't like the black Tannat tannic wines of Cahors