Runner Beans Dying why?

[fluffy]

Hi, don't know if anyone can help but I'm having trouble with my Runners. I took over this allotment end of the season last year, I put 50 Runner plants in & I keep having to replace dead ones with live ones. I know at the lower end there was lots of ant activity so been using ant powder & left the bottom half of the row for planting till last. Problem is, the plants keep dying but in sporadic areas. Most of the ones that die are at the bottom but, I've had plenty die in the middle & one at the top who was doing really well. The plants develop holes in the leaves & look like they're being eaten but can't tell what by. It looks like slugs but then no slug trails & there's holes in the middle of the leaves, also, there's no sign of any other critters eating them & the ant activity just isn't there. I've also had the whole plant just turn yellow & die. The latest one was at the top of the row & he was half way up the support before he just went. I've pulled it up & there's not much of a root system there. The plants have been fed on Miracle Grow, Fish blood & bone & various good feed. I don't feed them all the time just every so often. I have been told by the previous person working this plot that he used to grow Runners all the time where I put my set then, one day someone was spraying the path with round up & it was windy so the stuff got on his runners & killed them. Since then he's had problems growing them there so moved them to another part of the plot & had no problems. He said this is why the dead plants are somewhat sparodic in their placements. What do you think? What can I do? The ones that have started flowering are looking ok but the others well, who knows what'll happen. Your opinions greatly welcomed!

[Tom J]

i'm no expert but it does sound to me like there are still traces of roundup in the soil