I had some trouble with my 'climbing french beans', I noticed, what looked like mosaic virus on some of the older, lower leaves and when I checked under them, sure enough, as I suspected, there was clusters of black bean aphids that had transmitted the virus, also there was evidence of slug damage, as they seem to prefer old yellowing leaves.
Sorry I went off on a bit of a tangent there but I think that if your bean poles are thick enough, they may be able to get to the tops.