French beans - discoloured

[pops]

Some of my french beans have developed a black/grey mottled outer skin although others on the same row are perfect. Any ideas as I cannot find this 'disease' mentioned in any of my books.

[pops]

[redimp]

Mine have done this when they get to a certain size.  They are dwarf tendergreen.  I think they just do it, it does not affect the taste.

[pops]

These are affected from very young to mature - also as I said before, some on the same plant are excellent.

[jennym]

It may be wind/sun damage - I grow mine on a frame work made of two old rose arches with canes tied across to extend it and I notice discolouration at the edges, where the wind and sun get to them most.