This sounds like lichen, Fedup. It is harmless and indicates clean air conditions. I leave it alone on my shrubs. I dont know how you would go about removing it though.

As SBG says it sounds like lichen which is harmless to the shrub. As for removing I suppose you could pull it off but why bother? We have an old rhododendron that is covered in lichen - flowers beautifully every year.

This topic came up on GQT on Radio 4 today. It was explained that lichen is harmless and not parasitic at all, as it generates all its own 'food', so best to leave it alone on plants.

Quite a few of my bonsai and shrubs have a lot of lichen and algie on them.
It's a laborious process picking the lichen off and scrubbing the stems with a toothbrush (bonsai only), but I think its a fair trade off for clean air.

On the other hand it might not be lichen. A photo would be helpful.

I find that the stems of my small leaved acers do die off when covered with lichen. I think it's something to do with the fine branches being encased as it were, and no air can get to them. When I have removed it by hand, the place where it was is discoloured and looks scarred. So I think it can harm some plants, but I may be wrong:-)

No Bornagain, you are correct. Though this organism (actually two living in symbiosis) does not directly attack the plant, merely using it as a support, it can in severe cases cause problems to a plant as the breathing pores on twigs called lenticels can get blocked. This would have a detrimental effect on a plant.

Some plants have very obvious lenticels, just look at Birch trees, even heavy branches have them all over. Others are less obvious but still there on other types.

Thanks Fractal, vindication at last. I knew it was causing them to die off. I think my first question on here was about this problem. Do you think it best to cut out the worst affected branches and hope for re-growth?