raspberry problem??

Planted 3 types of raspberries this year and had a small crop, late variety did quite well. Have recently noted that the underside of the leaves are covered in small brownish black dots. It looks a little like tiny eggs but if you rub your thumb across them you can't feel anything and it almost seems to smudge as if it's peat. I realise the late variety will be cut to the ground soon and I can burn them but the earlier varieties will remain. Does anyone have any idea what this is and if so do I need to treat it and with what? Grateful for any help.

Regards Sheelagh :luv: