Have you been putting mushroom compost on your Rasps?
As far as treatment is concerned try adding an acid fertiliser containing ferrous sulphate or aluminium sulphate to the soil, to restore the nutrients the raspberries are missing.
You could also try using fertilisers which include chelated iron which are often recommended for rhododendrons, heathers and azaleas.
BBC - Beds, Herts and Bucks Why Don't You? - Gardening - Disease of the Week