The mite Epitrimerus gibbosus causes a white or greyish felty erineum along the veins of the bramble leaf undersides, and a corresponding discoloration on the upperside. 

Other organisms can cause discoloration of the leaf and whitish growths.  

Check that the growth along the veins is an erineum - a carpet of small outgrowths of the leaf surface, in which the mites live.  

Include a note of the host species

See Redfern & Shirley 2023 British Plant Galls for more info