Blackcurrant Big Bud Mite. This mite can cause the normally arrow-shaped dormant buds to become spherical. Individual buds can be picked off in light infestations, but there is no cure for the inversion virus that the mites carry.
PLEASE dig up and burn old and infested bushes. New members in particular should be wary of taking over infected bushes, often buried under mounds of couch grass - they are simply not worth the risk of saving