Big bud is a condition caused by microscopic mites known as Blackcurrant big bud mites (Cecidophyopsis ribis). It can be identified by cutting open a swollen bud. When examined under a microscope, the mites can be seen.
The mites live and breed in the buds, starting in July and remaining in the buds until the following spring, when they migrate to the leaves. The cycle then begins again.
Blackcurrants are susceptible to Big Bud. Redcurrants and gooseberries may also be attacked by the mites but will not exhibit swollen buds.