A condition caused by a pest, the blackcurrant gall mite, which also attacks red and whitecurrants although less frequently. The buds swell in the spring and inside this protective environment the mites breed. When the bud bursts, the mites emerge and move on to infect other buds. The mites spread reversion virus, an infection that results in a drastic reduction in the crop. There is no known cure for reversion, and the bushes must be removed and destroyed. During the winter and early spring, routinely examine currants for big bud, which is common in some areas. Hand picking and destroying the swollen buds is the most effective control.