Caused by a virus, which is introduced by the Big Bud Mite. It can first
be identified by some of the leaves being abnormal. Affected leaves are
narrower than normal and they have four or less pairs of veins.
easily identified are the buds which turn bright magenta rather than the
normal grey colour.
There is no cure. Dig up and burn the bushes. Do not plant new ones in the
same position for five years or more.