This is a fungal disease which can have a devastating effect on the blossom of Rhododendrons and Azaleas. It is asociated with infestations of the Rhododendron Leafhopper which acts as a vector. The infection may enter via the wounds made as they feed or when the females insert their eggs into the tissues of the plant. Starting in the autumn and progressing over the winter, affected buds gradually discolour. By spring they are dry, brown and covered with characteristic 'bristles'.
As the disease progresses it produces black, bristle-like fruiting bodies (coremia) on the surface of the bud, by this time the damage is done and cannot be treated. The fungus can remain active in the dead bud for more than three years, so it is essential to remove them - if practical.
Some cultivars appear to be more resistant, but the only treatment is removal of affected buds as soon as possible. Check with neighbours, as their Rhododendrons are most likely affected as well and will harbour the fungus. The Leafhoppers become active in mid-summer, so spraying against them may be effective in preventing the spread of the disease
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