Symptoms
Rhododendron flower buds fail to open although they remain on the bush looking silvery. Later in spring they become covered with black 'bristles'. If the buds don't rot but remain on the plant for two or three years this is symptomatic of rhododendron bud blast - a disease particularly common in southern England.
Cause
The black 'bristles' are the spore-bearing bodies of the fungal pathogen Pycnostysanus azalae. Infection is believed to occur by means of the rhododendron leafhopper (Graphocephala fennahi), which lays its eggs in slits in the flower buds and so provides a point of entry for the fungus. Adult leafhoppers occur on the foliage from mid-July until October. They are about 8mm long with yellow head and legs, a blue-green thorax and forewings, the latter have two red stripes.
Non-chemical control
The best way of removing the source of the infection is to pick off and dispose of the infected buds. Do not add them to the compost heap.
Chemical control
There is no fungicidal treatment available. If the infection is a severe problem you could try to control the leafhoppers in late summer with an insecticide, such as bifenthrin (Bayer Sprayday Greenfly Killer Plus, Scotts Bug Clear Gun or Doff All In One Garden Pest Killer).
Confusion
Don't confuse this disease with the general bud browning that results from drought during the preceding late August to early September. In this case the buds turn brown and dry, and fall off unopened, or are easily removed. Mulching and watering in late summer should prevent this problem.