I am sorry to hear your rhododendron plant is affected by bud blight (also known as bud blast). Bud blight is caused by the fungus, Seifertia azaleae. The fungus infection spreads to the terminal flower buds through overhead irrigation, rain, and insects. Affected terminal buds will turn black and fungal fruiting bodies will appear. The infection spread through fungal fruiting bodies called synnemeta.

There is no known treatment to permanently treat bud blight in rhododendron plants. Once a plant is infected with bud blight, it is only possible to control not eradicate the infection. If your plant is near other rhododendron or lilac plants that are not infected, I would recommend removing the plant. If the infected plant is not near other lilac or rhododendron plants, you can keep the plant if good cultural practices are used. I have included a few helpful recommended cultural management practices to help control bud blight in your rhododendron. 

Suggested Cultural Management Practices:

-Clean your tools prior to and after touching the infected rhododendron
-Remove and destroy infected flower buds as soon as noticed. Pick up infected terminal buds from the ground.
-Do not compost or mulch the infected flower buds.
-Avoid wetting the rhododendron buds while watering. Water with a drip, microsprinker, or soaker hose instead. Irrigate earlier in the day to encourage the foliage to dry.
-Keep your plant healthy overall. Keep your plant correctly fertilized, pruned, and watered.

Sources:
Rhododendron-Bud Blight

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