Answer: There are a couple of possible explanations. Rhododendron flower buds are initiated during late summer and early autumn. A short period of very dry conditions at the roots at this time may cause a check in this process. The plants may subsequently fail to produce flower buds, or buds may only partially form, eventually drying up and dropping unopened, or being easily detached.
Prevent this by keeping plants well mulched, watering thoroughly and regularly during dry periods, particularly from July onwards.
The disease rhododendron bud blast (Pycnostyanus azaleae) is believed to be spread by the rhododendron leafhopper (Graphocephala fennahi) which is a sap-feeding pest that causes little direct damage. It lays overwintering eggs in slits cut in the flower buds, providing a means of entry for the fungus. Affected buds do not rot and can remain on the bush for two to three years.
Control by picking off and destroying affected buds, which can be recognised by the fringe of black fungal spores produced
Compare your rhodie to the above information to see if they match. I think feeding with an acidified rhododendron/azalea food will help your rhodie, as will a mulch of peat moss or pine needles.
Best wishes with your rhododendron.
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