You find black buds on your rhododendron? Then the plant is probably suffering from bud browning or bud dieback: This is when the flower buds of rhododendrons brown over the course of winter and spring until they are black in color and no longer open and die. Dark, about 1 to 2 mm long hair-like fungal structures (coremium) are found on the bud surface.
Who is to blame: fungus or cicada?
Bud dieback is caused by a fungus (Pycnostysanus azaleae). This is usually transmitted when the rhododendron cicada (Graphocephala coccinea) lays its eggs and the wounds that occur with it.
The adults of the rhododendron cicada appear from about July to September. They are distinctly shiny green in color (about 1 cm long) and have some conspicuous orange stripes on their backs. They are quite active on sunny days, preferring to colonize the tips of shoots. The eggs of the cicada overwinter in the buds, in April the yellowish, flightless and very agile larvae hatch. The larvae as well as later the adults suck on the undersides of the leaves (consequence: partly bright speckles on the upper side of the leaves), also the moulting remains of the animals can be found here. The rhododendron cicada goes through only one generation a year.
The occurrence of the fungus is limited to the bud, thus it does not spread to the shoot and does not grow into it. Rhododendron cicada occurs mostly on large-flowered hybrids. Hairy as well as small-flowered species are less affected or not affected at all. Plants in unfavorable locations or with poor growing conditions are more easily infested.
Prevention and control
- Around July, you can hang yellow sheets between the rhododendron branches to trap some of the adults that are able to fly. The yellow color magically attracts the insects and they stick to the sticky coating (available at all garden centers).
- Also, carefully remove any infested buds (dispose of them in the residual trash, not the compost) by hand. To do this, simply break out the buds.
- Also see if you can optimize the site conditions (avoid sunny, drought-prone places). If necessary, transplant your rhododendron to a better place. Very important is the acid soil, low in lime and rich in humus. If the plants are well cared for, diseases or pest infestations are very rare.