Rhododendron bud and twig blight is caused by a fungus, Seifertia azalea. The flower buds turn brown and do not open in the spring. They are later covered by very noticeable black fruiting bodies called coremia the year after infection. Removal of the black buds is recommended if possible.
The rhododendron leafhopper, Graphocephala sp. is implicated in these infections by some researchers. It lays its eggs by making slits in rhododendron buds in the fall allowing entry of the fungus into these sites.
The feeding activities of the adults and nymphs have no obvious adverse effect on the appearance of the foliage or the plant's vigour. Female leafhoppers make egg incisions in next year's flowers buds in late summer-autumn. The fungal disease, known as bud blast (Seifertia azalea), is thought to infect the buds through the egg wounds, although RHS notes that some recent research has placed doubt on this assumption. The fact is: Infected flower buds turn brown and die.
The American Rhododendron Society notes Bud Blast does not seem to attack all plants. In fact, there are only a few, usually with R. catwabiense in their background, that are affected.
- We do not recommend chemical controls due to toxicity to other insects.
- Sunset Western Garden Problem Solver suggests control of leafhopper may be had by spraying the affected leaves with a strong blast of water from a hose. Natural predators of leafhoppers include birds, beneficial Green Lacewing larvae, and parasitoid wasps.
- Flower buds infected with bud blast should be picked off and disposed of to reduce the amount of fungal spores being released in the vicinity of the plants.
Rhs.org Rhododendron leafhopper and bud blast
Information compiled by: H. Nielsen, Vancouver Chapter