Hellebore black death
Hellebore black death is a serious disease of hellebores, probably caused by the virus Helleborus net necrosis virus (HeNNV), where plants become stunted, deformed and marked by black streaks and netting patterns.
Scientific name: Helleborus net necrosis virus (HeNNV)
Plants affected: Helleborus spp.
Main symptoms: Stunted and deformed growth, black streaks on leaves
Caused by: virus
Timing: mid-spring to summer
What is hellebore black death?
The disease known colloquially among hellebore growers as ‘black death’ causes stunting, distortion and black streaking and netting patterns on the leaves. It is probably caused by a virus called Helleborus net necrosis virus (HeNNV).
In the UK, the most seriously affected hellebore is Helleborus × hybridus (syn. H. orientalis) but similar symptoms have been seen in other species. New damage can be expected from mid-spring.
- Plants show stunting and distortion of the emerging new growth, the damage becoming progressively more pronounced as the season progresses
- Patterns of black streaks develop on the leaves, often following the veins, sometimes as rings
- Black streaks may also develop on stems and flowers
- All infected plants should be dug up promptly and destroyed
- Many viruses are not transmitted through seed, so raising new plants from seed is a possible way for gardeners to ensure disease-free plants
There are no chemical controls for plant virus infections.
Control of aphid vectors is not feasible with the products available to amateur gardeners because these are non-persistent and would need to be applied at unrealistically short intervals to give any protection against the arrival of winged aphids.
This disease has been recognised since the early 1990's in the UK. It is also known from mainland Europe, North America, New Zealand and Japan. A virus called Helleborus net necrosis virus (HeNNV) is associated with the disease and believed to be the cause. The virus is thought to be transmitted by the hellebore aphid, Macrosiphum hellebori. However, conclusive evidence for both suspicions is still lacking.
Aphids transmit viruses by feeding on the sap of plants with virus infection, and thus contaminating their mouthparts with virus particles. When they fly to healthy plants and begin to feed, they then infect the plant with virus. Seriously infected plants are so stunted they are unlikely to be attractive to aphids and the most dangerous plants are probably those that are still lightly infected and suitable for aphid feeding.
The Royal Horticultural Society is the UK’s leading gardening charity. We aim to enrich everyone’s life through plants, and make the UK a greener and more beautiful place.