Hellebore black death is a serious disease of hellebores, probably caused by a newly discovered virus, where plants become stunted, deformed and marked by black streaks and ring patterns.
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 newly described virus called hellebore net necrosis virus.
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.
You may see the following symptoms:
- 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 for about two decades in the UK, but is becoming progressively more serious. It is also known from mainland Europe and North America.
A new species of virus in the Carlavirus family has been detected in the UK by RHS scientists working with Horticulture Research International, and in the USA where the name Hellebore net necrosis virus (HeNNV) has been proposed. It is thought that this is the likely cause and also that it is transmitted by aphids, especially the hellebore aphid, Macrosiphum hellebori, but 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 which are still lightly infected and suitable for aphid feeding.