Hellebore black death

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 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.

Symptoms

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

Control

Non-chemical control

• 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

Chemical control

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.

Biology

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 newly identified species of virus, Helleborus net necrosis virus (HeNNV), is associated with the disease, which 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 which are still lightly infected and suitable for aphid feeding.

Symptoms not be confused with the general blackening caused by Botrytis cinerea or leaf spotting caused by Microsphaeropsis hellebori.

See also...

Hellebore aphid
Hellebore leaf miner
Hellebore leaf spot
Hellebores
Plant viruses