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.

Symptoms

You may see the following symptoms (not to be confused with general blackening caused by Botrytis cinerea, or leaf spotting caused by Microsphaeropsis hellebori):

• 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 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.