Helleborus net necrosis virus (HeNNV ) is known as black death of hellebores in the UK. It is not widely known or observed in the US, but it is present in multiple states. It is believed to be carried on the feeding parts (stylets) of aphids. The latency period, from when the virus is transferred to when it expresses itself, can be as long as 18 months. It is not currently believed to be transmissible to other plant varieties.

The delayed expression of the virus means that symptoms are often noticed on older, well-established plants. Typical symptoms include:

• Black streaks following the lines of the leaf veins
• Mottling between leaf veins
• Stunted, distorted and  blackened new growth
• Black lines on the petioles and stems of the plant
• Black streaks on sepals (flower petals) and carpel (female reproductive structure)
• The damage becomes worse as the season goes on.

There are some more common hellebore diseases that have similar symptoms including the much more common fungal leafspot caused by Microsphaeropsis hellebori  (syn. Coniothyrium hellebori blackspot), and Botrytis cinerea. Hellebore leaves are also commonly "burned" by the cold winds of early spring. Fungal leafspot most typically causes concentric ring lesions on leaves and Botrytis cineraria the blighting of leaf and flower, but the symptoms are not always clearly differentiable from hellebore net necrosis virus which can produce similar symptoms. These fungal conditions are less serious and can be treated by removing and destroying the impacted plant tissue. Helleborus net necrosis virus cannot be cured and requires removal and destruction of plants.

The black death virus has been observed in Europe since the early 1990’s. Studies of the disease by Washington State University, examining strains found in the United States, determined that this disease is a Carlavirus, a group of viruses transmitted by insects for which plants serve as hosts and distinct from any previously encountered viruses. The insect vector for this disease has been assessed to be the hellebore aphid, which proliferates on hellebores. Not all aphid species are able to transmit all viruses and other aphid species have not yet been demonstrated to transmit this disease. It is believed to be carried on the feeding parts (stylets) of aphids but is not internalized (non-persistent transmission), thus the disease is carried for a period no longer than a few minutes.

Because the virus is transmitted on the feeding parts of aphids, preventing feeding is the most promising management technique in nursery greenhouse settings. There are systemic insecticides that are licensed for these settings in some states. Use of insecticides on home gardens to deter aphids, however, would not be effective or practical as a very short-interval repeat of application in an open environment would be needed.

We are not able to confirm a diagnosis from your photos as the symptoms vary and mimic other diseases.  If you notice symptoms on your hellebores, you can contact your county's cooperative extension office as a first step to arrange to have the plant material tested by their labs. If you need help locating their contact details, we would be happy to help. Remove the damaged plant material present. If new growth continues to develop with evidence of the virus, or if your extension lab diagnoses hellebore net necrosis virus, remove and destroy the plant.

Courtesy of NYBG Plant Information Service