Why are my hellebore leaves turning black

Most often seen in winter and on new growth in spring, the blackening of hellebore leaves, stems and flowers is known as Hellebore Black Death. It is a serious disease, thought to be caused by hellebore net necrosis virus. This virus is most likely spread by the hellebore aphid. All parts of the plant become heavily marked and streaked with black, sometimes following the veins in leaves and petals. One of the early signs is a coarsening and puckering of the leaf surface around the veins.

The blackening of the plant tissues may be so extreme that it causes the stems and leaves to become twisted and distorted. New growth is also badly affected and in some cases the new leaves barely develop at all. The plant will eventually be weakened and wither away. Helleborus x hybridus appears to be most badly affected, but this may simply be because this and its varieties are most commonly planted.

What can I do about black leaves on hellebores

The only thing that can be done is to dig out the infected plants and bin or burn them as soon as they are seen. This should help to limit the spread of the virus to neighbouring hellebore plants by aphids. This can’t be guaranteed. There is no chemical treatment for Hellebore Black Death. Don’t mistake this for hellebore leaf spot which consists of distinctive reddish or brown spots on the leaves, most especially noticeable in autumn.

Leaf spot is caused by a fungus which is able to overwinter on the foliage from where it can re-infest soft, new growth in spring. As its name implies this fungal disease causes distinctive spotting on the leaves, particularly of H. niger and H. x hybridus. The spots are generally brown in colour, but may be edged with yellow or reddish tones as the tissues are gradually infected. This disease can be limited by cutting off the affected leaves in the autumn and mulching the surface of the soil around plants to prevent any spores splashing up onto new spring leaves.

How to keep hellebores healthy

When planting new hellebores, it is a good idea to improve the soil with the addition of plenty of well-rotted garden compost. This will help the plants get well established with a good root system. Make sure that you incorporate the compost to at least the depth of a spade and mix it well with the soil. Where possible plant hellebores where they get plenty of good air circulation and sun for part of the day. This will allow the foliage to dry off quickly and reduce the risk of fungal leaf spot infection.

Unfortunately this won’t protect hellebores from the viral infection that is thought to cause Hellebore Black Death. The aphid populations which most likely transfer the virus can be limited by a healthy populations of birds, ladybirds, lacewings and other beneficial insects. Soft foliage is most liable to aphid damage.

Should I feed hellebores

It is also worth feeding your new and existing plants with a fertiliser that contains plenty of potash. This will not only encourage the plants to produce a good crop of flowers, but it also toughens up the foliage which makes it less palatable to the aphids that spread the hellebore net necrosis virus. Good sources of potash are well weathered wood ashes, sulphate of potash or even liquid tomato fertiliser. The general-purpose granular fertiliser Vitax Q4 is high in potash too.

The solid or granular forms of potash can be applied twice in the growing season – once when plants start into new growth and then again in early summer. If you are using a liquid tomato feed, then make 4 applications from April through to July.

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