Hellebore problems, diseases and pests and disorders
By Howard Drury
Hellebores are generally easy to grow tough plants that suffer few pests and diseases or indeed other problems. Most problems seem to arise from trying to grow Hellebores in unsuitable plance such as ground that is too wet or where they get too hot and little shade in the summer. Paying attention to good hygiene and solving problems at an early stage are important.
Black Spotting of foliage
Hellebore leaf spot (Coniothyrium hellebori)
Large, irregular brown or black spots appear on the leaves and stems. These often coalesce, resulting in yellowing and death of the leaves. Spots also occur on the flowers and lower stems, and infected stems may wilt above the point of attack so that flower buds fail to open. Tiny black fruiting bodies may sometimes be visible within the leaf spots, particularly on the undersides.
A common and damaging fungus disease that particularly affects the Christmas rose, Helleborus niger.
Pick off all affected leaves and flowers as soon as possible and either bin or burn them.
Propagate only from healthy material.
There are no specific fungicides available to amateur gardeners with specific recommendations for the control of this disease. Fungus Fighter and Fungus Clear are labelled for the control of some other similar diseases on ornamental plants, and may give some control of hellebore leaf spot.
Hellebore black death
Swift action is necessary to help try and prevent the spread of this serious hellebore disease.
Cultivars and hybrids of Helleborus x hybridus.
The main symptoms are black streaking and mottling of the tissues along or between the leaf veins. The black marks may be in a ringspot pattern or in lines, which pass down the leaf stalks to the main stem of the plant. The blackening is associated with serious distortion and stunting of the leaves and stems, and the flowers may also be affected.
Hellebores suffer from several viral diseases. Hellebore black death caused by a recently identified virus (Helleborus net necrosis virus) produces black streaking and mottling of leaves, stems and flowers. The blackening is associated with serious distortion and stunting of the plant. Cultivars and hybrids of Helleborus x hybridus are particularly affected. It is believed that this virus is transmitted by the hellebore aphid, Macrosiphum persicae.
Cucumber mosaic virus, a widespread virus, can also affect Helleborus species causing yellow to white line patterns in leaves. It is transmitted mechanically or by aphids (Myzus persicae).
There is no cure for virus infections. Dig up all infected plants as soon as symptoms are noticed and destroy them immediately to reduce the risk of spread.
As aphids are often responsible for transmitting viruses it is good practive to control aphids at the very first signs to prevent transmission of viruses, there is a specific Hellebore AphidMacrosiphum hellebori which is a sap sucking greenish white aphid 2-4mm long that can form dense colonies on the underside of leaves in late spring and early summer. The honey dew aphides excrete can easily become a source of food for black sooty moulds, debilitating disease that limit a plants growth.
Cultural controls are not practical and it may be necessay to apply insecticides to stop the problem and restore plant health.
Hellebore Leaf Miner Phytomyza hellebori
This relativiely new pest was first discovered on Helleborus foetidus in the 1990's but has since been seen on more species and many hybrids causing considerable disfigurement to the underside of foliage. It often appears in late winter or early sping and can build up to be a major problem to plants by mid summer. Removal of infected leaves is a good plactive as the success of chemicals is limited due the burrowing nature of the pest and the fact many Hellebore leaves are glossy, meaning the chemical is not readily absorbed. Bug Clear Ultra has been noted to have some effect.
Occassionally seen in late summer and causing disfigurement resulting in poor groowth. Spray with an insecticide as early possible when first seen.
Hellebores are also known to be susceptible to root decay and in particular to the soil-borne pathogen Phytophthora. As this fungus thrives on waterlogged soils, it is best to improve drainage to inhibit its growth. There is no soil sterilant that is effective in killing the fungus which may form resistant structures to survive in the soil for long periods.
Hellebore species and cultivars maybe beome infected with a Hellebore cyst eelworm which is very difficult to control other than by practising sanitory removal and starting with fresh stock in another area as cysts can remain active in the soil for some years.
specific mildews attack many species of plants, one is thought to attack Hellebores and can easily be mistaken for other plant problems such as frost damage or nutrient imbalance. Leaves may become mottled and chlorotic, resembling nutritional deficiencies, but generally the lesions are angular and bounded by major leaf veins. Downy mildew sporulation develops on the underside of leaves and is usually gray, brown or purple and has a downy texture
Home gardeners in the uk have few chemicals availabe today as fungicides and the downy mildews are more difficult to control than the more common powdery mildews such as seen on roses. The alternative is to improve cultural conditions and remove any infected foliage and destroy it.
For several years growers had reported non flowering of Hellebores, however after more investigation it is possible to see a patern emarge. Most plants were left unpruned and often in meadows or wildlife areas where the buds are particularley attractive to mice when food is short in winter. This is perhaps one good reason for removing the foliage of acualescent types in December / January to remove cover for this pest. Trps might also be employed along with more planned careful management of wildlife areas. Mice will also devour young seedlings in pots and in the garden in spite of being slightly poisonous.
(C) January 2019 Howard Drury