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In general hellebores are relatively trouble-free, but it pays to be aware of possible problems.
Aphids A wide range of different aphids attack hellebores, including its very own species. They can be found inside the flower, this is often the first infestation of spring, on new and under old leaves. The first sign may be a patchy glossiness to the foliage from honeydew dripped down from flowers and foliage above, and a few empty white skins. Seedlings are also sometimes infested and growth may be slowed down as a result. Aphids also transmit virus diseases.
A contact spray based on fatty acids is effective but needs careful application, a systemic insecticide needs less precise application but open flowers should not be sprayed or bees may be harmed.
Black death A relatively recent, slightly mysterious, but highly destructive disease which shows as black foliage blotches bounded by leaf veins or by the veins in bracts. Black streaks may also be seen in flowers. A virus is the most likely culprit, in which case keeping aphids under control is a wise precaution. It may also be spread on secateurs. Digging up and burning infected plants is the only safe remedy, but in general this disease is more common in large collections and on nurseries than in gardens.
Black spot This is the most frequent and damaging of the diseases which infect hellebores. Black or brown blotches appear on the foliage, merging to create dead areas of the leaf; other parts of the plant turn yellow and much of the foliage and flowers may be damaged. Flowers can be devastated and a year’s display completely ruined. If the disease is present on the foliage, it is transferred to the emerging flower shoots as they grow through the leaves in winter. In some cases buds may rot and the flower stems collapse.
Always more prevalent in warm wet conditions, removing much of the foliage of H. hybridus and similar species in autumn greatly reduces the chances of infection of the flowers and the carry over of disease.
It usually pays to give two preventative sprays of a product containing mancozeb or myclobutanil, at two weekly intervals, both in the run up to Christmas and again in the New Year.
This disease infects the taller caulescent species in a slightly different way. Often, it attacks the flowering stems at the base during the winter and the stems rot before they can mature. Lesions may sometimes also appear higher on the stems and the upper foliage can also be damaged. Spraying the base of the plant undoubtedly helps, and the prompt removal of stems immediately after flowering combined with spraying the base of the plant immediately afterwards is usually effective.
Damping off Seedlings rot off at soil level and collapse. Caused by a variety of soil borne fungi, usually as a result of re-using old seed compost, also watering with water from a rain butt, badly compacted (and so waterlogged) compost, poor drainage under seed pots or physical damage to the seedlings during pricking out. Avoid all these mistakes. Water with a copper fungicide at the first sign of trouble.
Mice Mice can be troublesome in two ways. They can eat hundreds of seedlings in a single night and they also eat the buds and flowers of mature plants, often leaving distinctive neat piles of debris at the base of the plants. Traps, and a good cat, are the answers.
Slugs and snails These pests have two modes of attack. Seedlings are devoured in their pots; covering the sown seed with sharp grit (and then a pot cover) is a good preventative and pellets or other control methods should be in place alongside the seed pots from the first. Slugs, and small snails, can also climb the plant and eat the buds and stamens; they are sometimes seen inside the pendulous flowers of H. hybridus in wet spells. Flower damage is rarely extensive.
Smut An uncommon disease, but sometimes seen in large collections. The leaf or flower stems split vertically to reveal black dust-like spores. There is no treatment, other than to carefully cut off and burn infected parts of plants.
Vine weevil Increasingly common in many garden situations, mature garden plants usually tolerate infestation without revealing symptoms but vine weevil can be troublesome when plants are grown in pots for extended periods. Growth is poor, top growth may seem poorly anchored and may eventually simply break away from the roots when the grubs have eaten through the roots. Biological control is effective in consistently warm temperatures as is an insecticide containing imidacloprid.
Virus Apart from Black Death (see above), the occasional virus symptoms seen are unusually shiny, stiff and jaggedly toothed foliage combined with a general loss of vigour. Dig up and burn such plants.