Hellebore Diseases

Hellebores (Helleborus spp.) are a group of perennial plants that often serve as ornamental additions to decorative gardens in temperate regions of North America. While they are not native to the United States, several species have become established in the mid-Atlantic and Pacific Northwest regions of the country. Hellebores are not particularly disease-prone, but they do suffer from a handful of diseases caused by fungal and viral pathogens.

Hellebore black death is possibly the most serious disease that impacts hellebores in North America. According to the Royal Horticultural Society, this disease has been tentatively ascribed to an infection of the hellebore net necrosis virus. Symptoms of this ailment include distorted growth and discolored leaves. Unsightly black lines mark the surface of the leaves of infected plants. As the disease advances, the black markings on the leaves expand and may extend to other new growth, including flowers, buds and stems. The only way to manage an outbreak of hellebore black death is to remove and destroy infected plants.

Black spot disease of hellebore is caused by the fungus Coniothyrium hellebori. Symptoms of this condition include dark discoloration of foliage, which is easily confused with the damage done by the more serious black death disease. The black spots from this fungus begin near the edges of the infected leaf's surface and are generally circular or ovoid. The disease can spread to healthy foliage and to other plants relatively easily, so it is important to prune infected leaves quickly.

Hellebores are also vulnerable to the Botrytis blight, also called gray mold, which is caused by the Botrytis cinerea fungus and related fungi. This blight disease is highly contagious and can affect hundreds of tree, shrub and herbaceous plant species. It is a significant threat to commercially cultivated crops and nursery plants. It can attack nearly any type of plant tissue, including stems, leaves, flowers and fruit. Initial symptoms include wilting or deformed growth and small brown spots on flowers and leaves. As the fungus grows, it produces visible reproductive bodies -- which are usually gray or light brown -- on damaged plant tissue.

Hellebore leaf spot, caused by the Microsphaeropsis hellebori fungus, is another common disease of hellebores in North America. According to the Royal Horticultural Society, it is particularly virulent to the black hellebore (H. niger), but is not as destructive to Corsican hellebore (H. argutifolius). This fungal pathogen damages both the leaves and stems of its host, leaving patches of decaying tissue that can cause structural damage to the plant. Damage to leaves causes them to wilt and fall to the ground. The reproductive bodies of the fungus emerge as tiny black spots on the brown-colored infected regions.