Advice
RHS Help & Advice
Hellebore leaf spot (Coniothyrium hellebori)
Symptoms
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.
Cause
A common and damaging fungus disease that particularly affects the Christmas rose, Helleborus niger.
Non-chemical control
Pick off all affected leaves and flowers as soon as possible and either bin or burn them.
Propagate only from healthy material.
Chemical control
There are no fungicides available to amateur gardeners with specific recommendations for the control of this disease. The fungicides myclobutanil (e.g. Systhane Fungus Fighter) and penconazole (Scotts Fungus Clear) are labelled for the control of some other diseases on ornamental plants, and may give some control of hellebore leaf spot.
Root problems
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.