Hellebore leaf spot is caused by the fungus Microsphaeropsis hellebori and is a common disease on many hellebore species. You are most likely to see infections when new plant growth is occurring from late winter until summer.

It attacks most Helleborus spp. but is not so damaging on the tougher leaves of Helleborus argutifolius. H. niger is particularly badly affected by leaf spot.

You may see the following symptoms:

• On leaves: Infections cause more-or-less round, dead, brown patches on leaves and stems. Later, the dead leaf tissue may fall out, leading to a ragged appearance
• On stems: Infections on the stems may cause them to collapse
• On flowers: Dark spots and blotches can also develop on the flowers
• Tiny, black fruiting structures of the fungus may be visible in the dead plant tissue
• Heavy infection in early spring can lead to widespread blackening of leaves and stems that could be mistaken for damage from adverse weather conditions

The RHS believes that avoiding pests, diseases and weeds by good practice in cultivation methods, cultivar selection, garden hygiene and encouraging or introducing natural enemies, should be the first line of control. If chemical controls are used, they should be used only in a minimal and highly targeted manner.

Non-chemical control

• Remove all affected leaves promptly and destroy
• Do not allow dead, infected material to remain around the plants, since this will be a source of infection in the following season

Fungicides

The RHS recommends that you don't use fungicides. Fungicides (including organic types) may reduce biodiversity, impact soil health and have wider adverse environmental effects. If you do intend to use a fungicide, please read the information given in the link and download below to ensure that use, storage and disposal of the product is done in a responsible and legally compliant manner.
  
The products listed in the ‘Fungicides for gardeners’ document below are legally available for use by home gardeners in the UK. This information is provided to avoid misuse of legal products and the use of unauthorised and untested products, which potentially has more serious consequences for the environment and wildlife than when products are used legally. Homemade products are not recommended as they are unregulated and usually untested.

There is no specific information available on the efficacy of any home garden fungicide against hellebore leaf spot.

Download

Fungicides for gardeners (Adobe Acrobat pdf document outlining fungicides available to gardeners)

Link

Chemicals: storing and disposing safely

The fungus causing hellebore leaf spot produces minute spores from small black fruiting bodies that form in the dead, affected tissues. The spores are spread in water and wind-blown rain and thus wet conditions are required to initiate disease. The fungus perpetuates on the plant over the summer and autumn and a new round of infections is intiated at times when new plant growth is occurring.