Hellebore leaf spot

What is hellebore leaf spot?

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.

Symptoms

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
• Tiny round black fruiting structures of the fungus may be visible in the dead plant tissue

Control

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

Chemical control

The fungicides difenoconazole (Westland Plant Rescue Fungus Control), myclobutanil (Doff Systemic Fungus Control and other formulations), tebuconazole (Bayer Garden Multirose concentrate2) and triticonazole (Scotts Fungus Clear Ultra) are available to gardeners for control of several problems on ornamental plants. They may give useful control of hellebore leaf spot, but do not carry any recommendations for this problem.

Products containing plant and fish oil blends (Vitax Organic 2 in 1) may be used on any plant and carry a recommendation for ‘Helminthosporium,’ a broad term for a number of leaf spots; they may give useful control, but again, are not specifically recommended.

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

Biology

The fungus causing hellebore leaf spot produces minute spores from small black fruiting bodies which 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.