Cause Coniothyrium hellebori, a fungus that is favored by wet weather and factors that stress plants. The disease may spread rapidly during wet springs or falls. Found frequently in Oregon and Washington.
Symptoms Large irregular dark brown to black spots on both sides of leaves. Spots often run together and show a concentric zonation pattern, with tiny black fruiting bodies (pycnidia) of the fungus forming in the lesions over time. Leaves turn yellow prematurely and die. Cankers may develop on the stems, which shrivel and collapse. Flowers may wilt before buds open or petals may develop spots.
Cultural control Summer shade and winter protection are important for this crop.
- Avoid overhead irrigation that keeps foliage wet for extended periods. Careful as Hellebore do not like being watered in the hottest part of the day.
- Space plants for good air circulation.
- Grow in a well-drained media at a pH of 5.5.
- Remove and destroy infected plant debris.
Chemical control There are no chemicals specifically labeled for this plant or this disease. It is reported that bordeaux can be used, which would suggest that copper-based products (group M1 fungicides) may be helpful. Many other products can be used if not phytotoxic.
Reference Meiners, J., and Winkelmann, T. 2011. Morphological and genetic analyses of hellebore leaf spot disease isolates from different geographic origins show low variability and reveal molecular evidence for reclassification into Didymellaceae. Journal of Phytopathology 159:665-675.