Cause There are many fungi that can cause leaf spots on coral bells. Mycospaerella heucherae (formerly Phyllosticta excavata) has been reported from Idaho while Ramularia heucherae (formerly Ramularia mitellae var. heucherae) has been reported from Washington. The Oregon State University Plant Clinic has found Botrytis and Colletotrichum sp. from leaf spots as well as the bacterium Pseudomonas. The microcyclic, autoecious (can complete their life cycle on this one host) rust fungus Puccinia heucherae has been reported throughout the Pacific Northwest. Wet weather and overcrowding are generally favorable for leaf spot development.
Symptoms Colletotrichum produces roughly circular, purple-red, necrotic leaf spots that crossed major veins. Spots are at first small but expand to about 0.25 inches. A chlorotic halo is observed around spots. Numerous spots cause premature leaf senescence and defoliation.
Rust - Small, orange to reddish pustules of rust can generally be seen on the undersides of leaves.
Botrytis may cause irregular necrotic spots or blights along leaf margins or along leaf veins.
Pseudomonas leaf spots are sunken and angular then becoming necrotic.
Cultural control
- Space plants for increased air flow and drying.
- Plants do best in moist, well-drained soils rich in organic matter at a pH near 7.0.
Chemical control These fungicides are safe on this crop and may be useful for control of some leaf spots.
- Chipco 26019 FLO at 1 to 2.5 quarts/100 gal water. Group 2 fungicide. 12-hr reentry.
- Insignia SC at 3 to 6 fl oz/100 gal water. Do not use with organosilicate-based adjuvants. Use preventively only. Group 11 fungicide. 12-hr reentry.
- Mural at 4 to 7 oz/100 gal water. Group 7 + 11 fungicide. 12-hr reentry.
- Ortho Rose & Flower Disease Control at 2 fl oz/gal water. Group 3 fungicide. H
- Although not specifically registered on coral bells, according to IR-4 data, azoxystobin (Heritage) is safe on this crop. Group 11 fungicide.
Reference Henricot, B., Denton, G., and Lane, C. 2007. First report of Puccinia heucherae on Heuchera spp. in the UK. Plant Pathology 56:352.