Cause Several fungi can cause leaf spots. Ramularia rudbeckiae has been reported from Idaho, and Septoria rudbeckiae has been reported from Idaho and Washington. Generally, weather or irrigation that keeps leaves wet for extended periods favors these diseases.
Symptoms Septoria-Vein-bounded dark-purple to brown spots develop on foliage beginning in late spring. Lower leaves may become completely covered and wither while upper leaves are spotted. Small black pimple-like fruiting bodies (pycnidia) may develop in the necrotic areas of the spots. Black leaf spots with a yellow halo may occur depending on the cultivar.
Cultural control
- Irrigate so that leaves are not wet for extended periods.
- Remove and destroy plant debris at the end of the growing season.
- Grow resistant cultivars such as American Gold Rush for resistance against Septoria.
Chemical control
- Heritage at 1 to 4 oz/100 gal water plus a non-silicone-based wetter sticker. Group 11 fungicide. 4-hr reentry.
- Medallion WDG at 1 to 2 oz/100 gal water. Use with oils or adjuvants may damage plant. Group 12 fungicide. 12-hr reentry.
- Orkestra at 8 to 10 fl 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
- Pageant at 4 to 12 oz/100 gal water. Do not use with organosilicone-based adjuvants. Group 7 + 11 fungicide. 12-hr reentry.
- Postiva at 10 to 28 fl oz/100 gal water. Group 3 + 7 fungicide. 12-hr reentry.
- Spirato GHN at 1 to 2 fl oz/100 gal water. Use with oils or adjuvants may cause plant damage. Group 12 fungicide. 12-hr reentry.
Reference Daughtrey, M.L. 2013 Cornell Guide for the Integrated Management of Greenhouse Crops and Herbaceous Ornamentals. Cornell University Cooperative Extension.