Cause The fungus Sclerotinia sclerotiorum is frequently encountered, however, several other fungi including Pythium spp. may cause similar symptoms. The fungus produces hard black sclerotia that help it survive from season to season without a host. Isolates from several vegetable and ornamental hosts can attack dahlia with varying degrees of pathogenicity. Potato flowers, which may be similar, are very susceptible to infection and are the main source of stem infection. Infected petals fall onto stems allowing the fungus to gain entry into the stems. Direct penetration, of stems in contact with the soil, from germinating soilborne sclerotia also occurs.
Symptoms Infected dahlia stems look water soaked. A white fungal growth may completely encircle the base of the stem. Plants wilt and die suddenly. Cutting open the stem reveals numerous characteristic black sclerotia.
Cultural control
- Space plants for good air circulation.
- Soil pH should be 5.8 to 6.2.
- Avoid overhead watering and severe dry-downs of soil/media.
- Rotate with less susceptible crops such as alyssum, daffodil, salvia, pansy, or ornamental grasses. Resistant crops include rushes, sedges, and the fountain grass Pennisetum glaucum.
- Use a low ammonium fertilizer and irrigate thoroughly to avoid salt accumulation.
Chemical control Some fungicides applied to flowers have worked well in vegetable systems where the epidemiology is better understood. The exact timing for ornamental crops may be different.
- Chipco 26019 FLO at 1 to 2.5 quarts /100 gal water. Group 2 fungicide. 12-hr reentry.
- OHP 6672 4.5 F at 10 to 20 fl oz/100 gal water. Group 1 fungicide. 12-hr reentry.
- Pageant at 12 to 18 oz/100 gal water. Do not use with organosilicone-based adjuvants. Group 7 + 11 fungicide. 12-hr reentry.
- Palladium at 4 to 6 oz/100 gal water. Avoid excessive runoff to small plants, which may result in stunting and/or chlorosis. Group 9 + 12 fungicide. 12-hr reentry.
Biological control
- Contans WG (Coniothyrium minitans strain CON/M/91-08) at 1 to 4 lb/A. Spray soil surface and then incorporate 1 to 2 inches with water or disc. Less effective in soils over 85°F and under 35°F. 4-hr reentry. O
Reference Price, K., and Colhoun, J. 1975. Pathogenicity of isolates of Sclerotinia sclerotiorum (Lib.) de Bary to several hosts. Journal of Phytopathology (Phytopath Z.) 83:232-238.