Cause Sclerotinia sclerotiorum, a fungus that produces hard, black sclerotia, which can survive many years in soil. The disease de-velops in cool, wet conditions and is more prevalent when plants form a complete, compact canopy that holds high relative humidity. The fungus can colonize dead organic plant matter and survive many years. In the potato system, flowers 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. Echinacea purpurea (purple coneflower) is more resistant than E. angustifolia.
Symptoms Plant will wilt and show dark lesions near the soil line along with blackened and rotting roots. White mycelia and black sclerotia may be seen at the base of dying plants. Diseased stems and petioles often disintegrated, leaving only fibrous tissues intact. Flower heads may bend over and break where stems rot.
- Use sterile, well-drained soil media.
- Space plants for good air circulation.
- Remove and destroy infected plants.
- Rotate with less susceptible crops such as alyssum, daffodil, salvia, pansy, or ornamental grasses for at least 3 years.
- Resistant crops include rushes, sedges, and the fountain grass Pennisetum glaucum.
Chemical control Some fungicides applied to flowers have worked well in vegetable systems where the epidemiology is better understood. The following materials may provide protection; however, few ornamental trials have been done to demonstrate the best timing.
- Orkestra at 4 to 10 fl oz/100 gal water. Group 7 + 11 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.
- 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 Gleason, M.L., Daughtrey, M.L., Chase, A.R., Moorman, G.W., and Mueller, D.S. 2009. Diseases of Herbaceous Perennials. APS Press, St. Paul, MN.