Cause The fungus Sclerotinia sclerotiorum has an extremely wide host range infecting 383 plant species in at least 64 families. The disease develops in cool, wet conditions and is more prevalent when plants form a complete, compact canopy that holds high relative humidity. Individual sclerotia (survival structure of the fungus) can survive 3 years, but the fungus can colonize dead organic plant matter and survive many years. Burying sclerotia deeper than 2 inches prevents production of airborne spores, but burial as deep as 24 inches is needed to reduce sclerotia survival. Tilling can pull sclerotia back toward the surface.
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.
Symptoms The newer delphinium shoots and leaflets wilt first, followed by whole branches (usually lower ones), which progress to wilting of the entire plant. Cankers are on the lower stem and become extensive as the wilt progresses. The fungus may be white, fluffy mycelia at the base of the stem and/or internally in the pith. Black, irregularly oval sclerotia about 0.125 to 0.75 inch long form on the surface of the stem, on the soil, and in the stem.
Cultural control
- Remove infected plants when symptoms first appear.
- After the disease is detected, plant only tolerant plants such as alyssum, daffodil, salvia, pansy, or ornamental grasses. Resistant crops include rushes, sedges and the fountain grass Pennisetum glaucum.
- Space plants for good air circulation, quick drying and so they aren't crowded when they reach maximum size.
- Avoid frequent watering; let soils dry partially before watering again.
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.
- Chipco 26019 FLO at 1 to 2.5 quarts/100 gal water. Group 2 fungicide. 12-hr reentry.
- Cleary's 3336 EG at 12 to 16 oz/100 gal water plus another fungicide. Group 1 fungicide. 12-hr reentry.
- According to one IR-4 trial Pageant was safe on larkspur and may be useful for this disease. Use at 6 to 12 oz/100 gal water. Do not use with organosilicone-based adjuvants. Group 7 + 11 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 Symposium on Sclerotinia (=Whetzelinia): Taxonomy, biology, and pathology. (Eight articles) 1979. Phytopathology 69:873-910.