Cause Sclerotinia sclerotiorum, a fungus that produces hard, black sclerotia, which can survive many years in soil. Sclerotia germinate best when soil temperature is 55°F to 60°F. The fungus can colonize dead organic plant matter and survive many years. As a rule, these plants tend to be susceptible to overwatering. 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. Soilborne sclerotia can also directly penetrate stems that are in contact with the soil.
Interesting to note that this disease on this host has only been reported from Connecticut but has been found by several plant clinics.
Symptoms Off-color lesions develop near the root collar. Later, a white, cottony fungal growth appears on the stem. Hard, black sclerotia may develop in the white growth. Roots remain healthy, but the upper part of the plant beyond the stem lesion wilts and eventually dies. Roots may rot when they encounter sclerotia in the soil. Leaves then turn yellow, become necrotic as the plant wilts and eventually dies.
Cultural control
- Space plants for good air circulation.
- Avoid overhead irrigation or apply such that plants are not wet for extended periods of time.
- Use moderate but consistent watering avoiding either extreme of too wet or too dry. Keep pH around 5.8 to 6.2.
- Remove and destroy infected plants as soon as possible.
- Remove and destroy crop remains at the end of growing season.
- Rotate planting bed with less susceptible plants such as alyssum, daffodil, salvia, pansy, or ornamental grasses. 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.
- Pageant at 12 to 18 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.
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 Underwood, W., Gilley, M., Misar, C.G., Gulya, T.J., Seiler, G.J. and Markell, S.G. 2022. Multiple Species of Asteraceae Plants Are Susceptible to Root Infection by the Necrotrophic Fungal Pathogen Sclerotinia sclerotiorum. Plant Disease. 106:1366-1373.