Zinnia-Stem Rot

Cause Sclerotinia sclerotiorum, a fungus that produces hard, black sclerotia, which can survive many years in soil. The disease develops 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. 'Profusion White' zinnia is very susceptible. 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 Off-color lesions develop near the plant's base. Later a white, cottony fungal growth occurs on the stem. Hard, black sclerotia may develop with the white growth both outside and inside the stem. The upper part of the plant wilts and eventually dies. Damping-off of seedlings frequently occurs.

Cultural control

  • Use sterile, well-drained soil media.
  • Space plants for good air circulation.
  • Maintain the media pH at 5.8 to 6.2 for best results.
  • 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.

  • 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.
  • 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 Grabowski, M. and Malvick, D. 2013. Field trial evaluation of resistance to Sclerotinia sclerotiorum in annual bedding plants. Phytopathology 103:52. (abstract)