Southern blight of vegetables and herbaceous plants
Southern blight is one common name for a disease caused by the fungus Sclerotium rolfsii. Other common names include crown rot and white mold. At one time Southern blight was thought to be a disease problem only in warm climates. In recent years, however, incidents of the disease have been reported from Illinois, Iowa, and Michigan, as well as here in Minnesota. It was thought that the overwintering structures of this fungus were not cold hardy, but they have now been shown to survive winters when under cover of snow and/or mulch. Unfortunately, Southern blight has a host range of approximately 200 different genera including ornamental plants and field crops. Some of the more common hosts include daylily, astilbe, hostas, peony, phlox, ajuga, delphinium and potato.
Symptoms and signs
Fig 1. Mycelium and sclerotia growing on infected hosta stem
Fig 2. Daylily showing early signs of infection with S. rolfsii.
Fig 3. Sclerotia on astilbe
If S. rolfsii becomes established in your garden, there are some important cultural controls you can implement to reduce the spread of this disease. When dealing with Southern blight, sanitation is particularly important. Sclerotia can be transported around your garden with infected soil. For this reason, carefully clean soil off your tools and even your shoes. Transplanting infested plants is another way the fungus is spread around a garden. Because the fungus can overwinter in mulch, it is helpful to remove mulch from the crowns of the plants. Soil solarization, a process that heats the soil to levels sufficient to kill many fungi, is a control measure used in the south. Northern gardeners should be aware that the soil in our climate will not reach temperatures high enough to kill the fungus. The fungicides for Sclerotium rolfsii control are not available for homeowners to purchase at retail centers. If you have determined that your plants have Sclerotium rolfsii, and other control measures are unsuccessful, you may wish to contact a commercial applicator to discuss the use of chemical controls.
- Agrios, G.N. 1988. Plant Pathology. Academic Press, Inc. New York.
- Draheim, Jerry. 1997. Serious Hosta Disease. Michigan State University Extension.
- Iowa State University Extension. 2000. Crown Rot: A Serious Disease of Hosta and other Ornamentals.
- Pataky, Nancy. 2001. "Hosta" la Vista? University of Illinois Extension.
- UC Pest Management Guidelines. 2001. Floriculture and Ornamental Nurseries Southern Blight. University of California Statewide Integrated Pest Management Project.