Southern blight has a wide host range, which makes the disease especially difficult to prevent and manage and can result in severe plant loss. The pathogen is most active in tropical and subtropical regions, but occurs worldwide, especially in warmer regions and during summer months. Southern blight affects a wide range of both vegetable crops and ornamental crops, but this article will focus primarily on herbaceous ornamentals.

Southern blight is caused by the soil borne fungus Agroathelia rolfsii (=Sclerotium rolfsii). The fungus remains dormant in the soil as small (approximately 0.5 to 1 mm), round, brown- to tan-colored bodies called sclerotia (approximately the size of mustard seeds), and can survive in soil debris and diseased tissue for up to 5 years.

The southern blight pathogen has an extensive host range of over 500 plants worldwide. Herbaceous ornamental host plants include Aster, Hosta, peony, Petunia, rose, Salvia, black-eyed Susan, Impatiens, Dahlia, Gladiolus, Chrysanthemum, Coreopsis, Zinnia, Cosmos, Viola, Sedum, and daylily. The fungus mostly affects herbaceous plants, but woody plants like dogwood, azalea, Forsythia, and arborvitae are susceptible, as well.

Humid and warm conditions (around 80 - 95°F), as well as moist and acidic soils (pH of <7) favor disease caused by A. rolfsii. The pathogen most severely affects tropical and subtropical regions, though infection is also common in the northern hemisphere in warmer regions during the summer months.

At beginning stages of A. rolfsii infection, a water-soaked lesion commonly appears on the lower stem near the surface of the soil. The leaves often wilt and turn brown (Figure 1 and Figure 2) and the lesion will likely girdle the stem. Eventually, the entire plant may die. As the infection advances, white mycelial mats grow around the lesion on the stem and spread to the soil (Figure 3). The mats develop small (0.5 to 1 mm) sclerotia that can overwinter in plant and soil debris (Figure 4 and Figure 5). The sclerotia are spherical, smooth, and initially light colored but eventually become tan and then dark brown.

Figure 1. Wilted leaves of phlox with southern blight.

PDIC, Sample 25326, Pic. 1

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Figure 1. Wilted leaves of phlox with southern blight.

PDIC, Sample 25326, Pic. 1

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Figure 2. Symptoms of southern blight of zinnia.

PDIC, Sample 33293, Pic. 1

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Figure 2. Symptoms of southern blight of zinnia.

PDIC, Sample 33293, Pic. 1

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Figure 3. Mycelial mats and sclerotia (orange/tan colored round structures) on zinnia infected with the southern blight pathogen.

PDIC, Sample 33293, Pic. 2

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Figure 3. Mycelial mats and sclerotia (orange/tan colored round structures) on zinnia infected with the southern blight pathogen.

PDIC, Sample 33293, Pic. 2

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Figure 4. Close-up of sclerotia on peony infected with the southern blight pathogen.

PDIC, Sample 30854, Pic. 2.

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Figure 4. Close-up of sclerotia on peony infected with the southern blight pathogen.

PDIC, Sample 30854, Pic. 2.

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Figure 5. Southern blight on octopus tree. Stem rot, mycelial mat, and sclerotia visible.

Florida Division of Plant Industry, Florida Department of Agriculture and Consumer Services, Bugwood.org.

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Figure 5. Southern blight on octopus tree. Stem rot, mycelial mat, and sclerotia visible.

Florida Division of Plant Industry, Florida Department of Agriculture and Consumer Services, Bugwood.org.

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Agroathelium rolfsii is primarily transmitted by contaminated tools, equipment, or movement of infested soil. Outbreaks can also occur from infected transplants, although this is probably rare. When conditions are conducive, the sclerotia germinate and infect the host plant by penetrating the crown (the lower stem of the plant where it meets the roots). Above-ground wilting and/or yellowing are usually seen first and could start to appear 2 to 4 days after infection. Within days to weeks, white mycelial mats develop at the lesion around the stem and sclerotia begin to develop from the mycelia. As disease progresses, the lesion on the lower stem expands leading to the entire plant wilting and eventual mortality. The sclerotia overwinter in plant and soil debris, allowing the fungus to survive for multiple years without a host plant.

There is no cure for a plant infected with A. rolfsii. Due to the wide host range of the pathogen, southern blight can be difficult to control. Thus, it is important to employ as many of the following prevention and management strategies to limit the impact of this pathogen:

• Sanitize all containers, tools, pots, trays, work surfaces, greenhouse floor, benches, and equipment before planting.
• Avoid unthrifty looking plants. Unhealthy appearing plants, whether they have southern blight or not, should be discarded to avoid introducing the southern blight pathogen or other pathogens.
• Greenhouse growers: Use new potting media each season. Re-use of potting media increases your chances of carrying over several pathogens and, as potting media breaks down, chemical and physical properties can change and become unsuitable for plant growth.
• Avoid conducive conditions (warm and moist) among plants as these favor disease development. This includes practices such as shading, overwatering, and crowding plants. Increasing air flow in and around plants helps reduce southern blight occurrence.
• Increase soil pH to above 7. Studies have shown that raising soil pH can limit the germination of A. rolfsii and, thus, reduce the occurrence of disease.
• Till the soil to bury the sclerotia into deeper soil layers (8 to 10” deep) at the end of the crop season. Sclerotia require light to germinate so burying them reduces the number of sclerotia at the soil surface at the beginning of the crop season the following year.
• Rotate with plants that are less susceptible to southern blight. Unfortunately, grasses are the only known non-host of A. rolfsiii, although there may be others that have not been tested. Ideally, rotating away from a susceptible crop for 2 or more years will reduce, but not eliminate, the pathogen in the soil.
• Soil solarization can significantly reduce sclerotia in soil, but it will not eliminate the pathogen entirely as only the top 6 to 12” of soil are heated sufficiently to cause mortality of the southern blight pathogen. It is important to conduct solarization in the summer months for soil to reach temperatures that are lethal to the pathogen. Soil should be moist before solarizing for best results. This method also reduces weeds, pests, and other soilborne diseases.
• Consider replacement of bedding soil after the growing season since the fungus can remain dormant in the soil for up to 5 years. This option is expensive and may not be sustainable, but can be considered for small beds or beds that are considered high-value to the homeowner or commercial property.

Homeowners should follow the above recommendations. There are no effective fungicides that prevent southern blight on ornamentals available to homeowners.

In addition to the general disease control mentioned above, conventional growers may also find the following recommendations helpful.

• Use preventative fungicides. There are currently limited fungicides commercially available for the treatment of ornamentals with southern blight, and none are curative. For the latest fungicide recommendations for southern blight of ornamentals, see Commercial Landscape and Nursery Crops Disease Control. Be sure to always read and follow fungicide labels.

The 2025 North Carolina Agricultural Chemicals Manual provides information on the selection, application, and safe use of chemicals.

The NC State University Extension Plant Pathology website provides information on crop disease management.

The Southeastern US Pest Control Guide for Nursery Crops and Landscape Plantings provides information regarding control products used in nursery crops, greenhouse crops, and ornamental landscapes.

The NC State University Plant Disease and Insect Clinic (PDIC) provides diagnostics and control recommendations.

Benson, D.M., Commercial Landscape and Nursery Crops Disease Control. NC State Extension.

Chase, A.R., Daughtrey, Margery L., Cloyd, Raymond A. 2018. Compendium of Bedding Plant Diseases and Pests. APS Press.

Garcia-Gonzalez, J., Hansen, M., Strawn, L.K., Rideout, S.L. 2021. An Overview of Southern Blight, Caused by Sclerotium rolfsii. Virginia Cooperative Extension.

Joy, A., Hudelson, B. 2019. Southern Blight. Extension University of Wisconsin-Madison.

Mullen, J. 2001. Southern blight, Southern stem blight, White mold. The Plant Health Instructor. DOI: 10.1094/PHI-I-2001-0104-01.