Pathogen: Fungus Sclerotium rolfsii
Hosts: This aggressive fungal pathogen has been reported in over 500 annual, perennial and woody plants. Southern blight occurs in all temperate, tropical and subtropical regions with hot summers and plentiful rainfall. Unlike some other diseases that result in lower stem and crown rots, this one is more prevalent in well drained soils. Herbaceous plants are more susceptible to this disease than woody plants. However, juvenile trees and shrubs or those that lack a corky bark layer at the root collar at maturity are vulnerable to attack.
Symptom: The fungus colonizes the stem of its host one to two inches above or below ground. The initial symptom is wilting of the entire plant or individual shoots or branches, which is usually followed by rapid plant death. The symptoms are caused by brown, water-soaked, sometimes sunken, girdling cankers at the stem or root collar. Herbaceous plants generally die more quickly than woody ones. The fungus forms a profuse, white, often fan-shaped mycelial mat and round sclerotia, about the size of a mustard seed, on the stem, and on the soil surface. The sclerotia are white initially, but turn tan to dark brown within a few days of formation. When conditions are less favorable for fungal growth, the mycelium disappears, but the sclerotia remain and persist.
Pathogen cycle: The fungus survives in the soil as sclerotia, which can persist for several years. The pathogen is spread about by movement of sclerotia via splashing water or by transport of infested soil or plant materials. The disease tends to be a problem in soils with high levels of available carbon, from sources such as fresh mulch that has not been well composted, or dead or dying plants (weeds, grass, etc.). It often colonizes these materials before attacking live plants. However, the pathogen can spread more easily to neighboring plants when the soil is not mulched. The disease occurs most often from late spring through early fall when the weather is hot and humid. Plentiful moisture and temperatures exceeding 86F in the day and 70F at night favor disease development. The warm temperatures stimulate sclerotial germination and growth of the fungus. The disease can occur under lower temperatures, but all aspects of development will proceed more slowly.
Diagnosis: Diagnosis is confirmed by observation of spherical, tan to brown sclerotia which are often accompanied by a coarse white, fan-shaped mycelial mat.
Management: Use well-composted mulch. Avoid overwatering. Time irrigations, taking rainfall into account. Fallow affected areas for 6 months, if possible, as it’s hard to get rid of the fungus. There is one home-grower sized product available - Southern Ag. Terrachlor, which may be used to protect surrounding plants.