Plant Problem Diagnostic
Southern Blight: Diagnosing Sudden Collapse from a Soilborne Fungus
The short answer
Southern blight, caused by the soilborne fungus Sclerotium rolfsii (sexual stage Athelia rolfsii), attacks hundreds of plants, tomato, pepper, bean, hosta, and many other vegetables and ornamentals, causing sudden wilting and collapse. The diagnostic signs are a white, fan-like mat of fungal growth at the stem base and across the surrounding soil, followed by numerous tan-to-brown, mustard-seed-sized round sclerotia, with the stem girdled and rotting at the soil line.
It is a hot-weather disease: NC State notes it is favored by high temperatures of roughly 77 to 95F together with moist conditions and acidic soil, so it appears in the heat of summer in humid regions. The fungus survives in soil as sclerotia and on debris for years, mostly in the top few inches.
Because the sclerotia persist and the host range is so wide, control is cultural, not chemical, for home gardeners. Remove infected plants along with the surrounding infested soil, bury debris and sclerotia by deep cultivation, rotate host crops to non-hosts like corn or small grains, consider soil solarization, keep mulch pulled back from stems, and improve drainage; raising soil pH toward neutral also helps.
Recognizing southern blight
The collapse is fast and the signs are at the soil line. NC State Extension describes a brown-to-black lesion developing on the lower stem right at the soil line, which expands rapidly and can completely girdle the stem, causing a sudden and permanent wilt of everything above it, a healthy-looking plant one day can flop the next. Under moist conditions a dense mat or web of white mycelium develops at the stem base and can extend a few centimeters up the stem and out across the soil surface.
Then come the diagnostic sclerotia. Michigan State notes the fungus produces round sclerotia about half a millimeter to two millimeters across, at first white, then turning tan to reddish or dark brown, which resemble mustard seeds in size and color, hence the nickname mustard-seed fungus. Finding that white fan of growth plus scattered mustard-seed sclerotia at the base of a suddenly wilted plant in hot weather is what separates southern blight from a vascular wilt like fusarium or verticillium, which do not produce surface mycelium and sclerotia.
Conditions and survival
Heat plus moisture drives it. NC State states that high temperatures of about 77 to 95F together with aerobic, moist conditions and acidic soil favor disease development, which is why southern blight is a signature summer problem in warm, humid regions and largely spares cool-climate gardens. A spell of hot, wet weather after mulch or dense growth has kept the soil surface moist is the classic setup.
The reason it recurs is the sclerotia. The fungus survives and overwinters as sclerotia and on host debris in the soil for years, generally in the upper two to three inches, and its very wide host range, pepper, bean, carrot, potato, sweetpotato, watermelon, hosta, and many ornamentals, means there is almost always a susceptible plant for it to attack. Once a bed is infested, those long-lived sclerotia near the surface are what you are managing against for several seasons.
Managing southern blight
Home control is cultural, aimed at removing and burying inoculum. Remove symptomatic plants along with the surrounding infested soil, and control weeds that can host the fungus, avoiding moving contaminated soil to clean areas. Michigan State advises inspecting incoming plant material, removing and destroying affected plants, and limiting mulch on sites with a history of the disease, since mulch against the stem keeps the surface moist and favors the fungus.
Longer term, disrupt the sclerotia. NC State recommends deep plowing or cultivation to bury infected debris and sclerotia below the shallow zone where the fungus operates, rotating host crops with non-hosts such as corn, wheat, or barley, and raising soil pH toward neutral (above 7). Soil solarization during hot weather can reduce inoculum in the top layer of infested beds, and improving drainage removes the moisture the fungus needs. Chemical control is rarely practical for home gardeners, and any fungicide is used strictly per label. If you are unsure whether a sudden wilt is southern blight or a vascular wilt, an Extension diagnostic lab can confirm before you strip out a bed; the fusarium wilt and verticillium wilt guides cover the look-alikes.
Sources
Reviewed 2026-07-18. We cite university Extension, government, and established horticultural sources — and never invent pesticide rates or dangerous treatments.
- NC State Extension - Southern Blight of Vegetable Crops — Southern blight (Sclerotium rolfsii / Athelia rolfsii) affects hundreds of crops including tomato, pepper, bean, carrot, and potato; a brown-to-black lesion develops on the stem near the soil line and can completely girdle it, causing sudden permanent wilt; white mycelium develops on stem lesions and extends up the stem, with tan-to-reddish-brown spherical sclerotia one to two millimeters in diameter on the mat; high temperatures of 77 to 95F, moist aerobic conditions, and acidic soil favor it; the fungus survives as sclerotia and on debris for years in the upper two to three inches; management includes crop rotation with corn, wheat, or barley, deep plowing to bury debris and sclerotia, removing symptomatic plants and weeds, raising soil pH above 7, and fungicides per label.
- Michigan State University IPM - Southern Blight — In early disease a dense mat or web of white mycelium is evident at the base of the plant, and sclerotia are globular, half a millimeter to two millimeters in diameter, at first white then turning tan to reddish or dark brown, resembling mustard seeds; management includes inspecting incoming plant material, removing and destroying affected plants, avoiding spreading infested soil, limiting mulch use in sites with a history of the fungus, and deep plowing to bury sclerotia.