Southern blight identification & organic treatment
Sclerotium rolfsii
How to identify southern blight
A fast, heat-loving soil fungus that girdles the stem right at the soil line, identifiable by a white fan of fungal threads and mustard-seed-sized round sclerotia at the plant's base.
Symptoms
- Sudden wilting of an otherwise healthy-looking plant
- White, coarse, fan-shaped fungal growth (mycelium) at the stem base and on the soil surface
- Small round tan-to-brown 'sclerotia' (like mustard seeds) embedded in the mycelium
- Stem tissue at the soil line turns brown, water-soaked, and collapses
Host crops
Tomato, Pepper, Bush bean, Watermelon, Carrot, Potato.
How it spreads
Sclerotia survive years in soil and spread in contaminated soil, mulch, and on equipment; the fungus grows aggressively through moist surface soil and organic debris.
Favorable conditions
Hot weather (85-95°F) with high humidity and moist soil surfaces; especially common in the Southeast in mid-late summer.
Progression
- Soil-line colonization (Hot, humid weeks) — Mycelium spreads across mulch/soil surface unnoticed.
- Stem girdling (Days after contact) — Water-soaked lesion encircles the stem base.
- Collapse (Within a week) — Plant wilts and dies rapidly despite adequate soil moisture.
Organic treatment for southern blight
- 1. Remove plant and surrounding soil · medium effort
Dig out the dead plant plus 6 in of surrounding soil and the mulch touching it; bag and discard.
When: Immediately at diagnosis
- 2. Deep-bury or remove mulch near stems · low effort
Keep organic mulch a couple inches back from stems; the fungus thrives in the moist debris layer right at the crown.
When: At mulching
- 3. Solarize soil · medium effort
Clear plastic over moist soil for 6+ hot weeks kills sclerotia in the top few inches.
When: Fallow period, peak summer
Prevention
- Rotate to non-susceptible crops (corn, small grains) for 2-3 years.
- Deep-till to bury sclerotia below the germination zone.
- Avoid piling mulch or debris against stems.
- Improve drainage; avoid overwatering in hot weather.
Easy to confuse with
Fusarium/Verticillium wilt (no white mycelial mat or sclerotia), Damping off (younger seedlings, no sclerotia).