Southern Blight

Athelia rolfsii (syn. Sclerotium rolfsii)

Southern Blight is a warm-soil crown and lower-stem disease caused by Athelia rolfsii, widely known by its older name Sclerotium rolfsii. It has an exceptionally broad host range that includes vegetables, herbs, ornamentals, field crops, and young woody plants. North Florida’s hot, humid summers and organic surface debris can strongly favor outbreaks. Plants often wilt suddenly after a brown, girdling lesion develops at the soil line. A fan of coarse white mycelium spreads across stems, mulch, and soil, followed by numerous round sclerotia that begin white and mature tan to reddish brown, closely resembling mustard seed. The combination of high temperature, soil-line infection, fanlike mycelium, and small brown sclerotia separates southern blight from cool-weather Sclerotinia diseases. The pathogen can persist in the upper soil profile and move with transplants, potting mix, soil, tools, and runoff, making sanitation and exclusion more reliable than attempts to cure collapsed plants.

Identification

Inspect the crown without pulling the plant prematurely. Remove mulch from the soil line and look for a water-soaked to brown girdling lesion, coarse white strands or sheets, and spherical sclerotia approximately poppy- to mustard-seed size. Collect the entire crown with roots, attached mycelium, several sclerotia, and adjacent surface soil. The pathogen is most active near the warm soil surface, whereas onion white rot produces much smaller black sclerotia around Allium bulbs and Sclerotinia sclerotiorum commonly produces larger irregular black sclerotia under cooler, moist conditions. Phytophthora crown rot lacks the characteristic mustard-seed structures; Rhizoctonia can cause dry sunken lesions without the same profuse white fan. Culture or molecular confirmation is advisable in a new production site, valuable nursery block, or where treatment and disposal decisions are substantial.

Symptoms / Signs

A plant may show midday wilt before permanent collapse. Lower leaves yellow, the crown softens or browns, and a lesion encircles the stem at or just above soil level. White mycelial fans spread over the crown, mulch, media, or neighboring debris when humidity is high. Small round sclerotia form in clusters, first white, then cream, tan, and brown. Fruits or leaves touching infested soil can develop soft rots. In plug trays or densely spaced pots, neighboring plants may fail in a widening cluster. Roots can remain less damaged than the girdled crown, explaining abrupt wilt despite an apparently intact root system. Heat injury and drought do not produce organized mycelial fans or sclerotia.

Life Cycle / Process

Sclerotia survive in soil and organic debris and germinate when warm, moist conditions occur near susceptible tissue. The fungus grows through surface litter and contacts crowns, lower stems, fruit, or leaves. Temperatures roughly in the high 70s through 90s °F, moist soil, acidic conditions, dense plantings, and undecomposed organic matter commonly favor disease. New sclerotia develop on colonized tissue and return to the soil. Movement occurs with infected plants, contaminated soil or media, equipment, irrigation runoff, and plant debris. Unlike Sclerotinia stem rot, apothecia and airborne ascospore infection are not the central production pathway. Disease pressure can remain highly localized because the fungus is concentrated in the upper soil layer, yet contaminated transplant production can distribute it broadly.

Prevention

Purchase clean transplants and inspect crowns and media surfaces before introducing lots. Use pathogen-free potting components and keep reused containers, benches, and ground cloth free of soil and plant debris. Improve drainage and avoid maintaining constantly wet mulch against stems. Remove old crop residues that can serve as food bases, but do not move infested debris through clean production. Space plants for drying and avoid burying crowns. In known infested sites, rotate to less susceptible crops, consider soil solarization during the hottest season, and use physical separation or raised container systems. No short rotation reliably eradicates sclerotia because of the host range. Preventive fungicide or biological options are crop- and site-specific and must reach the crown zone before infection.

Management / Control

Quarantine affected flats or landscape zones and minimize traffic through wet soil. Remove collapsed plants with the surrounding surface soil or media while sclerotia are still contained; do not shake roots over the bed. Correct irrigation leaks, standing water, excess mulch, and buried crowns. In small areas, solarization or removal of heavily infested surface soil may lower inoculum, but neither guarantees eradication. For commercial crops, select only labeled products and apply preventively to the infection court; foliar-only coverage may miss the crown and upper soil where the fungus is active. Replanting immediately into the same pocket often results in recurrence. Track the site and inspect adjacent plants daily during warm wet weather.

Procedure Steps

1. Photograph the wilt pattern and soil surface before moving plants. 2. Gently expose the crown and search for white fanlike growth and tan-to-brown spherical sclerotia. 3. Collect the whole crown, roots, mycelial margin, sclerotia, and a small amount of adjacent soil in separate labeled packaging. 4. Compare with Phytophthora, Rhizoctonia, Sclerotinia, onion white rot, and waterlogging. 5. Remove diseased plants and the immediate food base without scattering sclerotia. 6. Clean soil from tools before disinfection and restrict movement from the infested zone. 7. Correct surface moisture and debris conditions. 8. Submit a sample when the site is newly affected or when a crop-specific treatment is contemplated.

Safety Notes

Wear gloves when handling decayed crowns and contaminated soil. Soil solarization films, fumigants, and fungicides have distinct worker-protection and site restrictions; none should be improvised. Never burn plastic pots or treated plant debris. Keep infested soil and sclerotia out of clean compost and propagation areas. Some products used in commercial production are not labeled for residential landscapes or edible crops, so legal use must be checked for the exact host and site.