Southern Blight
Sclerotium rolfsii (Athelia rolfsii)
Overview
A soil-borne fungal disease common in warm climates that attacks the base of stems, causing a white, fan-like mycelial growth and small, round sclerotia resembling mustard seeds. Southern Blight spreads mainly by spores on wind, splash, tools, and crop debris. Humid, crowded plantings and overhead watering favor outbreaks in home gardens and food forests. Causal agent: Sclerotium rolfsii (Athelia rolfsii). In regenerative gardens, prioritize sanitation, airflow, and unbroken soil cover over calendar spraying, and intervene early when symptoms appear.
Symptoms
White, fan-like mycelial growth at the base of stems at the soil line. Small (1-2mm), round, tan to brown sclerotia (resembling mustard seeds) in the mycelium. Wilting and collapse of plants. Stem girdling at soil level. Compare several plants and note whether symptoms spread after rain, insect feeding, or wet foliage. Photograph undersides of leaves and cut stems when wilt is present to refine the diagnosis. Rule out simple drought, fertilizer burn, and herbicide drift before assuming a pathogen - abiotic injury often mimics disease.
Environmental Conditions
Hot
Prevention
Rotate with non-host crops (grasses, corn). Deep plow to bury sclerotia. Avoid excessive mulch near stems. Maintain good air circulation. Solarize soil in summer. Mulch to reduce splash, clear season-end debris, and avoid working plants when wet. Design beds for airflow and diverse plantings that support beneficial insects without creating permanent humid pockets around susceptible crops.
Management
Remove infected plants and surrounding soil. Apply biological controls (Trichoderma). Lime soil to raise pH above 7.0. Deep tillage to bury sclerotia. Fungicide drenches around plant bases. Prefer drip irrigation, morning watering, and open spacing so leaves dry quickly. Disinfect pruners between cuts on woody plants. Use copper, sulfur, or biological products only as labeled protectants when pressure is high - they rarely reverse advanced infections alone. Destroy (do not casually compost) highly infectious debris.