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Southern blight

Also known as Sclerotium rolfsii, Athelia rolfsii, Agroathelia rolfsii, Mustard seed fungus, Sclerotium blight

DiseaseHarmful
Sclerotia of Athelia rolfsii (southern blight) on a tomato stem Photo: Persley D. & Cooke T · CC BY-SA 3.0

In short

Southern blight is a soil-borne fungal disease that rots the stem at the soil line, causing sudden wilting and death. It is common in warm, moist conditions in tropical and subtropical regions and attacks hundreds of plant species. It is recognised by a white fungal fan and small, round, mustard-seed-like sclerotia.

How it looks

At the stem base, a white mat of cottony fungal threads spreads out from the rotting stem onto the nearby soil surface. Within it form numerous round sclerotia about 1–2 mm (0.04–0.08 inch) across, tan to brown, looking like mustard seeds. The stem at the soil line shows a discoloured lesion.

Signs

A crown or stem lesion at the soil line, yellowing of lower leaves, wilting, collapse and death of the plant, often quickly in hot, wet weather.

Life cycle

The fungus survives in soil as sclerotia, which also travel in infested soil and in planting stock such as bulbs, tubers, rhizomes and corms. When conditions are warm and moist, sclerotia germinate and the fungus attacks the stem at the soil line.

What favours it

Warm, moist soil; growth is favoured above about 15 °C and the disease is most active in hot, wet summers and in tropical and subtropical climates.

Prevention

  • Use fresh, clean potting mix and do not move soil from affected beds.
  • Inspect bulbs, tubers and rhizomes before planting and reject any with white growth or sclerotia.
  • In beds, soil solarisation or steaming reduces sclerotia.

Treatment

Start with the first step and move down only if the problem persists.

  1. Remove infected plants with the surrounding soil or mix, bag them, and do not compost.
  2. Heat treatment of soil or mix (steaming or solarisation) kills sclerotia and is accepted in organic growing.
  3. Biological agents based on Trichoderma are used against this fungus.
  4. Some fungicide classes are used commercially against Sclerotium rolfsii; follow the product label.

Pros and cons

Cons

  • Very wide host range (over 500 species) and sclerotia persist in soil.
  • Plants can collapse quickly once the stem is girdled.

Look-alikes

Crown and stem rot
Other crown rots lack the conspicuous white fan and mustard-seed-like sclerotia.
Fusarium wilt
Fusarium wilt stains the internal vascular tissue and shows no external white fungal mat.
Slime mould or harmless surface moulds
Surface moulds on potting mix do not girdle and rot the stem.

Affected plants

Hundreds of vegetables, field crops and ornamentals, including tomatoes, onions, beans, peas, peanuts and many bedding and bulb plants.

Sources

  1. Southern Blight (Floriculture and Ornamental Nurseries) — UC IPM
  2. Agroathelia rolfsii (Sclerotium rolfsii) — Wikipedia

Written in our own words from these sources. Check a product label before using any treatment.