Southern blight

Athelia rolfsii

also called White mold of the crown, Sclerotium rot

A hot-weather crown rot: white fan-like threads at the soil line and mustard-seed grains among them, and a plant that wilts and dies in days.

Fungus No cure Soil-borneSpread by contactSpread on tools & hands Spreads to neighbors

What you'll see

Sudden, permanent wilting of a healthy-looking plant with no obvious cause above ground.
A ring of white, coarse, fan-shaped threads around the stem at soil level.
Round tan or brown grains the size of mustard seed in that white growth — the resting bodies, and the certain identification.
The stem at the soil line soft, brown and stripped of bark when scraped.
Neighboring plants going down one after another along the row as the fungus grows through the surface soil.

Why it happens

A soil fungus that lives on the surface layer of organic matter and attacks the stem where it meets the soil. It survives as those hard grains, which stay viable for years and are moved on tools, boots, transplants and washing water. Unlike most soil rots it grows across the surface rather than through the roots, which is why mulch piled against a stem is such an invitation.

A disease of heat: it needs soil above 25 °C, so it is a southern and mid-Atlantic problem, and it appears in the hottest weeks of summer. Heavy organic mulch touching the stems, and a bed that grew tomatoes or beans last year, are what tip it over.

Treating it

Pull the affected plant with a generous spadeful of the soil and the grains around it, and bag it. Do not compost it.
Pull the mulch back from the stems of every plant nearby so the soil surface dries.
Do not water overhead, which floats the fungal grains along the row.
Clean tools and boots before moving to another bed.
Solarize the spot under clear plastic through the hottest six weeks of the summer if you have lost several plants there.

Keeping it away

Bury crop debris deeply or remove it; the fungus lives on the surface and does badly buried.
Keep mulch two or three centimeters clear of every stem.
Rotate away from tomatoes, peppers and beans, and use a grain or brassica crop in between.
Wrap the lower stem of a valued tomato in foil at planting where the disease is known.
Raise the soil pH slightly and avoid heavy fresh organic matter worked in shallowly just before planting.

Who gets it

Tomatoes, peppers, beans, carrots, sweet potatoes, hostas and many perennials — well over 500 plants.