Rhizoctonia root and crown rot identification & organic treatment
Rhizoctonia solani
How to identify rhizoctonia root and crown rot
A very common soil fungus that attacks seedlings at the soil line (a major cause of damping off) and, in older plants, causes root/crown rot and, on potato, the black, tar-like 'black scurf' on tubers.
Symptoms
- Seedlings collapse at the soil line with a thin, brown, wire-like stem lesion ('wirestem')
- Older plants show reddish-brown lesions on roots and lower stems, sometimes girdling and wilting the plant
- On potato tubers, small hard black lumps ('black scurf', actually fungal resting structures) stick to the skin
- Poor stand establishment in cool, wet soil
Host crops
Bush bean, Potato, Cabbage, Lettuce, Tomato, Pepper.
How it spreads
Lives indefinitely in soil as resting structures (sclerotia) and organic matter; spreads via infested soil, seed pieces, and equipment.
Favorable conditions
Cool, wet soil, especially right after planting; heavy or compacted soils that stay damp make it worse.
Progression
- Seed/seedling attack (Cool, wet soil at emergence) — Stem girdling at soil line; classic damping-off.
- Root/crown rot in established plants (Ongoing in infested soil) — Lesions expand, can girdle stems and cause wilt.
- Tuber black scurf (On potato, at harvest) — Hard black flecks on skin; cosmetic but marks infested ground.
Organic treatment for rhizoctonia root and crown rot
- 1. Improve drainage and delay planting · low effort
Wait for soil to warm and dry slightly before direct-seeding into cool, wet ground.
When: At planting
- 2. Remove infected seedlings/plants · low effort
Pull and discard collapsed seedlings and severely rotted plants; thin dense stands to improve airflow at the soil line.
When: As seen
- 3. Build organic matter and biological activity · low effort
Compost additions support beneficial microbes (including Trichoderma) that compete with Rhizoctonia.
When: Bed prep
Prevention
- Use certified disease-free seed potatoes.
- Rotate out of susceptible crops for 2-3 years.
- Avoid planting into cold, wet soil; use raised beds for better drainage.
Easy to confuse with
- Pythium damping off (softer, more water-soaked, no wirestem lesion)
- Fusarium/Verticillium wilt (vascular browning, not girdled stem base)
Terms on this page
- Sclerotia
- Hard, seed-like resting bodies a fungus forms to survive winter and dry spells in the soil. They look like tan, brown or black BBs or mustard seeds.
- Vascular tissue
- The plant's plumbing inside the stem. Cut a stem lengthwise: brown streaks here point to a wilt disease.
Sources
New to a word on this page? See the plain-English glossary.