⚠ CriticalDiagnosing root rot requires unpotting: healthy roots are firm and white; Pythium-infected roots collapse to black, slimy strands that detach on contact.
Root Rot — Pythium & Phytophthora Infection
Commonly affects: Spathiphyllum wallisii (Peace Lily), Calathea orbifolia, Zamioculcas zamiifolia (ZZ Plant)
The plant wilts consistently even when the soil is moist or wet. Lower leaves yellow and drop rapidly, and stems near the soil surface may turn dark brown or black and feel hollow. On inspection, roots are black, slimy, and emit a distinctive fetid odour caused by anaerobic decomposition.
Potential Causes
- 1Chronically saturated substrate creating anaerobic conditions that favour Pythium and Phytophthora oomycetes
- 2Contaminated potting mix or recycled soil carrying dormant pathogen spores
- 3Vessels with no drainage trapping moisture indefinitely around the root zone
- 4Cold, waterlogged conditions during winter dormancy slowing root respiration
Solution Steps
- 1Unpot the plant immediately and rinse all soil from roots under running lukewarm water.
- 2Using sterile pruning shears, remove all blackened or mushy root tissue, cutting back to firm white tissue.
- 3Soak remaining roots for 15 minutes in a dilute hydrogen peroxide solution (3% H₂O₂, 1 part to 4 parts water) to eliminate residual pathogens.
- 4Allow roots to air-dry in a warm, bright spot for 1–2 hours before repotting.
- 5Repot into a completely fresh, sterile mix in a clean pot — never reuse contaminated substrate.
- 6Water sparingly for the first two weeks to encourage new root growth without re-saturating.
Prevention Tip
Always sterilise second-hand pots with a 10% bleach solution and rinse thoroughly before reuse. Avoid terracotta glazed on the inside — unglazed terracotta allows lateral moisture exchange and greatly reduces root rot risk.
Related Issues to Explore
Yellowing Leaves — Overwatering Stress
Soft, diffuse yellowing spreading from lower foliage — the plant's above-ground signal that roots are suffocating in waterlogged substrate.
Read guide
Stem & Crown Rot — Botrytis & Fusarium spp.
When root rot advances upward into the stem and crown, Botrytis and Fusarium cause rapid vascular collapse — often fatal within days.
Read guide
Interveinal Chlorosis — Iron or Magnesium Deficiency
Root damage from rot directly impairs micronutrient uptake, causing interveinal yellowing that mirrors deficiency even when nutrients are present in the soil.
Read guide
