Root Rot
Root rot is the destruction of the root system by soil-borne fungi and water-moulds (Phytophthora, Pythium) that thrive in waterlogged, oxygen-poor soil.
Root Rot at a glance
| Also known as | Phytophthora / Pythium root rot |
|---|---|
| Affected parts | Roots, root collar, then whole plant |
| Most at risk | All species in wet, airless soil; dry-loving trees especially (junipers, pines, jades, olive, desert rose) |
| Main cause | Soil-borne water-moulds attacking roots kept constantly wet/airless |
| Severity | Serious — a leading killer once advanced |
| Reversible? | Only if caught early and enough healthy root remains |
What to look for
- Leaves: Yellowing, wilting, and progressive branch dieback; weak, stunted growth.
- Soil: Constantly wet; a sour, rotten, 'eggy' or earthy smell.
- Roots: The key sign — healthy roots are firm and white/tan; rotted roots are brown or black, soft, mushy, slimy or stringy, and the outer layer slips off leaving a bare thread. A bad smell confirms it.
- Whole tree: Overall decline that doesn't respond to watering; the tree may suddenly collapse.
Overview
Root rot is the destruction of the root system by soil-borne fungi and water-moulds (Phytophthora, Pythium) that thrive in waterlogged, oxygen-poor soil. It is usually the consequence of overwatering or poor drainage. Once roots rot, the tree can't take up water or nutrients, so the canopy declines even though the soil is wet — and by the time the top looks bad, the damage below is often severe.
Causes
Overwatering and poor drainage are the root cause; compacted/old broken-down soil; oversized pots; no drainage; cold-wet soil in dormancy; and contaminated tools or reused unsterilised soil spreading the pathogens.
Commonly confused with
Overwatering (the cause) shows wet soil before roots actually rot. Underwatering wilts with dry soil and firm (if dry) roots. Drought and rot can both yellow/wilt the top — the only sure diagnosis is to inspect the roots: mushy, dark, smelly roots mean rot.
Which species are affected
Any species, but especially dangerous for dry-loving conifers and succulents (junipers, pines, spruce, jade, desert rose, olive, rosemary) and any tree left wet, particularly in winter.
Treatment
Act fast: remove the tree, wash off soil, and cut away all soft/black roots back to firm, pale tissue with sterilised shears. Reduce the top proportionally if much root is lost. Repot into fresh, open, free-draining inorganic mix (pumice/lava/akadama); water in, then keep just-moist (not wet) and place in bright, airy shade to recover. A soil drench with an appropriate fungicide can help. Don't fertilise until new growth shows.
Prevention
Free-draining soil, correct watering (let topsoil dry between waterings), good drainage, right-sized pots, reduced dormant-season watering, clean tools and fresh soil at repotting.