Diseases & rots

Root rot

Also called: Pythium, Phytophthora, wet feet

Roots suffocate in waterlogged mix and are then colonised by water moulds; the plant wilts while the soil is wet.

What you'd see

Wilting or drooping even though the soil is moist
Yellowing lower leaves, sometimes several at once
Soil that stays wet for far longer than usual; a sour or swampy smell
Roots brown, soft and slipping their outer layer when tipped out; healthy roots are firm and pale

Where to look

Pots without drainage, oversized pots, dense peat mixes, cool rooms and low light — anywhere water sits.

How it spreads

The moulds (Pythium, Phytophthora) live in wet mix and can travel in shared water trays.

Is it really this? Look-alikes

Underwatering — A dry, light pot with crisp leaves is thirst; a heavy wet pot with droop is rot.
Cold shock — Sudden collapse after a cold night, with roots still firm.

What's going on

Roots breathe. In waterlogged mix the air pockets fill with water, the roots run out of oxygen and begin to die. Water moulds then move into the dying tissue and finish the job.

Damaged roots can no longer take up water, so the plant wilts as if thirsty — which tempts people to water more. That is the trap.

Why?

Root rot is almost never a disease that arrived; it is a condition that was created. Fixing the roots' access to air fixes the cause, and the mould loses its foothold. This is why Sensei asks about the soil before it asks about a fungicide.

Grandmother's remedies

What people have always done — with an honest note on each.

Tip it out and look
Slide the plant out, rinse the roots, trim every brown mushy root with clean scissors.
Verdict: Essential. You cannot treat what you have not seen.
Cinnamon on cuts
Dust trimmed roots with ground cinnamon.
Verdict: Mild antifungal; harmless and possibly helpful. Not a substitute for fixing drainage.
Repot smaller in fresh chunky mix
A pot just big enough for the remaining roots, in a bark-and-perlite mix.
Verdict: The real treatment. Smaller pot, more air, faster dry-down.

Modern remedies

What growers and extension services reach for today.

Hydrogen peroxide rinse
3% peroxide diluted 1:3 in water poured through the mix or used as a root rinse.
Verdict: Adds oxygen and knocks back moulds; modest evidence, low risk.
Fungicides for water moulds
Products containing phosphorous acid or similar, used per label.
Verdict: Useful for valuable plants; pointless if the drainage problem remains.

Honest verdict

A plant with a few remaining healthy roots usually recovers if you fix the air problem and stop watering until the top layer dries. A plant with no white roots left is better propagated from a healthy top than nursed.

Prevention

Pots with drainage holes, always
Chunky mixes; add bark and perlite
Pot size one step up, never three
Check soil depth before watering, every time

Plants this commonly troubles

Monstera Pothos Snake plant Fiddle-leaf fig Philodendron