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Plant diseases
Root rot
Phytophthora and Pythium · Water mould
Also called Phytophthora dieback (Australia), Phytophthora root and crown rot (USA).
Illustration
Roots rotting in wet, poorly drained soil: the plant wilts and doesn't perk up when watered. There's no spray cure for gardeners, so the answer is drainage, careful watering and clean plants.
How to spot it
- Wilting that doesn't recover after watering.
- Yellowing, dull or sparse leaves and dieback from the branch tips; conifers fade from green to grey to brown.
- Roots brown or black inside and soft, with few fine roots; the outer layer slides off, leaving a thin strand.
- Dark bark at the base of the stem, with reddish-brown streaks underneath.
Easily confused with
Drought
A thirsty plant perks up after a good watering; one with root rot doesn't.
Honey fungus
Leaves a creamy-white sheet of fungus under the bark at the base, which root rot never does.
Vine weevil
Plump, C-shaped white grubs with brown heads among the roots.
Where it's a problem
- Worldwide: Mainly a problem of heavy or waterlogged soils and overwatered pots.
- Australia: Phytophthora dieback is listed as a key threat to native plants under national environment law.
- New Zealand: A related species causes kauri dieback: if your kauri trees look unhealthy, you must tell Tiakina Kauri or your regional council.
What to do
- Stop overwatering, and never leave pots standing in water.
- Improve drainage: fork the soil, add organic matter, or use raised beds or mounds.
- Plant at the right depth with the base of the stem showing, and keep mulch away from trunks.
- Remove badly affected plants, replace the soil around them, and replant with less susceptible plants.
- Clean tools and boots, check the roots before buying plants, and use fresh potting mix in pots.
There are no chemical treatments for gardeners in the UK, and elsewhere official guides warn against relying on sprays: fix the drainage first.
Follow your local council's guidance for disposing of diseased plant material.
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Sources
- RHS: Phytophthora root rot
- University of California IPM: Phytophthora root and crown rot
- University of Wisconsin–Madison Extension: Root rots in the garden
- Penn State Extension: Pythium
- Business Queensland: Phytophthora root rot
- Australian Government DCCEEW: Phytophthora cinnamomi
- Tiakina Kauri: National plan rules
Facts checked Sunday 27 September 2026. General guidance only: follow your local council, agency and product label.