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Root rot

Also known as Pythium root rot, Phytophthora root rot, Rhizoctonia root rot

DiseaseHarmful
Root rot of pineapple caused by Phytophthora cinnamomi Photo: Scot Nelson from Honolulu, Hawaii, USA · CC0

In short

Root rot is decay of the roots, most often caused by water moulds such as Pythium and Phytophthora and by some true fungi such as Rhizoctonia and Fusarium. These organisms thrive in wet, poorly drained soil and potting mix, so overwatering and pots that stay wet are the usual trigger.

How it looks

Rotted roots are brown to black and soft or mushy; with Pythium the outer layer pulls away easily, leaving a fine, hair-like core. Phytophthora turns root tips dark brown and soft and can blacken the lower stem. Rhizoctonia can show coarse reddish-brown threads, like spider webs, over roots and stem bases.

Signs

Wilting even though the potting mix is moist, stunting, yellowing and dropping of lower leaves, browning and dieback, and eventually collapse. When the plant is lifted, most of the rotted root system may stay behind in the pot.

Life cycle

Water moulds produce swimming spores that move through water films between soil particles and attack young root tips, especially of stressed plants. They spread in drainage water, contaminated soil or potting mix, dirty pots and infected plants. Long-lived resting spores can survive in soil for several years without a host.

What favours it

Waterlogged or heavy soil and potting mix, poor drainage, standing water in saucers or cachepots, reused unsterilised pots and media, and warm, wet conditions.

Prevention

  • Water only when the plant needs it and let excess drain away; empty saucers and cachepots.
  • Use a free-draining mix suited to the plant and a pot with drainage holes.
  • Use fresh or sterile potting mix; do not use garden soil in pots.
  • Clean previously used pots before reuse (a 10% bleach solution is one method).
  • Inspect and isolate new plants before adding them to a collection.

Treatment

Start with the first step and move down only if the problem persists.

  1. Unpot the plant and inspect the roots; cut away soft, brown roots with clean tools.
  2. If the infection is limited, repot into fresh, sterile mix in a clean pot, and water sparingly while it recovers.
  3. If most roots are gone, take stem cuttings from healthy tissue and discard the rest of the plant with its mix.
  4. Isolate the plant from others while you deal with it.
  5. Commercial growers use oomycete-specific fungicides; home growers in some countries have none available. Follow the product label.

Pros and cons

Cons

  • Often not noticed until the plant is badly damaged because the roots are hidden.
  • Pathogens persist in used mix and pots and can spread to other plants.

Look-alikes

Underwatering
An underwatered plant wilts when the mix is dry and recovers after watering; roots are dry but not mushy.
Overwatering
Overwatering is the condition that sets up root rot; roots may still be firm if caught early.
Fusarium wilt
Fusarium wilt stains the internal water-conducting tissue brown, often while roots look less decayed.
Salt and fertilizer burn
Fertiliser burn browns leaf tips and edges and may leave a white crust on the mix.

Affected plants

Almost any plant grown in wet conditions, including most houseplants; in gardens, yew, box and rhododendron are commonly affected by Phytophthora.

Sources

  1. Root Rots of Indoor Plants — University of Maryland Extension
  2. Phytophthora root rot — RHS
  3. Houseplant Diseases & Disorders — Clemson HGIC
  4. Root, Crown, and Stem Rots on Flowers — University of Maryland Extension

Written in our own words from these sources. Check a product label before using any treatment.