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Crown gall

Also known as Agrobacterium tumefaciens, Rhizobium radiobacter

DiseaseHarmful
Crown gall on crape myrtle Photo: Scot Nelson · CC0

In short

Crown gall is a bacterial disease that makes plants grow tumour-like swellings, usually where the stem meets the roots. The soil bacterium enters only through fresh wounds and inserts some of its own DNA into plant cells, which then grow out of control. Galls cannot be cured, so prevention centres on clean stock and avoiding wounds.

How it looks

Galls start as small swellings and enlarge into knobbly, rough, irregular lumps. They most often form at the crown (soil line) and on roots, but can appear on stems, branches and even leaves wherever there was a wound. On woody plants galls can be hard and persist for years; on soft-stemmed plants they soon decay and fall apart.

Signs

Rough swellings at the stem base or on roots; stunting and poor vigour in young plants; galls on stems of plants such as chrysanthemum and grape in moist conditions. Many infected plants show little effect above ground and root galls often go unnoticed until repotting.

Life cycle

The bacterium lives in soil for long periods. It enters through fresh wounds from pruning, grafting, cuttings or root damage, transfers a piece of its DNA (from the Ti plasmid) into the plant cell, and the altered cells produce growth hormones that drive gall formation. Tools, water splash, infested media and contaminated water can carry it.

What favours it

Wounding is the key factor: propagation, pruning, grafting and root damage during planting or cultivation. Infested soil or media and moist conditions favour infection.

Prevention

  • Buy healthy, certified clean plants and inspect the root crown before buying.
  • Avoid wounding stems and roots; handle roots gently when repotting.
  • Sterilise pruning and propagation tools between plants, for example with a hypochlorite (bleach) solution.
  • Use fresh or heat-treated potting media; do not reuse media from galled plants.
  • Commercial growers treat roots and cuttings with the protective bacterial strain K84 (or its derivative K1026) within 24 hours of wounding to block infection.

Treatment

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

  1. Remove and dispose of young or badly galled plants; do not compost them.
  2. On a valued woody plant, galls on branches can be pruned out below the gall with sterilised tools.
  3. If a plant is removed, the RHS suggests growing potatoes or other vegetables (not beetroot, which is also susceptible) in that spot for one or two years to reduce the bacteria; in pots, use fresh sterile media.
  4. A gall-treatment product based on xylenol and meta-cresol is registered for commercial nursery use in some places; follow the product label.

Pros and cons

Cons

  • No cure for existing galls.
  • The bacterium persists in soil for long periods.
  • Galls on woody plants can persist and weaken the plant.

Look-alikes

Root-knot nematode galls
Nematode galls are usually many small swellings along the roots rather than one or a few large knobbly lumps at the crown.

Affected plants

A very wide range of woody and soft-stemmed plants, including roses, fruit trees (apple, cherry, peach, plum), grapes, caneberries, willows, dahlias, hollyhocks, chrysanthemums, beetroot, runner beans and courgettes.

Sources

  1. Crown gall — RHS
  2. Crown Gall — Floriculture and Ornamental Nurseries, UC IPM
  3. Agrobacterium tumefaciens — Wikipedia

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