How to recognise it
This bacterial infection produces ‘galls’ on any part of the plant’s stem or roots, but they are most often found where the root and stem join. A gall is a knobbly swelling which is usually irregular in shape, but is sometimes spherical. On the branches of trees they can be very large, often over 1m in diameter.
On woody plants the galls are very hard but they are usually soft and fleshy on herbaceous plants, often rotting away as they get older. Galls can also appear on leaves, in a similar symptom to, although more discrete than, leafy gall.
Why it’s a problem
Whilst crown galls can be unsightly, they rarely cause any serious damage to the plant. Fruit trees are at greatest risk of reduced growth if infected.
The crown gall bacteria resides in the soil and only causes galls when blown by the wind, or splashed by rain, onto wounds in the plant’s stem or root. Once established it appears that the bacteria is able to move within the plant to cause a secondary gall to occur.
Where you are likely to find it
Crown gall is a very common disease affecting woody and herbaceous plants. Most often affected are almonds, apples, beetroot, begonias, blackberries, cherries, courgette, currants, dahlias, elms, euonymus, gooseberries, grapevines, hawthorns, hollyhocks, loganberries, lupins, marguerites, marigolds, marrows, peaches, pears, pelargoniums, phlox, plums, poplars, privet, quinces, raspberries, rhododendrons, roses, runner beans, swedes, sweet peas, tomatoes, wallflowers, walnuts and willows.
How to prevent it
As they are the worst affected by this disease, fruit trees shouldn’t be grown on land where plants have previously suffered from crown galls.
Good pruning practice, ensuring all cuts are cleanly made to hasten the healing process, will help reduce the risk of infection.
How to get rid of it
Disfiguring galls on ornamental plants can be cut out. The removed section should be burnt.
Growing potatoes or other vegetables (except those which are susceptible to the disease) on a contaminated area, for at least 2 years, can help to significantly reduce the bacterial levels. Equally, growing grass on the area for a year or more can have the same effect.
Applying sulphur to the soil may reduce the risk of infection from soil which is known to be contaminated, however on a domestic scale this is too great an expense and effort to go to for a relatively minor disease.
On a commercial scale copper is sometimes used to manage the disease and biocontrols are available which are used to prevent certain strains of the bacteria from infecting plants.
Is it good for anything?!
Other useful information
Galls are tumours caused by the bacterial DNA combining with the plant’s DNA to mutate the plant’s cells and create a nutrient rich environment in which the bacteria to live and grow. This ability to integrate with the plant’s DNA, thereby changing its genetic make up, has been harnessed for the genetic engineering of crops and has been used, for example, to produce more flavoursome tomatoes and cotton plants which are herbicide resistant.
Burr knots, which can occur on some types of apple, on willows and a few other plants, can be mistaken for crown gall. These growths, which form masses of hairy roots at the base of branches, are naturally occuring and not caused by a pathogen.