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Crown gall is a disease caused by the bacterium Rhizobium radiobacter (synonym Agrobacterium tumefaciens), which enters the plant through wounds in roots or stems and stimulates the plant tissues to grow in a disorganised way, producing swollen galls. Galls are present all year.

Crown gall affects many plants, both woody and herbaceous. These are some of the plants on which it is found most commonly:

• Fruit: Apples, cherries, currants, gooseberries, grapevines, blackberries, peaches, pears, plums and quince
• Vegetables: Beetroot, courgettes, runner beans and swedes
• Herbaceous plants: Alcea (hollyhock), Argyranthemum (marguerite), Begonia, Dahlia, Lathyrus (sweet pea), Lupinus (lupin) and Phlox
• Woody plants: Crataegus (hawthorn), Euonymus, Populus (poplar), Salix (willow), Rosa and Ulmus (elm)

Note: swellings caused by crown gall should not be confused with the harmless nitrogen-fixing nodules produced on the roots of many members of the pea family.

You may see the following:

• Swellings (galls) on the plant stems or roots. In severe cases almost all the root system may be replaced by massive, swollen tissues
• Galls on herbaceous plants decay and soon disintegrate, but those on woody plants may be hard and perennial
• Plant growth may be affected, but often there is little apparent damage and root galls may go unnoticed for long periods
• Occasionally galls may form higher up a plant on the branches

Non-chemical control

If crown gall is detected, lift and destroy affected plants. Grow crops of potatoes or other vegetables (except beetroot, which are also susceptible) over the next one or two years to help eliminate the bacteria from the soil, or grass the area over for one or more years.

Chemical control

There are no chemicals available for the control of crown gall.

The crown gall pathogen Rhizobium radiobacter inhabits the soil where it can survive for long periods. Not all strains of this bacterium are pathogenic (capable of causing disease) and there are several different pathogenic strains with differing host ranges.

The bacteria enter stems or roots through wounds. The bacterial DNA combines with the DNA of the plant host cell where it ‘transforms’ the cell, causing it to become tumour-forming (galls) and also to produce specific new materials on which the bacteria feed. As the gall grows, the plant tissues become disorganised and normal transport processes are disrupted.

In herbaceous plants, the gall rots and the bacterial cells return to the soil. On woody plants, the galls are woody and perennial and do not rot away.