The crown gall pathogen Agrobacterium tumefaciens 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.