Crown gall is now a relatively uncommon disease in chrysanthemums. It is caused by a microscopic bacterium Agrobacterium tumefaciens  which causes irregular growths on the base of the stem, usually below soil level. The presence of the disease may not become apparent until stools are lifted and prepared for boxing prior to over-wintering.

The disease is soil-borne and can survive for long periods, possibly up to two years in wet soils.

It can only enter injured roots and stems. Infection early in the season can lead to rapid gall formation within weeks. When it occurs in a perennial plant in the autumn, galls appear in the following spring.

On no account should infected plants be used for future propagation and all infected stools should be destroyed. Because the soil will contain bacteria which can survive long periods on small pieces of plant tissue the area should not be used again without sterilisation.

The bacterium carries a plasmid which switches genes on for the tumour production and this acts in a similar way to a virus but it is not a virus. This disease is used in recombinant DNA technology.

Impact/effects on chrysanthemums
Basal area of chrysanthemum stool where irregular growths are formed on the stool (usually below soil level). The main qualitative implications from this disease relate to future propagations from the infected stock and to perpetuating the disease by constantly cropping from the infected area.

Controls
There is no chemical cure for this disease.

Some actions that can be taken to minimise disease impact include:
- Drainage should be improved to reduce the survival of the bacterium in the soil.

- Inspect new stock to avoid contamination of the soil. Where the disease is present, a crop of potatoes or other vegetable (except beet) may help to eliminate the bacterium.

- Wounding should be avoided, particularly root wounding during planting