Crown Gall

Agrobacterium tumefaciens

Crown Gall

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A close up of a a crown gall formed in a tree branch
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A very common and widespread disease but largely superficial the damage caused. Crown Gall is more of an issue with ornamental plants as the galls can be unsightly.
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Identification

Irregular or sometimes almost spherical galls that can appear on almost any part of the plant. They can be very difficult to distinguish between Crown Gall and other types of gall due to the diversity of their appearance and that they can appear anywhere on the plant. Usually, however, they mainly occur at the junction between the roots and stems. Crown Gall can infect a massive range of woody and herbaceous plants. Galls are hard on woody plants but soft on herbaceous plants and can become an ideal place for rots to invade.

Symptoms

Initially small swellings appear on the roots and stem near the soil line.
Young tumours are soft and cream-coloured.
Older tumours darken and become more irregular in shape.
Gall can be crumbly or spongy, or woody and knot-like.
Necrosis of the leaves.
Stunted growth.
Plants may be more susceptible to secondary infestations.

Biological treatment

Carefully to avoid wounding, remove the galls from ornamental plants and burn the galls.

Chemical treatment

The application of sulphur to contaminated soil has been known to kill the bacteria but it is rarely cost effective since the disease is minor.

Lifecycle

The bacteria is spread by insects or splashing rain, entering plants through tiny wounds. The bacteria begin to replicate, stimulating the surrounding plant tissues. The surrounding tissues begin to grow, eventually forming a large tumour, called a gall. Unlike other, more healthy plant parts, the tumour isn't protected by a surrounding layer of tissues (the epidermis). The lack of epidermis makes plants susceptible to secondary infections by other bacteria, fungi and insects. Secondary infections cause the gall to darken, turning-blackish brown. At this point, new bacteria are released in water or into the soil, where they can lay dormant until the following growing season, or even for years after.

Prevention

Practise good tool hygiene. Sterilise tools between use, especially after dealing with diseased plants. Not growing on land that has supported the disease before as the bacteria may still be in the soil. Careful handling of stock to reduce the risk of wounding plants. Barberry, Boxwood, Holly, Mountain Laurel, and Elderberry show some resistance.
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