Crown gall—Agrobacterium tumefaciens
The crown gall bacterium causes distorted growths or galls on bark. Many plants can be infected, especially euonymus, fruit and nut trees, Prunus spp., rose, and willow. Herbaceous hosts include chrysanthemum, dahlia, geranium, marigold, peony, and snapdragon.
Agrobacterium tumefaciens usually does not seriously harm woody plants unless galls occur in the root crown when plants are young. Infected young plants may become stunted and subject to wind breakage and drought stress. If galls are large, water or nutrient transport can be inhibited to the extent that young plants are killed. Crown gall appears to have a relatively minor effect on most older plants.
Identification
Another bacterium, possibly Agrobacterium pseudotsugae, causes galls on stems of Douglas-fir. Crown galls on bark surfaces resemble damage from certain boring insects, woolly aphids, or other causes of plant galls. Certain boring insects colonize galled tissue, complicating diagnosis of the original cause.
Agrobacterium tumefaciens causes galling principally on the basal stem and root crown, at the soil line or just below the surface. Galls sometimes also develop on limbs, trunks, and roots.
The surface of crown galls and wood underneath is the same color as healthy bark and wood. However, when cut with a knife, crown galls are softer than normal wood and lack the typical pattern of annual growth rings. Galls can be tiny and smooth on young plants but usually are rough and sometimes massive on older trees.
Life cycle
The bacteria reproduce in galled tissue and may slough off and survive in soil for long periods. Bacteria in soil enter plants through wounds, commonly caused during handling in the nursery or when transplanting. The bacteria can also infect newly emerging roots, growth cracks, and wounds caused by sucker removal or string trimmers or other equipment.
Solutions
Where crown gall has been a problem, plant resistant species, including birch, cedar, coast redwood, holly, incense cedar, magnolia, pine, spruce, tulip tree, and zelkova. Purchase and plant only high-quality nursery stock and avoid injuring trees, especially around the soil line.
Dipping seeds or roots in the nursery in a solution of the biological control agents K-84 or K-1026 [nonpathogenic strains of A. tumefaciens (=A. radiobacter)] may reduce infections by most strains of pathogenic crown gall bacteria but not the strain commonly affecting grape. Solarization, covering moist, bare soil with clear plastic for 4 to 6 weeks during the sunny, dry season before planting may reduce crown gall bacteria in the upper soil.
Galls can be removed by cutting into healthy wood around galls and exposing the tissue to drying. Cut out galls only during the dry season and minimize the removal of healthy tissue. At least on certain fruit and nut trees, gall regrowth is frequently avoided by using a blowtorch to briefly heat and sterilize the edges where galls were removed. Do not burn or char tissue and use caution to avoid fire or injury.
If galls encompass much of the crown area, cutting them out causes other problems or tree death. Replace extensively galled trees if their growth is unsatisfactory.
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Swollen outgrowths found on stem
Galls on roots
Warty tumors of large roots near the crown area of tree
Galls on blackberry roots
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