Crown gall is caused by a soil-inhabiting bacterium, Agrobacterium tumefaciens, which occurs worldwide and attacks over 600 plant species in more than 90 plant families. The disease severity can vary depending on the host, but host mortality is usually caused by secondary pathogen invasion. Infected plants are also predisposed to drought and winter injury. Other growths caused by insects, adventitious growths, tissue proliferation and burls can sometimes be mistaken for crown gall symptoms.
The most obvious symptoms are the galls or growths that usually occur on the twigs, stems, and roots near the base of the plant at the soil line. Gall size can vary from small to large and are usually spongy when young, but then become hard and woody with age. Galls can also appear higher in the plant canopy, on twigs and branches, well above the soil line. The color of galls will vary from cream or green on young roots and stems, to normal bark color on older stems and roots. The outer tissue on older galls will often crack, become rough, and will frequently slough off. Plants that are infected late in the growing season may not show symptoms until the next season.
Crown gall bacteria can be present in soil, on contaminated tools or carried by water to susceptible host plants. These bacteria can also survive in contaminated soil for years without a susceptible host. Crown gall bacteria need fresh wounds to cause an infection, which can be caused by transplanting and cultivating activities, feeding damage from nematodes or soil insects. Galls form within 2-4 weeks after infection on actively growing plants. As outer surfaces of the galls slough off, bacteria are returned to the soil.
Removal of galls will not cure infected plants because bacterial genes already inserted into the host's cells will continue to transform additional cells throughout the plant and produce galls in other locations. Removal of infected plants will lower bacterial populations in the soil, however low populations of soil bacteria still persist as surface colonies on many plant species regardless of their susceptibility to crown gall. Exclusion of infected plants by inspection of all nursery stock prior to planting is critical to prevent the introduction into new planting beds. However, late infections on dormant nursery stock may not always show obvious symptoms. If crown gall symptoms are already present on existing plants consider the selection of other nonsusceptible plant material.