Crown gall occurs on a wide range of herbaceous and woody plant species including pome and stone fruit trees. The disease occurs worldwide and is especially troublesome in nurseries. Losses in orchards are sporadic. The disease is caused by the bacterium Agrobacterium tumefaciens.

Symptoms

The formation of galls on roots and crowns of plants are characteristic of crown gall. Galls are often located at pruning wounds, especially on apple. Small galls are initially smooth on the surface. As they enlarge they become dark, hard, woody tumors with gnarled, irregular surfaces. Gall shapes and sizes vary. The same root or crown may contain numerous galls. Secondary fungi and insects are attracted to galls as they age.

Disease Cycle

The bacterium causing crown gall is spread from infected nursery stock, which can contaminate orchard soils. These bacteria enter the roots and crowns of plants through wounds. The development of galls can occur in a few weeks or a few years, depending on various environmental factors (temperature) and host affected. The bacteria can escape from these galls and infect healthy roots and contaminate surrounding soil. The pathogen spreads from this contaminated soil to new sites and plants by irrigation water, splashing rain, tools, wind, insects, and plant parts used for propagation.

The development of crown gall is influenced by planting site and rootstock type. The most susceptible rootstock to crown gall is M.7 followed by M.9 and M.26. The disease is reported to be more severe on apple trees in sites previously planted in nursery crops such as peach, grape, raspberry, and rose. Planting trees in poorly drained soils seem to contribute to the incidence of crown gall.

Disease Management

Good sanitation and cultural practices are necessary for the control of crown gall. Planting disease-free nursery stock is essential to avoid the introduction of this disease. Choose rootstocks that are not susceptible. Adopt management practices that minimize wounding. Plant trees in well-drained soils.

In recent years, the use of a bacterial antagonist has been shown to be very effective in preventing crown gall on all tree fruit species, except apple. This antagonist is used on seeds, roots and stems of propagation material.

Fumigation is ineffective.