Crown gall is a plant disease caused by the bacterium Agrobacterium tumefaciens. The tumor-like galls that appear on the roots, trunks, branches or stems of trees and shrubs are unsightly but don’t necessarily kill mature plants. However, it’s crucial to diagnose and treat the disease early so it won’t spread, especially to young plants that can be killed by it.
Which Plants Can Get Crown Gall?
More than 600 plant species, both herbaceous as well as woody perennials, can get crown gall. It occurs most often in trees, including fruit trees (apple, apricot, cherry, pear, nectarine, peach, plum, and quince), willows and other hardwood shade trees, shrubs such as Euonymus, and roses.
The damage is most eye-catching in trees because crown gall is a perennial disease, and as the tree grows, the galls grow with it.
How to Identify Crown Gall
After a plant has been infected, the first signs of a gall may appear within two to four weeks during the growing season: swollen tissue that looks like warts, or light-colored, round galls of about 1/10 inch. As the galls grow, they get darker, harder and more irregular in shape. Old galls are hard, dry and dark, with a rough surface and numerous cracks.
Galls can appear on roots, trunks, branches or stems. A common place for galls to pop up is the root collar where the stem meets the soil.
How Plants Get Infected
The plant pathogen causing crown gall, Agrobacterium tumefaciens, is commonly found in many soils. It attacks a plant through fresh physical damage to the roots from digging, tilling, or planting, or from insects and nematodes that feed on the roots of the plant. Above-ground the disease can get transmitted by wounds from pruning or grafting, either through contaminated tools, or rain splashing contaminated soil to the injured parts.
The pathogen attaches to an exposed cell of the host plant and transfers some of its DNA to the cell. The host cell then incorporates these foreign genes with its own genetic material and becomes a tumor cell, dividing uncontrollably and forming galls.
The time during which a wounded cell is vulnerable to the crown gall pathogen can range between a few days during the growing season, to several months during the dormancy period.
Damage Caused by Grown Gall
Galls prevent the movement of water and nutrients within the plant, which leads to nutrient deficiently and reduced plant growth. The leaves of plants with a heavy crown gall infection are yellowed and smaller than those of healthy plants.
Plant death from crown gall only occurs when young plants are covered in galls, or a gall is fully girdling the trunk or stem. Mature trees can survive even a large amount of galls, but they are more susceptible to heat, drought, winter injury, and secondary diseases that can attack through the cracks in the gall.
Taking prompt action when you notice crown gall is very important because as galls age, they decay and decompose. The pathogen is then transmitted back to the soil where it survives for many years, continuing the disease cycle indefinitely.
If a crown gall appears on a recently planted tree or shrub, if at all feasible, dig up the plant and the soil immediately surrounding the roots. Safely dispose of it in the trash or by burning, and don’t compost it. Fill the planting hole in with new, healthy soil.
When an established tree and or shrub is infected, you can leave it in your yard but remember that crown gall cannot be removed, and the infected plant may be around for years, potentially spreading the disease to other plants.
If you decide to keep the plant, you can prune out and destroy infected branches or stems below galls. Make sure to sterilize your tool after each cut with a 10% bleach solution (one part bleach to nine parts water).
If a tree has enormous galls, it is best to cut it down. The pathogen will remain in the soil, and soil replacement or soil sterilization are daunting and usually not feasible for a home gardener. Your replacement should be a different species that is less prone to getting crown gall.
Prevention of Crown Gall
Once the grown gall pathogen is in the soil, it's very difficult to get rid of. Therefore, prevention is key.
When buying new plants, carefully inspect their roots, trunks, branches, and stems carefully for any galls.
If you already have a crown gall problem in your yard, stay away from planting roses, willows, poplars, and fruit trees, as these are the most susceptible to the disease.
Instead, choose resistant trees and shrubs, such as barberry, beech, birch, black gum, boxwood, catalpa, deutzia, firethorn, ginkgo, golden-rain tree, holly, hornbeam, larch, little-leaf linden, magnolia, mahonia, redbud, serviceberry, smoke tree, sweet gum, tulip tree, yellowwood, and zelkova. Conifers are also resistant to crown gall.