Crown Gall: Ugly Bumps On Shrubs And Trees
Crown gall impacts a wide variety of fruit trees and other shrubs. Our guide reveals the cause and explains prevention of this plant problem!
Crown gall, caused by the crown gall bacteria, Agrobacterium tumefaciens, is an unsightly plant disease found in soil. While in most cases crown gall is not fatal to plants, this disease disrupts water transport and nutrient flow throughout the plant and can lead to stunted growth and malnutrition. Crown gall impacts over 600 plant species, including many important crops like stone fruits and grapes.
These tumor-like growths were observed on grape vines and woody plants and documented starting in the 1800s throughout France, Italy, Germany, Canada, and the U.S. Many naturalists at the time thought the growths were spontaneous. It was not until 1897 that an Italian scientist, Fridiano Cavara, isolated the bacterium responsible for this disease.
Many woody plants like forsythias, maple, hickory, and others suffer from a different type of growth known as Woody Phomopsis Galls. This disease is fungal instead of bacterial, but cultural treatments for both galls are similar.
What Is Crown Gall?
Crown galls are fleshy plant tumors that form near the crown or soil line of an infected plant. Galls expand and become more corky and woody over time. As the galls enlarge, they continue to restrict the transport of water and nutrients, and the plant may be weakened and experience some tip dieback. Gall formation is most rapid under warm conditions.
Life Cycle of Crown Gall
The crown gall pathogen is a soil-inhibiting bacterium. The bacterium, Agrobacterium tumefaciens, can overwinter and survive for a long time in the soil and on infected plant material. Open fresh wounds on susceptible plants and roots expose them to a crown gall infection. Pruning and insects are two common ways to transmit the crown gall disease.
Once inside infected plants, the crown gall bacteria, Agrobacterium tumefaciens, transfers and integrates its own DNA into a host plant’s genome. The bacterial DNA instructs host plant cells to produce additional hormones which result in the proliferation of bacteria and cell hyperplasia and hypertrophy. The increase in cell count, hyperplasia, and the growth of cell size, hypertrophy, causes crown gall formation. Unlike other pathogenic bacteria, the crown gall bacterium does not trigger a hypersensitive immune response in infected plants which typically leads to rapid and localized death of plant cells. This means that the crown gall bacteria can survive systematically throughout the entire plant because it has essentially sneaked past the host plant’s immune defense.
Symptoms Of Crown Gall
Symptoms are easy to spot because of the telltale tumor growths commonly found near the soil line or towards the bottom of plant stems. Because the crown gall bacteria can travel throughout infected plants, occasionally, there will also be secondary galls or aerial galls on the upper branches. New galls are soft and white in appearance and older galls are corky or woody. The galls can range from half an inch to several inches in diameter. During the early stages of infection, there might not be visible signs of gall tissue on plant growth. Developing crown galls will show tumor formation once they burst free from the bark of the plant. Over time, plants with crown gall tumors may appear weak or sickly and become more susceptible to environmental stressors and winter injury.
What Plants Does It Impact?
Crown galls can affect many species of dicotyledonous plants including many ornamental plants and fruit trees. Bacterial crown gall is a major plant pathogen for the grape-growing industry worldwide. Common plants around the home and garden in North America affected by crown galls include roses and other Rosa spp.; bramble fruits like raspberries; apples; cherries; Prunus spp. like plums; and poplars to name a few.
Controlling Crown Gall
If you see early stages of infection on a young plant, immediately dig up the plant and remove as much of the surrounding soil as possible. An infection cannot be fully cured and the bacteria will travel throughout the plant. Do not compost any infected plant parts or the contaminated soil and try to plant a known resistant species to agrobacterium strains in its place. Established plants in the landscape such as infected trees with signs of crown galls can typically survive the infection but it is also recommended to plant resistant species in the surrounding area. Conifers, for example, are not host plants of the crown gall bacterium signs. Crown gall tumors induced by Agrobacterium tumefaciens cannot harm conifers as a result.
Specifically for many grape growers, it is recommended to select cultivars that are less susceptible to freeze damage and to plant vines on north-facing areas to minimize the stress of the freeze-thaw cycle. Proper pruning can also help maintain grape vine vigor and defense.
There are no effective chemical controls against this plant disease but some growers use a biological control (sold as Galltrol A, Norbac 84C, Nogall, or Diegall) made from the bacterium Agrobacterium radiobacter to protect propagated cuttings.
Preventing Crown Gall
Prevention is a key strategy to mitigate against crown galls because it is very hard to eliminate the soil bacteria once it establishes in your garden. A typical way that crown gall is introduced into a landscape is through infected transplants. Always double-check nursery stock or young plants before planting and purchase your plants from a reputable nursery to ensure that you have disease-free plants. Pay close attention to plant roots to check for any signs of infection. Disinfecting tools and workwear is also important to prevent the spread of many pathogens including crown gall. A solution of one part bleach to nine parts water can be used to sanitize tools.
Frequently Asked Questions
Q: What does crown gall do?
A: The crown gall pathogen is a bacteria (Agrobacterium tumefaciens) that infects host plant cells and triggers wart-like tumor formations near the base of the plant. While this plant pathogen is not fatal, it does cause a weakening of the plant and decrease the productivity of infected crops.
Q: Can crown gall affect humans?
A: The crown gall bacterium cannot affect humans but humans can play a role in the disease cycle by planting in contaminated soil, using contaminated pruning tools, and overall helping to transmit the pathogenic bacterium from one host plant to another.
Q: What plants are resistant to crown gall?
A: There are some resistant species of trees to crown gall such as conifers, magnolia, beech, birch, and sweet gum. Other resistant or tolerant woody ornamental species include boxwood and holly.