Crown gall, a bacterial disease that occurs throughout the world, infects several different plant hosts. In particular, it is a devastating disease in the Rosaceae (rose) family.
The specific bacterium, Agrobacterium tumefaciens, causes crown gall by inserting a tumor-inducing gene into the plant genome. Scientists have extensively studied this bacterium and used it for introducing desirable traits into many cultivated plant species. Since the 1970s, genetic engineers have exploited the bacteria’s ability to insert genes into other plants, and the agricultural industry has benefitted from creating genetically modified plants with crops that grow faster and bigger and are resistant to insects and other diseases.
Initial symptoms on roses appear as a small, light green spherical swelling around the crown of the plant. This swelling also occurs below the soil line (Fig. 1) on the roots as well as on higher branches, depending on the infection site. These growths should not be confused with normal callus growths that commonly form at wound and grafting sites.
Once the infection worsens, the shape of the galls becomes uneven, and they harden into a dark, woody mass (Fig. 2). In heavily infected plants, secondary tumors may develop near the first gall. There may be several tumors affecting a single plant in any given infection. Gall formation interferes with the plant’s ability to transport water and food supplies, producing other symptoms such as
- Discoloration of leaves
- Dieback of shoots
- Increased susceptibility to winter injury or secondary infection
- Wilt and, eventually in severe infections, death
It is easy to confuse crown gall with symptoms of hairy root, another root pathogen caused by the bacterium Agrobacterium rhizogenes. Hairy root causes the same gall formations; however, they are not woody and produce fibrous roots that project from the galls, giving them a “hairy” look.
The bacterium that causes crown gall lives in the soil that surrounds the host—in this case, roses. This bacterium can live in the soil as a decomposer for years without infecting a living host. When a plant is injured—either by mechanical transmission, insect feeding, or naturally—the damaged cells release compounds into the soil, attracting the bacterium to the wound site. Once inside, the bacterium replicates rapidly, forming the tumor-like gall by integrating some of its DNA (deoxyribonucleic acid), which is contained in a circular plasmid, into the host’s DNA. A plasmid is a small DNA molecule that is isolated from chromosomal DNA and can replicate on its own. Once the bacterial genomic material is incorporated into the host’s genome, the normal plant cells are altered. They multiply and form the gall structure.
Only the strains of these bacteria that contain these tumor-inducing plasmids (Ti plasmid) can cause disease. Some strains of A. tumefaciens lack the specific plasmid and remain in the soil without causing disease. When the outside tumor cells shed into the soil, replicated bacteria can live in the soil and be carried off by water to infect neighboring plants.
A.tumefaciens lives primarily in the upper layers of the soil closest to the plant (rhizosphere). It lives for years off of decaying organisms until conducive conditions allow it to inhabit a host plant. The bacterium can enter a plant only through a wound site or natural opening. Following favorable rose-growth conditions, A. tumefaciens becomes dormant during the cold winter months and is most active in summer months.
Managing crown gall disease includes, but is not limited to, these preventive cultural practices:
- Purchase healthy-looking plants.
- Avoid injuring the plant (especially around the roots and crown) while planting, and try to reduce the impact of chewing insects that can cause wounds.
- Plant in soils with no previous record of crown gall.
- Sterilize all pruning utensils before and after use.
- If symptoms develop after planting, dig up the whole plant and dispose of it properly, including the surrounding soil.
- Use insecticides to reduce the wounds on plant tissues that insects can cause. Be careful when undertaking any chemical control measures.
- Soak seeds or bare-rooted plants in a solution containing a closely related species, Agrobacterium radiobacter, as a preventive treatment. The bacterium
- A. radiobacter does not induce disease in plants; it uses similar resources and can prevent crown gall disease by typically out-competing A. tumefaciens.
Horst, R. Kenneth, and Raymond A. Cloyd. 2007.Compendium of Rose Diseases and Pests. St. Paul,MN: American Phytopathological Society.
Londeree, N. 2013. Garden Bad Guys–Crown Gall.Marin Rose Society. ttp://www.marinrose.org/crowngall.html.
University of Illinois Extension, Integrated Pest Management. 1999. “Crown Gall.” Reports on Plant Diseases No. 1006. http://extension.cropsciences.illinois.edu/fruitveg/pdfs/1006.pdf.
Watt, B. 2010. “Crown Gall.” University of Maine Cooperative Extension: Insect Pests, Ticks, and Plant Diseases. Pest Management Fact Sheet #5095.http://www.extension.umaine.edu/ipm/ipddl/publications/5095e/.
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