What is Crown Gall?
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Crown gall is a plant disease caused by Agrobacterium tumefaciens, a soil-dwelling bacterium. The bacterium induces irregular growths or galls on the roots, twigs, and branches of euonymus and other rose-related shrubs.
The bacterium causes the galls by stimulating the rapid growth of plant cells. The galls weaken and stunt the plant's growth, in addition to being unsightly. Despite the fact that galls can obstruct the flow of water and nutrients up the roots and branches, they seldom result in the death of the plant. Via polluted soil and equipment, the disease will spread to other plants. The majority of chemical therapies are ineffective.
Phomopsis sp. is thought to be the cause of galls on forsythia, viburnum, highbush blueberry, American elm, hickory, maple, oak, and privet. Since the cultural controls for bacterial crown gall are the same, both are viewed together here. Chemical therapies, on the other hand, can vary.
Crown Gall Symptom and Diagnosis
Galls are most commonly found on the roots and lower branches of plants near ground level. The galls become woody and hard as they grow larger. The outer layer darkens and becomes corky. With any branch or tip dieback, the plant may be weakened and stunted. Symptoms can not appear right away after an infection. Galls grow at their fastest throughout the summer months.
Life Cycle
Crown gall-forming bacteria can be found in the soil and can live for a long time. The bacterium is initially carried in by contaminated plants' roots. It spreads from there through soil and water movement, as well as polluted pruning tools. Bacteria enter the plant through wounds caused by chewing insects, cultivation damage, or grafting and pruning tools, among other sources.
In the absence of susceptible plants, the crown gall bacterium has been known to live in the soil for more than two years. It can survive decomposing galls buried in the soil for many years. Crown gall is more likely to be severe in limed soils than in acid soils, so soil pH can play a role in disease control.
Integrated Pest Management Strategies
1. Remove contaminated tissue with a pruner. Even if the infected plant lives for several years, the crown gall cannot be removed from it. Prune and destroy infected stems below the galled region to enhance the plant's appearance. After each cut, disinfect the pruning shears with a solution of one part bleach to nine parts water.
2. Eliminate contaminated plants. Ensure that the contaminated plant is destroyed. Since the bacterium can persist in the soil, it is important to plant a plant that is immune to it. If the same species must be planted in the same location, the soil should be removed and replaced, or soil sterilisation should be considered.
3. Cleanse the surface. Chemicals, fire, or antibiotics may be used to sterilise soils known to be contaminated with crown gall bacteria. Many home gardeners would find this impractical. A bacterium, Agrobacterium radiobacter strain 84, has been used as a biological control. Crown gall bacterium has been discovered to be antagonistic to this bacterium. It can be used as a pre-plant treatment by dipping nursery stock in a water-based suspension of live bacteria.
4. Rule out the problem. When buying forsythia and euonymus seeds, look for signs of galls. Plants with gall-forming signs should not be purchased.
Crown Gall of Apple
Apples are susceptible to a variety of galls, the worst of which is crown gall. The bacteria Rhizobium rhizogenes (old name Agrobacterium tumefaciens) causes this disease, which is likely to destroy your tree over time.
Crown gall can infect hundreds of different plants, so it can infect a variety of fruit trees. Apple, crabapple, peach, nectarine, mango, apricot, cherry, and pear are among them.
The bacteria genetically modify plants so that they can produce tumours using their own DNA. What you think of as a gall is actually a tumour that has developed on your tree.
The crown gall bacterium Agrobacterium tumefaciens can live saprophytically (living on dead organic matter) in soil for up to two years and can survive in decomposing plant debris for long periods of time. It will only continue its life cycle if it can infect and form galls on a new wound.
What are the Effects of the Infection?
The flow of nutrients and water up the stem is hampered by crown gall. Normal growth can be hampered, and your tree may begin to deteriorate. Drought stress or winter injury can make it more vulnerable.
Worst-case scenario: the growths will girdle the tree's stems or trunk, killing it completely.
What are Bacterial Galls?
The most successful cure for crown galls is to remove the gall and the bark tissue covering it after they have been exposed. Treatments that destroy or extract the bark that surrounds the gall produce excellent results. Careful surgery has been shown to be very successful in studies.
Some galls serve as "physiologic sinks," collecting resources from the surrounding plant parts and concentrating them in the gall. Galls can also provide physical protection to the insect from predators. Insect galls are typically caused by chemicals ingested into plants by insect larvae, as well as possible mechanical damage.
Plant material should be wound as little as possible. Planting too far is a bad idea. Mounding soil up around freshly planted trees is not a good idea. Keep the tree's crown as dry as possible; Agrobacterium thrives in moist conditions.
FAQs on Crown Gall
1. What exactly is crown gall?
Crown gall is a plant disease that results in the formation of large, tumour-like growths, called galls. These galls are typically found on the plant's stem near the soil line (the "crown") or on its roots. It is essentially a form of plant cancer caused by a specific bacterium.
2. What microorganism causes crown gall disease?
Crown gall is caused by a soil-dwelling bacterium named Agrobacterium tumefaciens. This bacterium infects plants through wounds, often caused by pruning, grafting, or insect damage. Once inside, it transfers a piece of its DNA into the plant's cells, forcing them to grow uncontrollably and produce the gall.
3. What are the main signs or symptoms of a crown gall infection?
The most obvious symptom is the appearance of rough, rounded galls on the plant. Key signs include:
- Galls that start as small, soft, and light-coloured swellings.
- Over time, these galls grow larger, becoming hard, woody, and dark.
- They usually appear at the crown (where the stem meets the soil), on roots, or at graft unions.
- Infected plants may also appear stunted, weak, or produce less fruit or fewer flowers.
4. How does crown gall spread from one plant to another?
The disease primarily spreads through the movement of contaminated soil, water, or gardening tools. The Agrobacterium tumefaciens bacteria can survive in the soil for years. Infection occurs when these bacteria enter a fresh wound on a susceptible plant. It can also be spread by planting already infected nursery stock.
5. Is crown gall harmful to the infected plant?
Yes, crown gall can be very harmful. While it may not kill a mature tree quickly, the galls disrupt the flow of water and nutrients through the plant's vascular system. This can lead to stunted growth, reduced vitality, and make the plant more vulnerable to other stresses like drought or secondary infections.
6. Why is the bacterium that causes crown gall so important in biotechnology?
The bacterium Agrobacterium tumefaciens is a key tool in genetic engineering. Scientists use its natural ability to transfer DNA into plant cells. They replace the tumour-causing genes on the bacterium's Ti plasmid with desirable genes (e.g., for pest resistance). The modified bacterium then acts as a natural vehicle to insert this new, beneficial DNA into the plant's genome, creating a genetically modified plant.
7. Can humans get sick from the bacteria that cause crown gall?
No, crown gall is strictly a plant disease. The bacterium Agrobacterium tumefaciens cannot infect humans or animals. The biological mechanism it uses to alter plant cells is specific to the plant kingdom and does not work in animal cells, posing no health risk to people or pets.
8. How can you tell the difference between crown gall and galls caused by insects?
While both can look like lumpy growths, there are key differences. Crown galls are solid, undifferentiated masses of tissue. In contrast, insect galls are often highly structured and may contain chambers or exit holes where the insect larvae lived. If you cut an insect gall open, you might find the insect or its remains, whereas a crown gall is just a solid mass of plant tissue.
