Crown gall, a serious plant disease caused by the bacterium Agrobacterium tumefaciens, is characterized by the formation of large, tumor-like growths. These galls typically develop at the crown of the plant (where the stem meets the roots) but can also appear on stems and roots.
By disrupting the plant’s water and nutrient transport systems, these galls can weaken and eventually kill the infected plant.
Crown gall is a widespread disease that thrives in temperate climates. It poses a serious threat to a diverse range of plants, both herbaceous and woody. Over 600 plant species are susceptible to this bacterial infection, including:
Fruit Trees: Almond, Apple, Pear, Plum, Peach
Ornamental Plants and Shrubs: Dahlia Roses, Chrysanthemum, Forsythia, Euonymus, Hydrangea, Geranium, Peony
Berries: Blackberry, Blueberry, Raspberry
Other Plants: Walnut, Willow, Poplar
While many species are susceptible to crown gall, only a select few develop visible tumors. Notably, conifers are generally resistant to this disease.
Bacterium Agrobacterium Tumefaciens
The soil bacterium Agrobacterium tumefaciens causes this disease. This clever bacterium has a unique ability to genetically modify its host plants. It inserts a piece of its own DNA into the plant’s genome, leading to the formation of tumors or galls.
Interestingly, while this bacterial trick can cause harm to plants, scientists have harnessed its power for genetic engineering. By understanding and controlling the process, researchers can introduce specific genes into plants to develop desirable traits like disease resistance or improved nutritional value.
Disease Cycle
Crown gall, a plant disease caused by the bacterium Agrobacterium tumefaciens, is a fascinating tale of bacterial ingenuity. This soil-dwelling bacterium survives by feeding on substances released by plant roots.
When a plant is wounded, whether by insects, nematodes, or human activities like pruning or transplanting, it releases sugars and phenolic compounds. These chemical signals act as a beacon, attracting the motile Agrobacterium bacteria.
Once the bacteria reach the wound site, they attach to the plant cells and inject a piece of their DNA, known as a plasmid. This plasmid carries genes that instruct the plant cells to produce excessive amounts of growth hormones.
As a result, the plant cells begin to divide uncontrollably, forming the characteristic tumor-like galls.
As the galls decompose, the bacteria are released back into the soil, ready to infect new hosts and continue the cycle. Crown gall thrives in alkaline conditions, making it more prevalent in limed soils. Therefore, maintaining a slightly acidic soil pH can help reduce the risk of this disease.
Spreading of Crown Gall
Rosser1954, Burr or Crown Gall tumour on beech, Anderson Plantation, Lainshaw Woods, East Ayrshire, CC BY-SA 4.0
Crown gall primarily spreads through the following phenomena:
Infected Plant Material: The most common way is through infected plant roots. When infected plants are moved or propagated, the bacteria can be transferred to new locations.
Contaminated Soil: The bacteria can survive in the soil for many years, and contaminated soil can spread the disease to healthy plants.
Contaminated Tools: Pruning tools, shovels, and other gardening equipment can carry the bacteria from infected plants to healthy ones.
Water: Water can spread the bacteria, especially in areas with irrigation systems.
It’s important to note that the bacteria enter plants through wounds, such as those caused by pruning, grafting, insect damage, or mechanical injury. Once inside the plant, the bacteria triggers the formation of galls.
Identification of Crown Gall
Eacf25fd-08e1-413c-8904-84f9742edce4, 根頭癌腫病に感染したリンゴの木, CC BY 4.0
Once you know what to look for, crown galls are pretty unmistakable. Here’s what to watch out for:
Unusual Growths: Look for irregular, tumor-like growths on stems. These can range from tiny to quite large, often starting soft and spongy before hardening and darkening over time.
Stem Base: Galls frequently form at the base of the stem, where it meets the soil. However, they can also appear on other parts of the plant.
Multiple Galls: In some cases, you might see multiple galls forming in rows along the stem.
Identification of Underground Galls
While root galls are harder to spot, their effects on the plant can be a giveaway. Look for:
Premature Leaf Drop: Yellowing and early leaf loss.
Reduced Vigor: A general decline in plant health.
Lack of Bloom: Fewer flowers, fruits, or buds.
Distorted Growth: Unusual shapes of leaves and stems.
If you suspect a root gall problem, don’t hesitate to dig up the plant and examine the roots. Look for abnormal growths or swellings.
Prevention
Here are some effective ways to prevent crown gall:
Prevention is Key:
Purchase Healthy Plants: Always buy plants from reputable sources and inspect them thoroughly for any signs of galls before planting.
Avoid Wounding Plants: Be gentle when handling plants to minimize wounds, as these are entry points for the bacteria.
Sanitize Tools: Disinfect pruning and gardening tools with a 10% bleach solution or 70% isopropyl alcohol before and after use to prevent the spread of the bacteria.
Practice Good Soil Hygiene: Avoid working in infected soil and clean your shoes before moving between different planting areas.
Use Biological Control: Some biological control agents, such as Agrobacterium radiobacter strain K84, can help prevent crown gall by competing with the harmful bacteria.
Soil solarization: This involves covering the soil with a clear plastic sheet to trap solar energy. The heat generated raises the soil temperature to levels that eliminate bacteria and other soilborne pathogens.
Shield from Damage: Use Tree Wrap to safeguard your trees from string trimmer damage and maintain clean gardening tools.
Winterize Effectively: Employ natural burlap to shield tree bark from cracking during harsh winter conditions.
Remember, while these methods can help manage crown gall, they may not completely eliminate the disease. It’s essential to practice good plant hygiene and monitor your plants regularly for signs of infection
Treating Crown Galls
Unfortunately, there’s no cure for crown gall once galls have formed. However, you can manage the disease and prevent its spread:
Managing Infected Plants
Pruning: Carefully prune away infected branches or stems, cutting well below the gall. Remember to disinfect pruning tools before and after each cut to prevent the spread of the bacteria.
Plant Removal: In severe cases, especially for young plants, it might be necessary to remove and destroy the entire plant, including the roots. Dispose of infected plant material responsibly to avoid spreading the disease.
Soil Replacement: If the infection is severe, consider replacing the soil in the affected area with fresh, uncontaminated soil.
In some cases, you can manage this disease as follows:
Surgical Removal: For plants already showing signs of crown gall, the first step is to carefully cut away the galls using a sharp, clean pruning knife. The goal should be to remove the gall entirely while preserving surrounding healthy tissue.
Wound Care: Effective wound care is essential to prevent additional infection. Dispose of the infected tissue immediately, and apply a pruning sealant to the wound. This helps protect the exposed area and reduces the risk of re-infection.
Monitor and Remove: Keep an eye on the plant’s recovery. If the plant continues to show signs of stress or additional galls form, it may be necessary to remove and dispose of the plant entirely. This helps to prevent the bacteria from spreading to other plants in the area.
With attentive care, managing crown gall can help your garden stay healthy and thriving.
Controlling Crown Galls
A Non-Chemical Approach
To reduce the bacterial population in the soil, consider planting crops like potatoes or other vegetables (excluding beetroot) for one or two years. Alternatively, maintaining a grass cover over the affected area for a year or more can also help.
Chemical Control Limitations
Unfortunately, there are currently no chemical treatments available to directly control crown gall.
Resilient Plants
A recent study by the University of Illinois has shed light on a group of plants that seem to be more resistant to the devastating crown gall disease. These resilient plants include:
Hornbeam, Ginkgo, True Cedars, Golden Raintree, Mahonia, Tulip Tree, Spruce, Boxwood, Linden, Beech, Catalpa, Holly, Larch, Black Gum, Magnolia, Pine, Bald Cypress, Douglas Fir, Hemlock, Firethorn, Birch, Redbud, Sweet Gum, Smoke Tree, Deutzia, Yellowwood, Serviceberry, Yew, Zelkova.
A Note on Regulations
It’s important to note that crown gall is a regulated pest in Indiana. Any plant material exhibiting symptoms of crown gall must be removed from sale and destroyed to prevent further spread.
By understanding these resilient plant species and adhering to regulatory guidelines, we can help protect our landscapes from this destructive disease.
The Persistent Threat of Crown Gall: A Complex Challenge
Laurent Bélanger, Broussin sur un peuplier deltoïde (2), CC BY-SA 4.0
Crown gall, a bacterial disease caused by Agrobacterium tumefaciens, presents a significant challenge due to its wide host range, persistence in the environment, and the lack of highly effective control measures.
Understanding the Enemy
To combat this disease, researchers are delving deeper into the complex interactions between the bacterium and its host plants. By studying the molecular mechanisms involved in infection and gall formation, scientists hope to identify new strategies for prevention and control.
Breeding for Resilience
Another promising approach is the development of resistant or tolerant plant varieties. Traditional breeding techniques and modern genetic engineering tools are being employed to create plants that can better withstand the onslaught of crown gall.
Early Detection, Early Action
Quick and precise diagnosis is essential for the successful management of crown gall. Scientists are working on developing innovative diagnostic tools that can quickly identify infected plants, allowing for timely intervention and reducing the spread of the disease.
A Fascinating Biological Phenomenon
Beyond its impact on plant health, crown gall offers a fascinating glimpse into the intricate world of plant-microbe interactions. By studying this disease, we can gain valuable insights into the mechanisms of bacterial infection, plant defense responses, and the evolution of plant-microbe associations.
The ongoing battle against crown gall highlights the importance of continuous research, innovation, and collaboration among scientists, growers, and gardeners.
By working together, we can hope to develop effective strategies to mitigate the impact of this disease and ensure the health and vitality of our plant populations.
Frequently Asked Questions
Currently, there is no definitive cure for crown gall. However, there are various methods to manage and control the disease effectively.
To control and prevent crown gall, follow these steps:
Select Disease-Free Plants: Choose healthy, certified disease-free plants.
Practice Good Hygiene: Clean tools regularly to avoid spreading the bacteria.
Use Soil Solarization: Employ this technique to eliminate pathogens in the soil.
Avoid Wounding Plants: Be careful not to damage plants, as wounds can be entry points for bacteria.
The best way to remove crown gall is to carefully cut out the infected tissue, disinfect the wound, and protect it from further infection.
Crown gall can be a serious disease, especially for young plants. It can stunt growth, make plants more susceptible to other diseases, and in severe cases, even lead to death. However, mature trees often tolerate the disease with minimal impact.
The bacteria that causes crown gall, Agrobacterium tumefaciens, is a soil-borne bacterium. It can survive in the soil for many years and infect plants through wounds in their stems or roots.
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