What is Crown Gall?
Crown gall is a widespread plant disease caused by the soil-borne bacterium Agrobacterium tumefaciens. This pathogen induces the formation of tumors, primarily at the base of the stem (crown) of the host plant. However, galls can also appear on roots and branches.
Crown gall affects a wide variety of herbaceous and woody plants. In total, more than 600 plant species are susceptible.
- Fruit Trees: Apple, Pear, Peach, Cherry, Plum, Almond
- Ornamental Plants and Shrubs: Roses, Chrysanthemum, Dahlia, Euonymus, Forsythia, Geranium, Hydrangea, Peony.
- Berries: Raspberry, Blackberry, Blueberry
- Others: Willow, Poplar, Walnut
The disease has a global distribution, impacting plants wherever conditions are favorable. It’s particularly prevalent in temperate regions where stone fruits, nuts, and grapes are grown.
What Causes Crown Gall?
The bacterium A. tumefaciens enters the plant through wounds, which can be caused by cultivation tools, insect bites, or natural cracks. Once inside, it introduces a piece of its DNA into the plant cells. This DNA fragment causes the plant cells to proliferate uncontrollably, leading to gall formation.
What are the Symptoms?
Identifying crown gall involves observing the physical symptoms on the plant and understanding the conditions that might promote its occurrence. Here’s how to identify crown gall:
- Appearance of Galls:
- Galls are the primary symptom of the disease. They start as small, white, soft overgrowths on the roots or base of the stem (crown).
- Over time, these galls harden and become woody. They can vary in size, from small nodules less than an inch in diameter to large masses several inches across.
- On older galls, the outer surface might be rough and corky, resembling a cauliflower.
- Location of Galls:
- While they commonly form at the plant’s crown (where the stem meets the root) or on the roots themselves, galls can also form further up the stem, especially at wound sites or where the plant was pruned.
- Galls might also appear at sites of leaf attachment or on branches.
- Plant Vigor:
- Affected plants often show reduced vigor. Since the galls can disrupt water and nutrient flow, plants with significant infections might exhibit stunted growth, yellowing leaves, or wilting.
- Plants with extensive root galling may decline and eventually die.
- When sliced open, young galls are often white or pale in color and have a soft, spongy texture. Older galls become harder and woodier.
- Associated Wounds:
- The bacterium often enters the plant through wounds, so the presence of galls near sites of injury, such as pruning cuts, is a strong indicator.
- Lab Testing:
- If you are unsure, the most definitive way to identify crown gall is to send a sample to a diagnostic laboratory where technicians can isolate and identify the pathogenic bacteria.
How to Control and Prevent Crown Gall?
Preventing and controlling crown gall is vital because once plants are infected, it’s challenging to eliminate the disease completely. Here are strategies for managing crown gall:
- Purchase Disease-Free Plants:
- Always buy plants from reputable sources. Inspect them thoroughly for galls before planting.
- Avoid Wounding:
- Galls often develop at wound sites. Be cautious when cultivating around plants not to damage them. The bacterium can enter plants through fresh wounds, so be careful when pruning or handling plants.
- Site Selection and Rotation:
- Avoid planting in sites previously known to have infected plants.
- For agricultural settings, crop rotation is beneficial, especially with non-host plants.
- Sanitize Tools:
- Regularly disinfect garden tools, especially after working with infected plants. This can be done using a 10% bleach solution or rubbing alcohol.
- Maintain Healthy Soil:
- Encourage good soil health and drainage. The bacterium thrives in poorly-drained soils.
- Remove and Destroy Infected Plants:
- For heavily infected plants, especially young ones, it’s often best to remove and destroy them. This prevents the disease from spreading.
- When removing, get as much of the root system as possible. Do not add infected plant material to compost piles.
- Beneficial Microorganisms:
- Some products contain beneficial microorganisms that compete with Agrobacterium tumefaciens. These can be applied to wounds or planting sites to help protect against infection.
- Chemical Control:
- Chemical control is not typically very effective against crown gall, but some products can offer limited protection when applied to wounds or pruning cuts. Always read and follow label instructions.
- Biocontrol Agents:
- There are some commercially available biocontrol agents, like Agrobacterium radiobacter strain K84, which can prevent crown gall by competing with the pathogenic Agrobacterium tumefaciens for space and resources.
- In areas where crown gall is a persistent problem, soil solarization (using clear plastic to heat the soil and kill pathogens) might be effective.
- Limit Overhead Watering:
- Wet conditions can facilitate the spread of the bacterium. Use drip or soaker hose irrigation to keep foliage dry and reduce splash.
- Proper Pruning:
- Prune during dry conditions to minimize the risk of infections through open wounds.
Remember, while these measures can significantly reduce the incidence of crown gall, it’s challenging to ensure complete protection due to the ubiquity of the bacteria in many environments. However, with vigilance and the proper practices, gardeners can keep this disease in check.
While every effort has been made to describe these plants accurately, please keep in mind that height, bloom time, and color may differ in various climates. The description of these plants has been written based on numerous outside resources.