Crown gall is a plant disease caused by the bacterium Agrobacterium tumefaciens. It causes tumor-like growths called galls to form on the stems and roots of infected plants. Crown gall can affect a wide range of woody and herbaceous plants, including fruit trees, vines, roses, and ornamentals. If left unchecked, crown gall can stunt plant growth, reduce yields, and even kill affected plants. The good news is that crown gall is preventable and treatable if properly identified and managed.

What Causes Crown Gall?

Crown gall is caused by the bacterium Agrobacterium tumefaciens. This pathogen lives in the soil and infects plants through wounds and injuries to the stem or root tissues. Agrobacterium tumefaciens contains a plasmid (a small, circular strand of DNA) called the Ti (tumor-inducing) plasmid. When the bacteria infect plant tissues, they transfer DNA from this plasmid into the plant cells. This inserted bacterial DNA causes the plant cells to grow uncontrollably, resulting in tumor-like galls.

Some key facts about Agrobacterium tumefaciens:

• It is a gram-negative, rod-shaped bacterium found commonly in soil and plant debris.
• The bacteria can survive in the soil for several years, even in the absence of a host plant.
• Agrobacterium enters plant tissues through fresh wounds, such as those made by pruning shears, cultivators, insects, or hail.
• Warm temperatures between 75-85°F favor crown gall development.
• Once inside the plant tissues, the bacteria transfer part of the Ti plasmid into the host plant’s cells.
• The transferred DNA triggers overproduction of plant growth hormones, leading to uncontrolled cell growth and gall formation.

What Do Crown Galls Look Like?

Crown galls are usually easy to recognize on infected plants. Here are their key identifying features:

• They appear as round, tumor-like growths on stems, at the soil line, or on roots.
• The galls are typically light brown, greenish, or whitish in color.
• They can range in size from just a few millimeters to several inches across.
• The surface may be smooth, rough, or hairy.
• On woody plants, the galls are hard, woody, and permanent.
• On herbaceous plants, the galls are soft, fleshy, and may rot away.
• Galls often leak excessive sap or ooze from wounds.
• Affected plants often display stunted growth, yellowing leaves, early leaf drop, and reduced flowering/fruiting.
• Multiple galls may form on a single plant.
• The galls tend to form at sites of injury to the stem or roots.

Carefully inspect the base of plant stems at the soil line as this is a prime location for crown gall development. Also, check for galls on root crowns if you gently excavate around the roots. A hand lens may help identify small, developing galls.

Susceptible Plants

Crown gall can occur on a wide range of dicotyledonous plants. Some of the most commonly affected garden and landscape plants include:

• Fruit trees – apple, apricot, cherry, peach, nectarine, plum, pear
• Nut trees – almond, pecan, walnut
• Berries – blackberry, boysenberry, raspberry, strawberry
• Grapes
• Roses
• Ornamental trees and shrubs – euonymus, hydrangea, lilac, pyracantha, willow
• Herbaceous plants – chrysanthemum, dahlia, marigold, sunflower, zinnia

Monocotyledonous plants like corn, grasses, irises, and lilies are not affected by crown gall.

Among fruit and nut crops, stone fruits (peaches, cherries, etc.) and grapes tend to be the most susceptible. Young plants are more prone to infection than mature, established plants.

How to Prevent Crown Gall

Prevention is the best management approach against crown gall disease. Here are some key prevention tips:

Purchase Healthy, Certified Plants

• Inspect plants carefully before purchase and reject any plants showing crown galls or other abnormalities.
• Purchase plants from reputable nurseries that sell certified, disease-free stock.

Avoid Wounding Plants

• Practice careful handling of plants to avoid unnecessary wounds during transplanting, staking, or pruning.
• Sterilize pruning tools before and after each cut using diluted bleach, isopropyl alcohol, or other disinfectants.
• Control insects like borers that create entry points for the bacteria.

Rotate Crops

• Avoid repeatedly growing crown gall susceptible plants in the same location.
• Rotate to a non-host plant for 2-3 years to lower pathogen levels in the soil.

Solarize Soil

• Solarization uses clear plastic sheeting to heat and pasteurize the top layers of garden soil.
• Leave the plastic in place for 4-6 weeks during warm weather to help reduce Agrobacterium populations.

Manage Soil pH

• Agrobacterium prefers acidic soils below pH 6.0.
• Liming to maintain a soil pH of 6.0-7.0 can help discourage this pathogen.

Disinfect Tools and Wounds

• As mentioned above, always sterilize pruning tools before use.
• If you must prune during wet seasons, protect resulting wounds with disinfectants or pruning paint.
• When transplanting, water newly planted roots with a dilute bleach solution (1 part bleach to 9 parts water) to kill any bacteria on freshly cut roots.

How to Treat and Control Crown Gall

If crown gall is detected on plants, take action right away to avoid spread. Here are some effective treatment options:

Prune Out Small Galls

• For small, isolated galls, prune them out promptly.
• Sterilize pruning tools before each cut to avoid spreading the bacteria.
• Destroy all gall tissue – do not compost it.

Remove and Destroy Heavily Infected Plants

• If a plant displays large, spreading galls or general decline, it is often best to remove and destroy it entirely.
• Pull or dig up the whole plant including the root system.
• Dispose of the plant material safely by burning, deep burial, or double-bagging and sending to a landfill.

Apply Antibiotic Treatments

• Injecting crown galls directly with antibiotic agents can eradicate the bacteria and cause galls to detach.
• Streptomycin sulfate is an effective antibiotic treatment for crown gall. It is mixed with water and injected into the gall tissue.
• Follow product label directions carefully when applying antibiotic treatments. Repeat applications may be needed for best results.

Grow Resistant Rootstocks

• For valuable trees like fruit and nut crops, cultivating them on resistant rootstock varieties can limit crown gall severity.
• Consider resistant rootstocks like Krymsk® for stone fruits, Geneva® for apples, or Natalia® for almonds.
• Graft susceptible scion wood onto the resistant rootstock to confer protection while maintaining desired fruit varieties.

Improve Air Circulation

• Promote good air movement through proper spacing, pruning, and weed control to keep plants dry.
• Drip or subsurface irrigation is preferable to overhead watering.
• Avoid excessive nitrogen fertilization which can encourage soft, susceptible growth.

Crop Rotation

• As mentioned for prevention, rotate plantings to non-host species for 2-3 years to lower pathogen levels in soil.
• Avoid successive plantings of crown gall prone crops like stone fruits, berries, and roses.

How to Identify Crown Gall

Being able to accurately identify crown gall is important for effective management. Here are tips on distinguishing it from other abnormal plant growths:

Compare Location on Plant

• Crown galls originate at wound sites on stems and roots.
• Burls arise spontaneously on above-ground woody tissue.
• Cancker sores appear on trunks and branches.

Check Texture

• Crown galls are hard, rough textured, and firmly attached.
• Burls have a gnarled but smooth texture.
• Cankers feel sunken, cracked, or softened.

Observe Shape and Growth

• Crown galls are round, tumor-like, and often spread into clusters.
• Burls maintain the shape of surrounding bark and branches.
• Cankers expand in elongated, sunken lesions.

Review Color

• Crown galls are usually light brown, greenish, or white.
• Burls match the bark color but appear twisted.
• Cankers show dead bark and reddish-brown lesions.

Assess Impact on Plant Health

• Crown gall can stunt growth but isn’t usually lethal alone.
• Burls are unsightly but usually don’t affect plant health or productivity.
• Cankers can eventually girdle and kill branches or whole trees.

Carefully considering all these factors – location, texture, shape, color, and plant health effects – will help distinguish crown gall from similar maladies. When in doubt, have a plant expert or diagnostic lab analyze a sample.

Frequently Asked Questions

Can I save a plant once it has crown gall?

Plants with minor galls can often be saved by promptly pruning out all infected tissues followed by disinfecting tools and wounds. Heavily infected plants with large spreading galls are often best removed entirely. For valuable plants, antibiotic injection of galls might be warranted to try saving them.

Does crown gall affect fruit production?

If galls are limited to lower trunks and roots, they may not directly reduce yields, but they can stunt plants and indirectly lower vigor and productivity over time. Galls on canes or cordons can more directly decrease fruit yields.

Should I replace a plant damaged by crown gall disease?

Replanting in the same site is not advisable unless you first implement solarization to reduce Agrobacterium levels in the soil. Otherwise, the new plant will likely develop crown gall as well. Better options are rotating to a non-host plant, amending soil pH, or cultivating replacement trees on resistant rootstock.

Are there biological controls for crown gall?

Some Agrobacterium-attacking viruses and bacteria show promise for biocontrol of crown gall disease. More research is needed, but integrating these natural enemies into soil management could provide a biological means of protecting susceptible plants.

How long do crown gall bacteria persist in the soil?

Agrobacterium can overwinter in plant debris and soil for 2-4 years even without a host plant present. Rotation and soil solarization help reduce pathogen levels more quickly.

Why do some plants like roses get crown gall while other plants don’t?

The bacteria responsible for crown gall only infect dicotyledonous plants – those with two seed leaves. Monocots with one seed leaf like corn, grasses, and lilies do not develop crown gall. Also, certain dicot species have greater inherent susceptibility, such as roses, vine fruits, and stone fruits.

Conclusion

Crown gall is a prevalent disease, but one that can be effectively prevented and managed. Carefully inspect all new plant introductions. Promote plant health through proper care and cultivation techniques. Disinfect tools and wounds promptly. Rogue out diseased plants or branches before the infection can spread. Rotate plantings to non-host species to lower pathogen populations in the soil over time. Seek resistant rootstocks when available. With vigilance and prompt action at the first sign of infection, you can help keep crown gall under control and maintain healthy, productive plants.

How to Identify Crown Gall

Crown gall can be reliably identified by its characteristic symptoms. Here are the keys to accurate identification:

Location on Plant

• Inspect the base of stems and root collars near the soil line. This is the most common place for crown galls to initiate.
• Also check along roots if you gently excavate the root system.
• Crown galls arise at sites of wounding from pruning, transplanting, cultivation, etc.

Shape and Texture

• Galls are rounded, tumor-like swellings that can range from pea-sized to several inches across.
• They have a firm, rough texture and appear securely attached to the stem or root.
• Multiple galls may merge into a large, distorted mass.

Color

• Young galls are greenish, creamy white, pink, or reddish.
• Older galls turn brownish in color.
• Excessive sap may leak from cracks or wounds on the gall surface.

Effect on Plant Health

• Crown gall can stunt and weaken plant growth.
• It only directly kills the host plant in severe cases.
• Carefully assess the overall vigor and productivity of affected plants.

Confirmation of Cause

• To conclusively identify that crown gall is caused specifically by Agrobacterium tumefaciens infection, submit a sample to a plant diagnostic laboratory for isolation and culture of the pathogenic bacteria.

Carefully observing where on the plant the abnormal growth occurs, its texture, color, and effect on plant health will allow confident field diagnosis of crown gall in most cases. When in doubt, laboratory analysis can provide definitive confirmation.

How to Treat Crown Gall

If you detect crown gall on your plants, taking prompt action provides the best chance of saving affected plants and avoiding spread. Here are effective treatment methods:

Prune Out Small Galls

For plants with only a few small galls:

• Prune out each gall entirely, leaving no diseased tissue behind.
• Sterilize pruning tools in a 10% bleach solution before each cut to prevent reinfection.
• Remove a few inches into healthy wood surrounding the gall.
• Destroy all gall tissue – do not compost.

Remove Heavily Infected Plants

For plants with large, spreading galls or poor vigor:

• Remove and destroy the entire plant including the root system.
• Pull small plants by hand or use a shovel to carefully dig out trees and shrubs.
• Dispose of the plant by burning, burying, or sending to a landfill.
• Do not compost diseased material.

Apply Antibiotic Injections

For valuable plants with isolated galls:

• Injecting the gall itself with antibiotics like streptomycin sulfate may be effective.
• Make holes around the margins of each gall using a drill or syringe.
• Fill the holes with antibiotic solution.
• Repeat applications may be needed for best results.

Implement Preventative Measures

For any affected plant:

• Sterilize tools before each use to restrict spread.
• Improve air circulation and avoid excess nitrogen fertilization.
• Consider solarizing soil before replanting to reduce pathogen levels.
• Select resistant rootstocks when replanting susceptible crops.

Prompt surgery, removal of incurable plants, antibiotics, and preventative steps offer the best treatment strategy for combating crown gall.

How to Prevent Crown Gall

Prevention is key to avoiding crown gall issues in your garden and landscape plants. Here are some effective preventive measures:

Start with Healthy Plants

• Carefully inspect all new plant introductions for signs of galls or poor health.
• Obtain plants from reputable nurseries that sell certified, disease-free stocks.

Avoid Unnecessary Wounding

• Practice careful handling techniques when transplanting and pruning to minimize wounds.
• Disinfect tools before and after each use.
• Control borers and other pests that create entry points for infection.

Rotate Plantings

• Avoid repeat plantings of crown gall prone species in the same sites.
• Rotate to unrelated plant types for 2-3 years to break disease cycles.

Solarize Soil

• Covering moist soil with clear plastic during hot weather can pasteurize the top layers and reduce pathogen populations.

Manage Soil pH

• Liming acidic soils to pH 6.0-7.0 can help deter Agrobacterium bacteria.

Graft onto Resistant Rootstocks

• Grafting susceptible scion varieties onto resistant rootstocks limits crown gall severity.
• For fruit trees, select rootstocks labeled GF (gall-free) or RG (resistant to gall).

Promote Air Circulation

• Allow adequate spacing and prune plants to encourage air movement and rapid drying.
• Drip or subsurface irrigation is preferable to overhead watering.

Implementing preventive measures focused on wound avoidance, crop rotation, soil health, and plant vigor provides the first line of defense against destructive crown gall infections.

Conclusion

Crown gall is a challenging disease, but one that can be successfully prevented and managed. Carefully inspect all new plants, and reject those showing any signs of galls or poor health. Handle plants gently, and sterilize tools between plants to avoid spreading the Agrobacterium bacteria. Remove small galls promptly by pruning well below the diseased tissue, and destroy galls – do not compost them. For severely affected plants, complete removal is warranted. Valuable plants may be treated with antibiotic injections, but this is most effective on isolated galls. For long term prevention, improve air circulation, avoid excess nitrogen, rotate plantings, solarize soil, graft onto resistant rootstock, and promote overall plant health through proper care. Vigilance and prompt action at the first sign of infection offer the best protection against crown gall disease. With proper identification, treatment, and prevention, you can minimize losses and maintain productive, thriving plants.