Crown gall disease: tumors caused by Agrobacterium tumefaciens

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Rough, pale swellings form where the trunk meets the soil, spongy at first, later hard, dark and deeply cracked. Crown gall (Agrobacterium tumefaciens) is a bacterial disease recorded on over 600 plant species, roses, willows, poplars and fruit trees above all. The bacteria enter only through wounds, so clean tools and gall-free stock keep it out.

Identifying crown gall

Spotting crown gall is pretty straightforward, given how the tumor-stricken wood stands out. Most of the time, you’re looking at an Agrobacterium bacterial infestation.

• Tumor-like growths called galls appear on stems and roots. Their size ranges from ¼ inch (a few mm) to several dozens of inches (half a meter) in diameter.
• New galls are round, rough-textured, light-colored, and can even feel a tad spongy.
• Older ones turn hard and dry. They usually have a darker hue with lots of rough cracks.
• Mostly, you’ll find galls on the main stem or trunk where it meets soil – hence the common name “crown gall”, meaning root crown.
• They can also appear on roots.
• For some plants, the galls might line up along stems or branches.

Plants impacted by crown gall

Crown gall can impact over 600 different plant species across the world. This list includes many common vegetables, trees, and shrubs. You’ll often spot crown gall caused by Agrobacterium tumefaciens on:

• Roses
• Willows
• Poplar trees
• Fruit trees like apples, cherries, plums, and apricots

Clubroot (Plasmodiophora brassicae) infects brassicas, not grasses or cereals, so sowing turf over ground cleared of clubbed plants is safe for the grass, though it will not eradicate the soil’s resting spores.

Which plants are affected, and where the galls show

Crown gall affects over 600 plant species, but the bacterium involved and the part of the plant that swells are not the same from one host to the next. Use this table to work out what you are looking at.

If you already have infected plants, treat these hosts as the ones to keep out of the affected ground.

How crown gall survives and spreads

• Phytopathogenic bacterium Agrobacterium tumefaciens causes crown gall.
• Crown gall bacteria enter plant roots through injuries.

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  Injuries can come from planting, grafting, ground insect feeding, or root damage from digging. And remember: Watch out for dirty tools!

• Wounded roots release chemicals, and these chemicals actually attract the bacteria like a beacon.
• Newly injured cells are vulnerable to bacterial infection for a few days during the growth season. But if a plant is dormant, this period stretches to months. A wound in fall is more likely to lead to contamination than one in spring.
• Over time, bacteria form secondary galls, which in time creates those quirky lumps.
• In some plants, like roses, willows, and poplars, bacteria travel inside stems and branches, leading to above-ground galls.
• Eventually, galls start breaking down. As bacteria return to soil, water or equipment can scatter them around.

Preventing crown gall

The single best control method for crown gall is prevention. Indeed, once it sets up shop in an area, eliminating this bacterium is nearly impossible.

• Always inspect new plants closely.
• Most importantly, don’t plant any tree or shrub with galls on roots or stems, however small.
• Be extra careful when you’re dealing with roses, fruit trees, poplar, and willow.

• Ease the root ball into the hole without scraping the crown, because wounds to bark or stem tissue can give pathogens an entry point, so handle the plant gently.
• Keep strimmer lines and hoe blades well away from the base of roses and young fruit trees, and mulch that zone instead of scratching the soil.
• At the nursery, inspect the graft union: knobby or uneven callusing is often normal, but reject plants whose union is soft, oozing, cracked, or clearly not knitted firmly together.

Treatment for crown gall

For a young tree or shrub:

• Dig up the plant and all the soil around its roots. Toss it all out!
• Next, don’t add infected plant material to compost piles.
• Lastly, burn remains to completely get rid of infected woody plants.

For mature trees and shrubs:

• Mature trees and shrubs can handle a crown gall infection and still stay standing in your landscaping.
• When working on these trees, be sure to sanitize pruning tools with a 10% bleach solution after use.
• This is especially important when trimming trees with crown gall.
• But if you have infected plants on your property, steer clear of planting ultra-sensitive species like roses, willow, poplar, and fruit trees.

Good to know

Besides Agrobacterium tumefaciens, two other Agrobacterium species cause woody gall formation on sensitive plants.

• Agrobacterium vitis gets vines into trouble.
• Agrobacterium rubi targets brambles like raspberries; galls grow on their stems.

And this doesn’t include the wide range of fungus or insect-caused galls. Note that cypress canker is yet another type of bacteria that sometimes causes canker galls as well.

• Nursery roots and cuttings can be dipped in the harmless relative Agrobacterium radiobacter strain K84, which protects against crown gall but not against Agrobacterium vitis on vines.
• On beans, peas and other legumes, do not mistake nitrogen-fixing root nodules for crown gall: nodules are small, pink inside and rub off easily.