How to Identify and Prevent Crown Gall on Apple

Rhizobium rhizogenes

Does your apple tree have weird growths on it? Apples are prone to several different kinds of galls, but crown gall is the worst of them.

This disease, caused by the bacteria Rhizobium rhizogenes (old name Agrobacterium tumefaciens), is likely to be fatal to your tree over time.

Since crown gall can infect hundreds of kinds of plants, a number of fruit trees can contract this infection. They include apple, crabapple, peach, nectarine, plum, apricot, cherry, and pear.

The trunk of an apple tree with a crown gall growth.

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The bacteria genetically engineers plants to use their own DNA to make tumors. What you see as the gall is actually a tumor that your tree has grown.

How to Tell if Your Tree Has Crown Gall

If your tree looks like it has warts, it is probably crown gall. The galls start out light in color but turn dark over time.

A young apple tree trunk with crown gall on its base.
N.S. Luepschen, Bugwood.org via CC 3.0.

The growths vary in size – typically pea-sized to several inches – but they can grow to as much as a foot in diameter.

Most of them will be at or just below the soil line, but some could be on the upper stems.

Since the galls are caused by the production of hormones, which can travel through the plant, these growths can be some distance from the actual infection.

A crown gall on a apple trunk with a blue sky background.
A growth on a young tree trunk. Photo by University of Georgia Plant Pathology , University of Georgia, Bugwood.org via CC 3.0.

Sometimes people get crown gall confused with growths caused by woolly apple aphids or with burrs (aka burl). The woolly apple aphid galls appear in a greater number in an infected tree and they are smaller.

And more importantly, the aphid variety produces a white cottony wax.

Effects of an Infection

Crown gall interferes with the flow of nutrients and water up the stem. This can affect normal growth, and your tree might start to decline.

It will be more sensitive to drought stress or winter injury.

Worst-case scenario – the growths can girdle the stems or trunk and kill the tree outright.

How Plants Get Infected and What To Do About It

The bacteria are very common in soils, since they infect so many plants.

And they can enter through wounds! So many types of wounds – pruning wounds, those introduced by mowing or cultivating, freeze damage, wind damage, or insect injuries.

There are some treatment options that may or may not work. You can paint the growths on the stems with paint that contains antibiotics.

Don’t paint more than half of the area around the stem.

Another option is Gallex. You can treat small growths with this compound, and it may help to reduce them.

What To Do If Your Tree is Badly Infected

If your tree is severely declining, you should remove it and burn the stump and as many of the roots as you can.

You will have to wait about three years before you can plant another fruit tree there.

One thing that will help to cleanse the soil is to grow grasses on the land during this period.

How to Prevent Crown Gall

Your best bet is to avoid the disease in the first place.

Unfortunately, this can be difficult.

These bacteria are very common in nursery soils, so buy certified disease-free rootstock and be careful not to damage the roots or crowns when you plant your tree!

A young tree rootstock showing the effects of crown gall infection.
This seedling is infected. Be sure to buy rootstock from respected, disease-free nurseries. Photo by University of Georgia Plant Pathology , University of Georgia, Bugwood.org via CC 3.0.

Also be careful not to injure your tree while mowing or cultivating.

Control insects and root-knot nematodes that feed on the roots, so they won’t cause injuries that the bacteria could use to colonize the plants.

Keep your plants actively growing with the proper amount of water and fertilizer.

Some people swear by dipping the roots in strains of Rhizobium radiobacter that cannot cause disease with the idea that it will outcompete the bacteria that cause crown gall.

However, this treatment is not effective against every type of crown gall and often doesn’t work in soils that have a heavy infestation of the bacteria.

A Common Worldwide Problem

Soils throughout the world are infested with the bacteria that cause crown gall.  This is bad news for fruit tree growers, since these bacteria have a very wide host range.

Your apple, crabapple, peach, pear, apricot, nectarine, and plum trees are all susceptible.

Do your best to prevent wounds that the bacteria can colonize and be prepared to remove a tree if it develops a severe infection.

Have you encountered crown gall? Let us know how you fared in the comments below!

And for more information on apple diseases, check out our comprehensive guide where we delve into topics such as:

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About Helga George, PhD

One of Helga George’s greatest childhood joys was reading about rare and greenhouse plants that would not grow in Delaware. Now that she lives near Santa Barbara, California, she is delighted that many of these grow right outside! Fascinated by the childhood discovery that plants make chemicals to defend themselves, Helga embarked on further academic study and obtained two degrees, studying plant diseases as a plant pathology major. She holds a BS in agriculture from Cornell University, and an MS from the University of Massachusetts Amherst. Helga then returned to Cornell to obtain a PhD, studying one of the model systems of plant defense. She transitioned to full-time writing in 2009.

5 thoughts on “How to Identify and Prevent Crown Gall on Apple”

  1. Is it possible to take a cutting from an apple tree that has crown gall? My tree produces lovely apples but has a severe infection and I would like to take a cutting to graft onto another tree before destroying the infected tree. There is plenty of new growth at the top of the tree, well away from the infected areas. Many thanks

    Reply
    • That is an excellent question! The infection does not spread throughout the whole tree, although the galls can form some distance from the bacteria, because the hormones can spread throughout the tree. If your new growth is far from the infected areas, it should be safe to use them as cuttings. I hope it goes well for you!

      Reply
      • Hello Helga, I have been researching Crown Gall and I can’t seem to find any images or evidence that this bacterial disease affects the actual fruit of the apple. Does it?
        Also if the bacteria can actually affect the DNA of the host wouldn’t it stand to reason that a cutting taken from the infected tree would also carry the bacteria?

        Reply
        • Hi Marita,

          Thanks so much for your excellent questions!

          Crown gall doesn’t directly affect the fruit. It might indirectly if it causes the tree to decline, but the fruit should be safe to eat or sell.

          The changes in the DNA of the tree are localized to where the bacteria infect the trees. Since they are soil-borne, it is the lower part of the tree that has its DNA altered. It doesn’t spread throughout the whole tree, so it should be safe to take cuttings from an infected tree.

          Tumors can form at other places, but that is due to hormones as a result of an infection, not to actual growth of the bacteria or dissemination of its DNA.

          Reply

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