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A tell-tale sign of tree stress: Causes of gummosis on your cherry tree

Published: January 8, 2023
Signs of gummosis on the trunk of a cherry tree. Bacterial canker is one of the causes of gummosis on cherry trees.
Bacterial canker is one of the causes of gummosis on a cherry tree. Photo credit: OrchardPeople.com.

"What's wrong with my cherry tree?"

That's what a neighbour asked me recently. A huge amount of amber coloured, jelly-like goop was oozing out of the trunk and branches of his cherry tree as you can see in the image above.

That sticky goop is called gummosis and it's a sign that a tree is under stress. But what causes gummosis? And how can you prevent and treat it? We'll explore that in this article.

Stub on a cherry tree oozing with gummosis. Bacterial canker is one common cause of gummosis in cherry trees.
Bad pruning practices can cause gummosis. Photo credit: OrchardPeople.com.

There are many possible causes of gummosis in cherry trees

So, what are the causes of gummosis in cherry trees? There are a number of possibilities:

  • A string trimmer or mechanical injury can cause gummosis.
  • Gummosis can be the result of a broken, torn or incorrectly pruned branch.
  • Gummosis can form in a sun scald wound on the trunk of the tree.
  • Or it can be triggered by an insect like peach tree borers.
  • But often the cause of gummosis is a fungal or bacterial infection. And how do those infections get into the tree? Well, they are opportunists! And these pathogens infiltrate when there is a wound in the tree resulting from one of the situations above!
Gummosis in cherry trees start off with a canker, or a sunken patch on the tree's bark. Bacterial canker is one of the causes of gummosis on cherry tree
When a tree is injured, a sunken patch (canker) may form. Eventually, the tree's sap will start leaking out . The production and release of this sap in diseased trees is called gummosis. Bacterial cankers are very common on cherry trees. Photo credit: OrchardPeople.com.

Let's see how tree wounds develop.

  • The tree is wounded by a string trimmer, a bad pruning cut, or a pest that breaks through its protective bark.
  • A canker, or sunken patch, forms on the branch, bark or trunk of the tree. That canker may be a darker colour than the trunk and it may harden and form a corky edge.
  • The branch may swell, and the tree starts exuding sticky sap.

It may be easy to overlook a fungal or bacterial canker in the early stages, but when the tree's sap starts leaking out, these cankers are hard to miss. Think of the gummosis as your fruit tree's call for help. As the grower you need to resolve this problem...before it is too late!

Bacterial canker as a common cause of gummosis in cherry trees

While there are many things that can trigger a wound in your cherry tree, the factor that makes things worse is a bacterial disease called Pseudomonas syringae which is also known as bacterial canker. This pathogen affects other stone fruit trees as well like plums, apricots and peaches.

Other symptoms of bacterial canker include brown spots or holes on the leaves and the death of new shoots and young branches.

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How to treat gummosis in cherry trees

The best way to treat gummosis or bacterial canker in cherry trees is to remove it early on. When you remove it, you will remove the bacterial pathogen.

To remove bacterial canker on your cherry tree, prune off the diseased branch. Be sure to cut at least two inches (5 cm) before the canker appears on the branch as the pathogen may also be hidden inside the branch.

Once you remove the diseased wood, dispose of it in a sealed bag. Do not not put it in the compost, where bacterial canker can continue to spread. 

After pruning out the diseased branches, disinfect your pruning tool with isopropyl alcohol and wipe it with a clean rag. This reduces the risk of spreading the pathogen to other trees.

If you let things go too far, the bacterial canker and gummosis will take over the tree. When there is gummosis all over the tree, in its trunk, branches and even roots, you will have to cut the tree down to stop the problem from spreading to other trees nearby.

And if you're planning to plant another tree, don't use the same location as the pathogen may remain in the soil.

Plastic garbage bags containing cherry tree branches that were infected with bacterial canker. Causes of gummosis on cherry tree
If you find bacterial canker on your cherry tree, prune out the diseased branches, bag them up, and remove them from the site - do not put them in your compost pile. Photo credit: OrchardPeople.com.

How to treat gummosis in the tree's trunk

Are you seeing early signs of gummosis on your cherry tree? There are times when you can save a mature tree that has a small patch of bacterial canker in its trunk. Here's how:

  • Spread some sheets of newspaper out around the tree to gather all the infected tissue. You do not want to leave any of the pathogen in the soil as it can continue to spread.
  • Using a boxcutter with a new blade, cut a circle around the wound and scrape out the infected tissue inside the circle. Keep scraping until you expose the heartwood of the tree.
  • Put the newspaper and infected tissue in a bag and throw it away so that the pathogen can't continue to spread.
  • Spray the wound with a horticultural copper spray or Bordo to prevent reinfection.
Infographic with the four steps to removing bacterial caner form the trunk of a cherry trees. Bacterial canker is one of the causes of gummosis on cherry trees.

In Ben Nobleman Park Community Orchard, we successfully treated an apricot tree in this way and our tree has recovered and is thriving today.

And the moral of the story for organic fruit tree growers? It's important to learn to recognize fruit tree diseases early on, so that you can nip them in the bud. If you want to learn how to do that and how to keep your fruit trees healthy and productive, check out our award-winning online courses at OrchardPeople.com.

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Susan Poizner

Director, OrchardPeople.com Fruit Tree Care Education Online

Susan Poizner is an urban orchardist in Toronto, Canada and the author of three books on fruit tree care including Grow Fruit Trees Fast, Growing Urban Orchards, and Fruit Tree Grafting for Everyone. Susan trains new growers worldwide through her award-winning fruit tree care training program at Orchardpeople.com. Susan is also the host of the Orchard People radio show and podcast and is an ISA Certified Arborist.

Susan Poizner and the cover of her eBook Grow Fruit Trees That Thrive

FRUIT TREE CARE NEWSLETTER

Sign up for our monthly newsletter and we will send you our eBook "Growing Fruit Trees That Thrive." You can unsubscribe at any time.
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