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Mountain Ash Trees

Q.I Have A Mountain Ash Tree With Yellow And Dying Leaves

Zone Calgary, Alberta | breithar added on August 6, 2020 | Answered

I live in Calgary Alberta and have a Mountain Ash tree that is 25 years old. In the last 3 years it has started to get yellow leaves immediately in the spring. They will eventually prematurely yellow and then die. This is not fire blight and nor is it mold or fungus. I have had an arborist look at it and he has used ground injection fertilizer, but that has not solved the problem. He has looked at the tree and said it is not bugs. We have had a wet spring, and not hot (average for Calgary). This yellow is now spreading to larger sections of the tree. The tree used to be nice dark green but it is now lighter every year. The rborist told me that he is seeing this in birch and Mountain Ashe trees all around the city. The city has stopped spraying herbicide for at least 3 years and my yard has not had herbicide sprayed for 3 years either. Do you have any advice as to what might be the problem and how I can solve it? Thanks.

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Certified GKH Gardening Expert
Answered on August 7, 2020

I notice that the tree has one side that is largely affected. This is the tell-sign that these symptoms are attributed to Verticillium Wilt Disease.

This is not able to be cured, and is getting more and more common with Mountain Ash trees. You can provide the best care possible, and keep infected matter pruned and discarded of properly. This will give the tree the best defense against this disease.

Keep in mind that this does not rule out fire blight. If the arborist tested the disease to confirm, rather than by looking with the eye, then you may be able to rule it out.

Here is an article for more information:


This article will help you to care for the tree:


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