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Arborvitae trunk decay/fungal growth? #946396

Asked October 02, 2026, 7:52 PM EDT

Hello, I'm hoping you can help us identify what is happening to one of our arborvitae. We do have another one with similar problem though not as bad. We've also noticed brown foliage dropping, although we're seeing similar brown foliage and dropping on other arborvitae in our area, so we're wondering if that part may be seasonal. There is a significant amount of a white/cream-colored fungal?-looking growth--please see attached photos. The exposed wood underneath is soft and spongy when gently pressed. We are super concerned about it. The tree in the photo has also had significant amount of ants around it this past spring/summer. If you need anymore details, please let me know I would be happy to provide more photos and or description. Any insights or information into what this is and steps we can take to help this tree would be greatly apprecaited! Thank you!

Wadena County Minnesota

Expert Response

Thank you for your question.

The white material is attributable to fungal infection. I suspect that this arbor vitae incurred some type of physical injury near its base that then led to this fungal growth. I can’t identify the fungus with any degree of certainty. Armillaria root rot is a possibility. It is common for arbor vitae to experience needle drop at this time of the year (early October 2026). I don’t think this needle drop is directly related to the fungal infection. The ants that are present are probably attracted to the “soft and spongy” wood. They are not causing the problem; just reacting to it. Unfortunately, there is no fungicide that will restore wood that has reached this soft, spongy stage. Once structural wood has decomposed, it cannot regenerate. Painting the cavity, applying wound dressing, spraying fungicide into it, bleach, peroxide, copper products, horticultural oil, etc., will not reverse this decay.

To protect your other arbor vitae, you should have a professional, licensed arborist make an onsite inspection and assessment.

See the following for further information on these and related points:

https://pddc.wisc.edu/wp-content/uploads/sites/39/2024/11/ArmillariaRootDiseaseAdultsFactSheet.pdf

https://apps.extension.umn.edu/garden/diagnose/plant/evergreen/arborvitae/treebreaks.html

https://extension.umn.edu/garden-and-home/yard-and-garden/gardening-in-minnesota/yard-and-garden-problems/armillaria-root-rot

https://www.canr.msu.edu/news/diagnosing_armillaria_root_rot

https://extension.umn.edu/garden-and-home/yard-and-garden/gardening-in-minnesota/how-to-hire-a-tree-care-professional

https://www.treesaregood.org/Find-an-Arborist

Good luck. Thanks for consulting us.

An Ask Extension Expert Replied October 03, 2026, 6:26 PM EDT
Thank you so much for the response. 
Are there any professional, licensed arborists in the Menahga or Park Rapids area that you would recommend? 

Thank you again, 
Laura

On Sat, Oct 3, 2026 at 5:26 PM Ask Extension <<personal data hidden>> wrote:
The Question Asker Replied October 05, 2026, 4:10 PM EDT

Thanks for getting back to us.

Since we are affiliated with the University of Minnesota, it must be our policy to not endorse specific commercial enterprises. I would suggest that you do a web search for arborists in the Wadena-Hubbard County areas. I just did that, and several are available. When contacting them explain fully your situation. Indicate that you do not want to remove the arbor vitae but want an on-site assessment of the situation. Such an assessment should include various treatment options.

Good luck. Thanks for consulting our forum.

An Ask Extension Expert Replied October 06, 2026, 12:21 AM EDT

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