What can I do about armillaria in my yard?

Asked July 15, 2017, 5:54 AM EDT

For the last two years in the fall, mushrooms have appeared in the strip between the sidewalk and the street. Last year nearly 400 of these mushrooms appeared, predominantly clustered around an English walnut that's in the strip but also entering the yard proper. I took photos and made a spore print and believe the mushrooms are armillaria spp. Now the walnut is looking sick (many branches and twigs without leaves). I would like to save the walnut if at all possible; I would also like to stave off the armillaria from further entering my yard and the yards of my neighbors. What can I do? I'm in Portland if that helps.

Multnomah County Oregon armillaria mushrooms armillaria fungi walnut tree

3 Responses

Thank you for your question about armillaria. This fungus is known to infect English walnut trees; it is the most common conifer root rot in the Pacific Northwest, and attacks virtually every woody species. Like other fungi, it is the result, rather than the problem. That is to say, it survives and grows in old stumps or dead trees. By the time you see the mushrooms, the body of the fungus, which lives underground, has already 'eaten' much of the fibrous material that makes up the tree's root, trunk and often stem system. This progression leads to the gradual demise of the tree. Since you did not send photos of the tree itself, I cannot tell whether there are already symptoms apparent of this fungus' 'work.'

Armillaria root rot does not spread by spores (through the air.) It is spread root to root, so if you want to prevent it from spreading to other weakened trees in your neighborhood, the best thing you can do is to remove the tree. Here is a link to a very thorough article about the disease. Please read carefully the "Integrated Pest Management Strategies" paragraph of the article.

I hope this is helpful, and I'm sorry about your tree.

Thanks for your response. If I understand you right, you are saying my walnut is doomed and I should cut it down.


Three questions:

1) If armillaria doesn't travel by spores, how did it get into my parking strip in the first place?
2) I read that there are things people do to try and save trees (like "air-spading" the roots). If I do things like that rather than immediately cut it down, am I potentially worsening the armillaria situation for my other trees and plants?
3) If I do cut the tree down and dispose carefully of the stump, what about my other trees and plants? There is a crabapple several feet from the walnut, and the mushrooms have also appeared in my side yard where there are lots of bushes and other plants (huckleberries, ferns etc).

Thanks for your help!

I think that, based on the science I provided, (1) the fungus is incurable and (2) the presence of mushrooms indicates the fungus has progressed far into its development and (3) it's only a matter of time and your tree may become a hazard. (A) Spores may be brought in through infected stock. It may have been in the soil for decades, just waiting for a host (dead tissue). Pathogens have a triad in order to grow and survive: the pathogen, the environment and the host. Until there is necrotic (dead) tissue, many fungi are unable to break down complex material (through enzymes) and so they linger in the soil. (B) I have been unable to find any scientific research that promotes 'air-spading' as a cure for this fungus. (Please see prior article's recomendations.) (C) Anywhere you see mushrooms, there is a fungus and necrotic tissue--but the fungus may be different, and it can be ID'd by reference to the mushrooms. Because some plants are resistant to some fungi, what you have growing needs to be diagnosed on a plant by plant basis. Then you can decide what to do. I'm sorry there's no blanket diagnosis nor cure. Nature is complex.