Shoestring Root Rot

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armillaria

Shoestring Root Rot

Scientific Name
Armillaria
The Humongous Fungus!
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Hero Nate

This fungus affects many trees and can grow to humongous proportions! 

It can cause trouble to native trees all across North America.

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Field Guide
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Shoestring Root Rot
Disease Cycle

Armillaria spreads by producing mushrooms or fruiting bodies. They appear around infected trees and produce spores that can travel great distances when picked up by wind.

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Once a new tree is infected, the fungus attacks the “cambium” or living tissue. The tree can no longer transport the resources needed to the upper parts of the tree, and this process eventually kills it.

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Armillaria moves through dead trees, roots, and soil by growing these rhizomorphs, which is how it can reach new trees. They can’t spread very far, but this is how the fungus usually spreads from  tree to tree.

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Armillaria mellea's Favorite Trees

Several species of Armillaria threaten North American forests. Armillaria mellea mostly infects deciduous trees and can be found all over the country, but is doing the most damage in the southeastern U.S.

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Forests with high numbers of hickory (Carya spp.) and oak (Quercus spp.) trees are very common in the southeast U.S. Both oak and hickory  are hosts of shoestring root rot. Beech (Fagus spp.) is another very important forest tree in this region, and is also a host of A. mellea.

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Forest trees aren’t the only ones threatened by Armillaria mellea. Recently, this fungus has been found to be a threat to stone fruit (peach, pear, cherry, etc.) orchards.

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Armillaria solidipes' Favorite Trees

Armillaria solidipes is another species of shoestring root rot and is a major problem for conifers in the U.S., especially the Pacific Northwest and the Rocky Mountains due to the high numbers of conifer forests in those regions.

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Sitka spruce, grand fir, and douglas-fir are all important forest trees in the pacific northwest, and are all hosts of A. solidipes. Douglas-fir is a very important commercial species and a very popular Christmas tree species—it’s even the state tree of Oregon!

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The U.S. Forest Service has reported finding A. solidipes on almost all common tree species and in all major forest types in the Rocky Mountain Region. It is very widespread, probably occurring in all forests in the region. 

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Damage

Malheur National Forest in Oregon is home to a very large infestation of A. solidipes. Because the rhizomorphs stretch from tree to tree underground, the fungus is considered one of the largest living things on the planet and is called the "Humongous Fungus ."

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This ponderosa pine has been infected with shoestring root rot. Notice the brown needles, a sure sign of an unhealthy tree!

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These dead standing and fallen trees are typical of large Armillaria disease centers. The fungus rots the base of the trunk and roots. This causes trees to die, become unstable, and eventually break at the base.

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Management

Removing infected trees from the area is the first step in managing this disease. “Destumping” is the removal of old infected stumps. Since the fungus can survive in dead wood and roots, this needs to be done before the area can be replanted.

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Once the infection is stopped or slowed, new resistant trees can be planted. If the same trees are planted that were there before, any Armillaria fungus persisting on the site could infect them again. 

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Control

Firebreaks (or sections of forests without any plants) are used to stop forest fires from spreading. The same method can be applied to stopping Armillaria from spreading. If the break is large enough, the rhizomorphs will not be able to find new host trees, and will eventually die out.

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Birch and larch have both been found to be resistant to Armillaria, and do well in disturbed soil. After growing resistant trees on the site, less resistant (host) trees can be replanted, as the fungus will have died out.

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