Shoestring Root Rot

Shoestring Root Rot starts off small, but can turn into a BIG problem in our forests. Its different life stages aren't easy to spot, but the Plant Heroes are digging up whatever they can find on this nasty fungus. Check out the activity books and field guide to learn more!

 

Printable Field Guide: PDF

  • Armillaria sometimes appears as honey-brown mushrooms at the base of infected trees. The caps of the mushrooms average 9cm across – about the size of a sticky note.

    Photo Credit: Linda Hagen, USDA Forest Service

  • These rhizomorphs are like the roots of the fungus – they grow in dead wood and through the ground. They are what give it the name Shoestring Root Rot. They can grow up to several feet long, and are about 1-5mm in diameter. That’s around the thickness of a single piece of spaghetti. 

    Photo Credit: Bruce Watt, University of Maine

  • These rhizomorphs are like the roots of the fungus – they grow in dead wood and through the ground. They are what give it the name Shoestring Root Rot. They can grow up to several feet long, and are about 1-5mm in diameter. That’s around the thickness of a single piece of spaghetti. 

    Photo Credit: Tim Tigner, Virginia Dept. of Forestry

  • These fans of mycelia are another good way to identify Armillaria – this is the part of the fungus that absorbs nutrients from the tree

    Photo Credit: William Jacobi, Colorado State University

  • Under certain conditions, the fans of mycelia that grow from Armillaria can glow in the dark; this is called bioluminescence.

    Photo Credit: Martin Livezey, MushroomObserver.org

  • Armillaria spreads two different ways:

     

    The first is by producing mushrooms, or fruiting bodies. Fruiting bodies, which appear around infected trees in late summer or autumn, produce spores. The spores can travel great distances when picked up by the wind.

    Photo Credit: USDA Forest Service, Northern and Intermountain Region

  • These spores are about 8 μm (micrometers) across. The thickness of a single strand of human hair is about 100 μm!

    Photo Credit: Malcolm Storey, bioimages.org.uk

  • Once a new tree is infected, these fans of mycelia appear. This part of the fungus secretes chemicals that break down the wood. This rotting wood is what the fungus feeds on, and it is this process that will eventually kill the tree.

    Photo Credit: Joseph O'Brien, USDA Forest Service

  • Armillaria can move through dead trees, roots, and soil by growing these rhizomorphs. It can infect new host trees by growing these stringy structures into them. They can’t spread very far, but this is how the fungus spreads most of the time from tree to tree.

    Photo Credit: Kathie Hodge, Cornell University

  • There are several species of Armillaria that threaten North American forests. Armillaria mellea is mostly found infecting hardwood decisduous tree. This species can be found all over the country, but is doing the most damage in the south-eastern US.

  • Oak-hickory forests are one of the most common forest types in the south-east. In Virginia, 61% of hardwood forests are oak-hickory. Both oak (Quercus spp.) and hickory (Carya spp.) are hosts of Shoestring Root Rot.

    Photo Credit: Paul Bolstad, University of Minnesota

  • Beech (Fagus spp.) is another very important tree in the eastern US, and is also a host of Armillaria mellea

    Photo Credit: Andrej Kunca, National Forest Centre - Slovakia

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

    Photo Credit: Karen Blaha, flickr.com

  • Armillaria solidipes is another pest species of Shoestring Root Rot. It is a major pest of conifers in the US (and around the world). The Pacific Northwest and the Rocky Mountain region are at the greatest risk for damage from A. solidipes because of their high number of conifer forests.

  • Sitka spruce, grand fir (pictured), and douglas-fir are all important forest trees in the pacific northwest, and are all hosts of A. solidipes

    Photo Credit: Dave Powell, USDA Forest Service (retired)

  • Mixed conifer forests of Oregon and Washington are at risk of A. solidipes infection.

    Photo Credit: Dave Powell, SUDA Forest Service (retired)

  • The US 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 also very widespread, probably occurring in all forests in the region. That means this sub-alpine forest pictured here, and all the forests like it, are at risk of developing Shoestring Root Rot, and most of them already have…

    Photo Credit: Dave Powell, USDA Forest Service (retired)

  • Malheur National Forest in eastern Oregon is home to a very large infestation of Armillaria solidipes. Because the rhizomorphs connect the fungus together from tree to tree, it is considered one of the largest living things on the planet – this giant has been named the “Humongous Fungus”.

  • The Humongous Fungus covers almost 2400 acres, and it’s over two miles across – that’s about as wide as the island of Manhattan in New York City! Scientists estimate that it could be up to 4000 years old. It is by far the largest known root disease center in the world…

    Map Data: ©Google, Digital Globe

  • These dead standing and fallen trees are typical of large Armillaria disease centers. The fungus rots the base of the trunk and the roots – this causes them to die and to become unstable – this is why so many infected trees snap at the base.

    Photo Credit: Nicola Twilley for Venue (v-e-n-u-e.com)

  • These dead standing and fallen trees are typical of large Armillaria disease centers. The fungus rots the base of the trunk and the roots – this causes them to die and to become unstable – this is why so many infected trees snap at the base.

    Photo Credit: Nicola Twilley for Venue (v-e-n-u-e.com)

  • This ponderosa pine (Pinus ponderosa) has been infected with Shoestring Root Rot – notice the brown needles and the white mycelia at the base of the trunk.

    Photo Credit: William Jacobi, Colorado State University

  • 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.

    Photo Credit: USDA Forest Service - Ogden

  • 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.

    Photo Credit: Fred Baker, Utah State University

  • Firebreaks 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…

    Photo Credit: Billy Humphries, Forest Resource Consultants, Inc.

  • Once the infection is stopped or slowed, new resistant trees can be planted. If the same tree types are planted that were there before, any fungus that’s still in the soil could infect them again.

    Photo Credit: William Jacobi, Colorado State University

Scientific Name: Armillaria mellea, Armillaria solidipes

 

Description: Shoestring Root Rot is a fungal disease that attacks many different kinds of trees. It sometimes appears as honey-brown colored mushrooms around the base of infected trees. These mushrooms are visible in late summer or fall if conditions are right. It also appears as white fans of mycelia under the bark of infected trees, and as black rhizomorph roots under bark, in and around roots, and in the ground. The fans of mycelia are bioluminescent – sometimes they glow in the dark.

 

Native Range: Shoestring Root Rot is found all over the world in temperate climates.

 

Introduced range: In the U.S., shoestring root rot is doing the most damage in the southeast and the northwest. Armillaria solidipes attacks coniferous tree species in the northwest, while Armillaria mellea is more of an issue in the hardwood forests of the southeast.  

 

Habitat: Both species of Armillaria are forest pests. In addition to threatening forest trees, Armillaria mellea has been found in stone fruit orchards in the southeast.

 

Host Trees: Both species have very wide host ranges, and have been found to attack many important tree species – this includes sitka spruce and douglas-fir in the northwest, and most major hardwood species in the southeast (oak, beech, hickory, etc.).

 

The Facts: Shoestring Root Rot is a fungus that attacks many species of trees. It infects new host trees in one of two ways – either by growing rhizomorph “roots” out from an infected tree and into a new host tree, or by producing spores which land on host trees and cause a new infection. The white fans of mycelia that the fungus produce under the bark of the tree are what cause the damage. They produce a chemical that breaks down the wood – this is what the fungus feeds on. Because the fungus breaks down the parts of the tree that transport water and nutrients up to the leaves, it can cause yellowing leaves and leaf drop. The breakdown of wood around the base of the tree also makes the tree unstable, and can cause it to snap and fall. Stopping Shoestring Root Rot from spreading is a challenge, but it is possible! By removing dead stumps (and the remaining fungus along with them) and then planting resistant trees in their place, forests that were infected with Shoestring Root Rot can be productive again.