How to Fight Armillaria Root and Butt Rot

By Erin Marissa Russell and Matt Gibson

Armillaria root and butt rot is also commonly known as bootlace fungus, shoestring root rot, mushroom root rot, honey mushroom rot, or honey toadstool fungus.

Armillaria root and butt rot is a fungal infection caused by a group of Armillaria fungal species, including A. sinapina, A. gemina, A. gallica. A. mellea, A. ostoyae, and A. calvescens.

A common wood-rotting fungus among forest trees, Armillaria has a tendency to attack both hardwood and conifer tree types. Certain species prefer hardwoods, while others prefer conifers, but every species of Armillaria has the capability of attacking either tree type.

If you have trees on your property that are already stressed by insect infestations, mechanical injuries, exposure to extended periods of drought, or to ice and wind storms, they are more likely to contract Armillaria rot, and Armillaria rot is more assertive when attacking trees that are already compromised in some way.

Young, vigorous trees are much less likely to develop Armillaria than are older, weaker trees. However, physically-stressed young trees and young trees planted next to inoculum stumps are highly susceptible to Armillaria infections.

How to Identify Armillaria Root and Butt Rot

The symptoms of Armillaria generally look like many other root issues. Symptoms include fading, wilted, or thinned leaves, general signs of decline, dead branches, branch and trunk failure, dieback, stunted growth, and death. The foliage that remains on the trees will appear stunted and yellow.

Abnormal sap flow can be spotted along the root collar. If soil is taken from around the bottom of the trunk, black strands may be found in the soil and attached to large roots. Evidence of fungal growth can be found between the bark and wood of infected trunks in the form of fan-like sheets made of fungal strands.

The most commonly-spotted sign of infection is the formation of amber-colored mushroom caps which form at the infected tree’s soil line. These mushrooms rise four to six inches high with one inch caps that can grow as large as two to four inches wide.

How to Prevent Armillaria Root and Butt Rot

Armillaria root and butt rot is so widespread in certain areas that it is impossible to establish control measures that will prevent it completely. Part of the reason it spreads so well is due to the rhizomorphs it creates under the surface of the soil that we cannot see. Most wood-rotting diseases don’t spread via rhizomorphs but instead use the wind to spread their spores, making them simpler for gardeners to control.

If Armillaria is a serious concern in your garden, it’s vital for you to use the prevention measures listed here to minimize the risk of your trees contracting the disease as much as you can. However, Armillaria is not as prevalent in most of the world as many other tree diseases, so unless you know it is a concern in your area or have seen signs of it around your trees, you don’t need to be too concerned with it.

The good news is that one of the best things you can do to prevent Armillaria from taking hold in your orchard is to boost the health of your trees in general, which is something all gardens can benefit from.

Armillaria root and butt rot can persist above the ground for many years in the form of a saprophyte that lives on large tree roots or stumps. For this reason, gardeners should grind stumps and remove large lateral roots if at all possible to avoid providing a home for Armillaria saprophytes.

Even when Armillaria is present on your property, it’s possible for trees that are in good health to fight the disease off. Often, trees only become infected by Armillaria if they’re made vulnerable by other types of stress, even when the disease has existed around them for years and launched several attacks. Anything you can do to meet the care and maintenance needs of your trees and make them more generally healthy or prevent other types of stress from affecting them will help your trees to stave off Armillaria infection.

Sugar maple trees benefit from liming to make them more healthy and vigorous. For all your trees, take steps to protect them from infestation with insects that cause defoliation. (Especially take measures to prevent gypsy moth damage in oak trees located in the eastern United States.) Gardeners should also avoid wounds from gardening tools and other mechanical injury, and keep soil pH at the level that your trees prefer. You should also help your trees avoid the stress associated with drought by ensuring they get plenty of water from you when it isn’t provided by nature via rainfall.

When you select a site for a new orchard, be aware of the symptoms of Armillaria so you can attempt to choose a spot that has no signs or few signs of infection. There may be other factors that contribute to Armillaria infection in your area as well, such as soil type, that you should also be aware of in your search.

If you are clearing an area to establish a new orchard, a three-step approach can help prevent Armillaria from being a problem where you’ll be planting. First, girdle any existing trees on the property by using ring-barking, in which you remove a strip of bark all the way around the trunk. Allow the ring-barked trees to remain for at least six months before you remove them. Once the six-month waiting period has passed, rip out all the trees where your orchard will be, pulling up stumps and roots as you go and burning them. Finally, let the area go to pasture for two or three years so that any small roots that may be infected will decay.

Whenever possible, choose resistant species of trees when you’re making new plantings. Use local seed sources when you can, as these will have the highest resistance in the local environment.

How to Treat Armillaria Root and Butt Rot

As we’ve already discussed, healthy trees can live in the presence of Armillaria uninfected as long as they are not made vulnerable by other stressors or diseases, which can allow the disease to take hold. For this reason, you don’t need to roll out treatments against Armillaria simply because you see it in the environment. However, if your trees start showing the symptoms of infection, follow these steps.

Remove the roots or stumps of infected trees so the Armillaria won’t be able to stick around in them and continue its spread. Root raking can help to remove smaller roots from the area.

Use trenching around trees infected with Armillaria to slow down its progress by severing the roots and halting the spread of rhizomorphs underground. To use trenching around your trees, dig a trench that’s a meter deep (about three feet deep), then line it with a plastic material and backfill it. For trenching to work, Armillaria needs to be missing from the side of the trench you are protecting against the disease.

Install root barriers around your trees so the Armillaria infection can’t reach them.

Excavate a bit around infected trees to expose the collar (where the roots meet the main trunk). Remove the soil to the depth where the main roots of the tree come from. Studies have shown collar exposure of infected trees extends their productive life and protects the roots of the tree from vascular decay. This approach is only really effective on trees where the zone of infection is small and appears in the root system or on the root collar.

You can fight back against Armillaria with other types of fungi that are antagonistic to the Armillaria pathogens, such as Trichoderma. Note that the Italian study on which recommendation of this control measure is based tested an area that had 12 percent infection rate, which is not a particularly severe infection. The 12 percent infection was seen to go down to just 2 percent using Trichoderma, however. An attempt by Clemson University Cooperative Extension to test Trichoderma in an area with higher infection rates left too few surviving trees for meaningful results, so it’s likely that antagonistic fungi work best in areas where Armillaria infection is still low.

Make sure to clean and sterilize your gardening tools as you move from tree to tree when you are working in areas infected with Armillaria. Disinfecting your tools between each tree you work on will help prevent spreading the infection.

To keep Armillaria from spreading in the area surrounding an infected tree that you’ve removed, you may wish to consider also removing any susceptible trees or shrubs that are near to the original infected tree. This technique is recommended because it can take one to three years for symptoms of infection to be apparent, so trees that appear healthy but are near to an infection site may actually be infected themselves.

Although Armillaria can be widespread in certain areas and is a potentially lethal fungal disease, there are some very interesting things to know about the fungus. Many strains of the Armillaria pathogen are bioluminescent, which means they naturally glow in the dark. The glow that comes from wood decaying due to Armillaria is colloquially called foxfire, and humans have been in awe of it for a long time, as evidenced by its being mentioned in the oldest English literature still known today, Beowulf.

You can easily observe the glow by chopping out pieces of wood from a decaying tree infected with Armillaria, choosing a bright white area that does not have signs of other fungi. Place the chunks of wood into a plastic zipper bag, selecting several pieces because the amount of glow is highly variable. Take the bag outside on a dark, cloudy night, and once your eyes have had some time to adjust to the darkness, remove it from your backpack to observe the foxfire.

Armillaria is also the organism behind the “humongous fungus” news story that swept the world in the early 1990s. UHaul trucks even feature Armillaria in some of their artwork, but sticklers should know that the artist took a bit of creative license in making the pictured fungi in the illustration pink. They are in fact tan or yellowish.)

The clones of Armillaria (also called genets) can reach very large sizes, with the largest known genet in eastern Oregon measuring 900 hectares (which translates to 2,200 acres, or 3.4 square miles—the size of 1,665 football fields). That particular structure is estimated to be more than 2,400 years old.