Butt Rots: Have You Checked Lately?

Let's clear up any personal anxieties right away. These butt rots won't affect your own posterior. The term "butt rot" refers to decay fungi that attack the base ("butt") and roots of trees. All sorts of trees. In fact, butt and root rots may be the most damaging but least understood tree diseases. In this column, you'll meet some butt rotters and find out how and why they do their thing.

Many fungi in the group Basidiomycetes (the mushroom makers) can cause butt and root rots. These decay fungi set up shop in living trees, which makes them different from the sap-rotters, which attack dead trees and wood products (the siding on your house, your wooden fence poles, etc.). So we're talking about fungi that can weaken and even kill your trees prematurely.

How do butt and root rotters find their way into trees? Since they are wound invaders, they can slip in through openings. Basically, this means anyplace the bark has been removed, for whatever reason. For urban trees, wounds are all too common. Mower and string-trimmer injuries, whether to surface roots or to the base of the trunk, are so common it's hard to find landscapes without many obvious examples. Each wound is an open door to ravenous decay fungi. But mower wounds are just the tip of the iceberg. Add in limb-pruning wounds, roots severed by construction (trenching, driveways, foundations, sidewalks), ice and wind damage, and bark-stripping squirrels, and the possibilities quickly become limitless.

Once decay fungi have invaded, the story really gets interesting. Sometimes the invaders gradually grow through the cambium (the wood-making tissue) of the roots and trunk. The result is often declining tree health - yellowed, smaller-then-normal leaves, dying branches, and loosening bark. Other invaders may grow into the heartwood at the center of the tree, softening and rotting the core for many feet up and down the trunk. Have you ever noticed hollows in branches or trunks? How about woody-appearing shelfs or blobs of fungus, often dark-colored above and lighter-colored underneath, poking out of a trunk? These are the calling cards of butt rotters. Eventually, so much internal wood becomes rotted that the tree becomes unstable and keels over - maybe onto your brand-new Lexus.

Fortunately, these dire scenarios don't always play out after decay fungi invade. It turns out that vigorous trees - those with lots of energy - can defend themselves pretty effectively.

They use both chemical weapons, such as natural compounds that slow down the fungi, and physical weapons, such as barriers of new wood thrown up to block the invaders' path. Mother Nature designed this clever containment strategy to prevent the fungus from getting very far into the tree. But even Mother can't always save the day. Sometimes the containment strategy fails when the tree runs out of energy. If there are too many wounds to defend, or other stresses sap the tree's energy, the decay fungi triumph and butt and root rots are the results.

To save your Lexus from butt rot, you need to think like a butt-rot fungus. As an ambitious fungus, what do you want? First, you crave lots of bark wounds. Second, you want a weak victim with little energy to defend itself.

Back in your role as Lexus owner, you want to deny the fungus its desires. Strategy 1: Minimize wounds. You can do this by careful, annual pruning while the tree is young. This reduces the need for removal of large limbs later on, which can create wounds too big for the tree to close. When trenching or other construction projects are planned, work with the contractors to insure that the root zone is protected to the maximum extent possible. The best way to keep mowers and weed-whackers away from trunks is to keep turfgrass away, too. Mulching with wood or bark chips will not only back off the grass, but also encourage tree root growth and improve tree vigor.

Which brings us to Strategy 2: Maximize tree vigor. Like healthy people, healthy trees can often fight off diseases. So think about wellness for your trees as well as yourself. Mulching is a good start. In addition, it's very helpful to water your trees deeply, once per week, during dry periods in the summer and fall.

Mulching and watering are fairly inexpensive and can help stress-proof your trees. They're practical, commonsense strategies to beat butt rots and keep your trees off your Lexus.

This article originally appeared in the May 8, 1998 issue, pp. 52-53.


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