Wood rot could hide your tree’s decay

When you see mushrooms or other fungi growing out of the trunk of a tree, pay attention. It’s a sign that the wood inside the tree is decaying.

“It’s something you want to have a professional look at promptly,” said Sharon Yiesla, plant knowledge specialist in the Plant Clinic at The Morton Arboretum in Lisle. ”A certified arborist has the training to assess whether the wood rot has affected the structure of the tree.”

Many kinds of fungi can cause wood rot. Some are readily visible as shelf or bracket fungi — roughly semicircular projections that stick out horizontally, like a shelf. These fungi slowly expand, adding a ring of growth every year.

Other kinds of fungi may be at work undetected inside the tree for years. “You won’t know the fungus is there until it is ready to reproduce and sends out mushrooms,” Yiesla said. The mushrooms may grow from the trunk or from the roots at the base of the tree.

“Mushrooms and shelf fungi may look insignificant, and it’s easy to knock them off and forget about them,” she said. “That would be a mistake. They are the tip of the iceberg and can indicate serious decay.”

Shelf lichens growing on a tree’s bark indicate that wood is decaying in the interior of the trunk. A professional arborist should assess the condition of the tree. (Beth Botts / The Morton Arboretum)

The fungal organisms enter a tree through a wound or a crack in the bark that lets in water. The fungus lives as a network of very fine fibers that penetrate the wood and consume its nutrients, causing it to break down.

“If only one branch is rotting, we can just remove the branch,” Yiesla said. “If the decay has compromised the main trunk, the whole tree may need to come down.”

Although fungal organisms growing on trees suggest wood rot, there’s no need to be concerned when mushrooms pop up in the lawn or flower beds. Those mushrooms are harmless to plants, and usually indicate that the soil is healthy, with enough organic matter to support underground fungi.

A decaying tree may look fine, with plenty of healthy green leaves. “The decay fungi don’t affect the tissue just under the bark that conducts water and nutrients, so the tree can still grow,” Yiesla said.

Meanwhile, the fungi are at work in the heartwood, the hard older wood at the center of the tree. If that stabilizing core rots out, the tree may become a hazard.

“Any fungal structure growing on a tree should be taken seriously,” she said. Have a trained, certified arborist examine the tree to see how far the wood rot has gone and recommend a course of action.

To find a certified arborist, go to illinoisarborist.org or treesaregood.org.

For tree and plant advice, contact the Plant Clinic at The Morton Arboretum (630-719-2424, mortonarb.org/plant-clinic, or [email protected]). Beth Botts is a staff writer at the Arboretum.