Tree Facts: Heart rot of trees

Farm Forum

Do you have any mature trees that appear to have Styrofoam coming out of the trunk and main stems? The tree is infected with heart rot a fungal disease that causes the wood to decay at the center of the trunk and branches. The fungi are classed either as white-rot or brown-rot. There are 11 types of fungi that infect and decay hardwood tree and shrub species in the Great Plains.

Different fungi are specific to certain tree species. Throughout the Great Plains different types of fungi attack Green Ash, White Ash, Bur Oak and Box Elder, Poplar, Maple, Honey Locust and etc. In some areas heart rot fungi have killed out Buffalo Berry trees and they are no longer recommended.

Trees damaged from machinery, other falling trees and storms are susceptible to heart rot. The fungus enters the tree through wounds in the bark and broken branches. The fungus spreads within the tree and slowly rots the heartwood. Old trees with substantial decay tend to spread the disease into younger trees that would normally not be as susceptible. Decaying trees also become habitat for animals, insects and other microorganisms. The infection is a very slow process taking years to spread throughout a tree and eventually causing mushrooms or conks to grow on the tree.

Heart rot fungi have both a huge economic and environmental impact. The fungi only affect the heartwood and do not affect the living sapwood. Initially, the heartwood becomes discolored and eventually the branches and trunk become structurally soft and weak. Typically the fungus advances into the heartwood about two to three inches per year. The sapwood of the tree continues to grow creating more weight in the weakened areas of the tree, causing branches and trunks to be more susceptible to breakage from high winds and trunk fracture.

There are things that can be done to prevent or a least minimize heart rot. It helps to keep trees healthy, minimize wounds and properly prune branches. A healthy tree naturally combats heart rot through a process called compartmentalization. The tree grows new wood around the decayed wood tissue and preventing fungus from spreading. Providing a tree with the necessary nutrients, water and growing conditions will promote healthy growth and minimize rot. The bark is the tree’s main defense against disease; reducing the amount of large wounds and bare wood, especially in older trees, helps prevent rot.

Pruning focuses on removing dead or diseased branches with minimal damage done to the tree. Avoid pruning wounds which expose large areas of wood. Shape trees when young so major branch removal will not be necessary later. Remove broken branch stubs following storm damage. Make clean cuts of branches at the branch collar and do not leave stubs more than three inches long.

Damage to hardwood trees from heart rot becomes increasingly serious when trees get old and their vigor declines. Trees infected with heart rot near homes, businesses and high use areas should be inspected every few years by trained professionals to determine if sufficient live wood is present for structural safety. Large trunks and main branches with extensive decay may not have enough sound wood to support them.

My sources for this news release were the U.S. Forest Service and the SD Department of Agriculture Division of Resource Conservation and Forestry. If you would like more information about “Heart Rot of Trees,” contact Bob Drown @605-244-5222 Extension 109 or by e-mail at