We have received many telephone inquiries and several samples of trees that are declining and dying for no apparent reason. We can rule out cankers, vascular wilts, and insect causes, but we cannot assess the root situation from the lab. Here are a few suggestions on factors that might be involved and tips for investigating the possibilities. Just a few of the tree species we have had complaints about include maple, magnolia, birch, and dogwood. In these cases, the entire tree was suddenly affected. When the entire tree is affected, the injury, disease, or insect logically must be affecting the trunk or the roots—areas that could cut off water to the entire tree.
There are, of course, many belowground reasons for the decline of a tree. Drought, flooding, compaction of the root zone, poor soils, planting too deeply, inadequate space for roots, and many more factors could be involved. Often, the diagnosis of such a problem is a process of elimination, ruling out possibilities that might cause similar symptoms. One of the more difficult possibilities to eliminate is root rot. Most gardeners believe that they cannot possibly know the health of a mature tree’s roots.
The first sign of any root problem is top decline. Look for a few clues to determine whether a tree is growing well. You can see a tree’s annual growth by looking at the trunk cross-section. Most of us have done this as children. We counted the number of rings to tell us how old a tree was when it died. We have also looked at the thickness of these rings to compare relative growth between years. A less destructive way to determine amount of growth is to look at the stems. Follow the stem tip back to the first set of closely aligned rings (about 1/8 inch apart) around the stem. That is 1 year’s growth. Continue down the stem to the next set of rings for the next year’s growth. Most trees will grow anywhere from 6 inches to 18 inches of twig length in 1 year. Of course, this will vary with the species and whether you are looking at a shady or sunny part of the tree. If the tree has only grown 1 inch of twig growth for the last 2 years and 8 inches 3 years ago, it is safe to say that the tree is under stress and that the stress was initiated 2 years ago. Cankers on the stems, stem-tip dieback, off-color foliage, early fall color, and early defoliation are also clues to tree stress from underground causes.
To detect the pathogenic wood rots and root rots, look for mushroomlike fungi growing at the base of the tree or shrub. In the case of wood-rot fungi, the conks (also called shelf fungi or fruiting bodies) may be found growing on the trunk or main branches. These are signs of the pathogen. The actual mycelia of the fungus are probably growing in or on the roots or internally in the wood. One of the most common examples is Ganoderma root rot, which produces a shelf-type fungal structure at the base of many trees, especially honeylocust. The structure is reddish brown and appears to have been varnished. Its presence indicates that a root rot has invaded. Other fungi may indicate wood rots. Wet weather often triggers the formation of these structures. They could easily be confused with fungi growing on dead organic debris near a tree. If, however, they are growing from the tree itself, they are excellent signs of wood rot or root rot.
You can also do some careful digging in the root zone of a tree to determine the health of the roots. Dig near the drip line at two or three spots. Healthy roots will be brown on the outside but will be white internally or at the very tips of the roots. If the roots have a soft, brown outer layer that easily pulls off the center of the root, then a root rot may be involved. Digging with a hand trowel a few inches below ground around the trunk may reveal girdling roots, mechanical injuries, or other clues to the cause of tree decline.
Some experts say that the presence of conks on a tree or root rot in the root system means that a tree will soon die. That may be the case, but trees and shrubs may survive for many years with wood or root rot. Do not remove a tree simply because it has a conk. Instead, use this as a diagnostic tool in determining the true problem with the tree. If the tree becomes a threat to life or property because of its potential to fall or blow over, remove the tree as soon as possible.
No chemicals will help a tree in decline. Use approved cultural practices to improve tree vitality, including watering in periods of extended drought. Also, cut out dead branches in the dormant season, fertilize in late fall or early spring, and keep traffic off the root system. For very old or very large trees, fertilization and watering may have no benefit. These measures may help the tree continue to live for many years. The wood rot and decline fungi do not have to be fatal.