As risky as this may be, I am going to predict that we will see an increase in root rot problems of trees next year. The extensive wet weather experienced in much of the state in May, June, and July of 1997 and 1998 has undoubtedly caused root injury. You may not be able to see the roots, but there are many clues to a root problem.
There are, of course, many below-ground 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 other factors could be involved. Often the process of diagnosing 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 ascertain the health of a mature tree's roots.
The first sign of any root problem is top decline. Look for a few signs to determine whether a tree is growing well. You can get an idea of how well a tree has been growing by observing the amount of stem growth over the past few years. Start at the tip of the stem and follow it back to the first set of closely aligned rings (about 1/8 inch apart) around the stem. That is one year's growth. Continue down the stem to the next set of rings for the next year's growth. Most trees will show anywhere from six to eighteen inches of twig growth in one year. Of course, this varies by species and whether you are looking at a shady or sunny part of the tree. If the tree shows only one inch of twig growth for the last two years and eight inches three years ago, it is safe to say that the tree is under stress and that the stress was initiated two years ago. Cankers on the stems, stem tip dieback, off-color foliage, early fall color, and early defoliation are also signs that a tree may be stressed by underground causes.
To detect 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 out of the trunk or main branches in the canopy of the tree. These are signs of the pathogen. The actual mycelium of the fungus is 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 of fungal structure at the base of many trees, especially honey locust. 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 out of 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 try to determine the health of the roots. Do this near the dripline at two or three spots. Healthy roots are brown on the outside and 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.
Some experts say that the presence of conks on a tree or a 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 rots. Do not remove a tree simply because it has a conk. Instead, use this as a diagnostic tool to determine the true problem with the tree. If the tree becomes a threat to life or property because of potential to fall or blow over, remove the tree as soon as possible.
No chemicals can help a tree in decline. Use approved cultural practices to improve tree vitality, including weekly waterings of one to two inches of water during periods of extended drought, and fertilize in late fall or early spring. (For very old or very large trees, fertilization and watering may have no benefit). Also, cut out dead branches in the dormant season, and keep traffic (pedestrian and vehicle) off the root system. These measures may help the tree continue to live for many years.