Diagnosing disease and insect problems can really be fun. The task becomes more difficult when the problem is not something infectious or related to insects. It takes time to wade through all of the site, environmental, and cultural factors involved, and often this sort of information is not provided. Here are some tips on diagnosing tree root problems—things to look for before you send a sample to the Plant Clinic.
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, diagnosing such a problem involves 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 growth between years. A less destructive way to determine the 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 one year’s growth. Continue down the stem to the next set of rings for the next year’s growth. Most trees grow anywhere from 6 to 18 inches of twig length in one year. Of course, this varies 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 for the last two years and 8 inches three years ago, it is safe to say that the tree is under stress and that the stress began two years ago. Cankers on the stems, stem tip dieback, off-color foliage, early fall color, and early defoliation are also clues 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 aerially on the trunk or main branches. These are signs of the pathogen. The actual mycelia 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 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 try to determine the health of the roots. Do this near the drip line at two or three spots. Healthy roots are brown on the outside but 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, a root rot may be involved.
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 rots. 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 it as soon as possible.
No chemicals will help a tree in decline. Use approved cultural practices to improve tree vitality, including weekly waterings of 1 to 2 inches of water 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. On most trees, these measures may help the tree continue to live for many years. Wood rot and decline fungi do not have to be fatal.