There are many cultural, environmental, and site-related problems that can stress—and even kill—trees. Last year, many of us predicted that trees would have root problems this year, and that was certainly the case. The problem comes in knowing what symptoms to look for in diagnosing root decline. 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 other 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. We regularly tell Plant Clinic clients how to check roots for problems. I am always amazed when they call back and tell us how few roots they found or how rotten the roots were.
The first sign of any root problem is top decline. Look for a few clues to determine if a tree is growing well. You can get an idea of how well a tree has been growing by looking at the amount of stem growth over the past few years. You can even tell approximately when a tree was transplanted or when it was fertilized. 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 grow anywhere from 6 to 18 inches of twig length in one 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 1 inch of twig growth in 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 started 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 the pathogenic wood rots and root rots, look for mushroom-like 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. I am sure you have seen these structures on trees in the forest. Polyporus sulphureus is a common heart rot fungus on oaks. It produces a bright yellow structure that protrudes from the tree 6 to 8 inches. Other fungi may indicate root rots, such as Ganoderma root rot of honeylocust, with its reddish brown conk growing off the base of the trunk. 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 its roots. Do this near the drip line at two or three spots. Healthy roots are brown on the outside but white on the inside 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 true, but trees and shrubs can survive for many years with wood or root rots. Do not remove a tree simply because it has a conk. Instead, use this diagnostic tool to determine 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, do remove it as soon as possible. Some arborists can make a determination on internal tree integrity to help with this decision.
No chemicals will help a tree in decline. Use approved cultural practices to improve tree vitality, including weekly watering of 1 to 2 inches 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. These measures may help the tree continue to live for many years. But, for very old or very large trees, fertilization and watering may have no benefit. The wood rot and decline fungi do not have to be fatal. Consult Report on Plant Disease No. 642 for more information on the wood rots and decay of trees.