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What Is Killing My Tree?

August 15, 2001

Many clients bring samples to the Plant Clinic with dead leaves, stems that have lost all leaves, stems with cankered growth, or stems indicating years of poor growth. Laboratory diagnosticians can work only with the sample received; and nine times out of ten, the actual pathogen or insect is not on the sample submitted. We often (usually) need more information on what is happening at the site. Often, however, the person onsite does not know what to look for or what to convey to get the best help. Maybe this will help.

When the entire tree is affected, the injury or disease or insect logically must be affecting the trunk or the roots--an area that could cut off water to the entire tree. Look at the entire tree and compare it to nearby trees. Also consider when the problem started and what changed on the site about that same time. Healthy trees don't suddenly die because they are old. Many below-ground reasons may cause tree decline. 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 is a process of elimination. One of the possibilities more difficult 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 see if a tree is growing well. You can see a tree's annual growth by looking at the trunk cross-section. Most of us did this as children. We counted the number of rings to tell us how old a tree was when it died. We 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 a year's growth. Continue down the stem to the next set of rings for the next yea's growth. Most trees grow from 6 to 18 inches of twig length in a 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 grown only an 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 that was initiated 2 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 mushroomlike fungi growing at the base of the tree or shrub. In 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 fungal mycelium 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 honeylocust. The structure is reddish brown and appears to have been varnished. Its presence indicates invasion by a root rot. 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 root health. Do this near the drip line at two or three spots. Healthy roots are brown outside but white internally or at the tips. If roots have a soft, brown outer layer that easily pulls off, 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 decline, such as deep planting. In issue no. 14, we explained how to determine if the tree was planted too deeply. Some experts say that conks on a tree or root rot in the root system means a tree will soon die. That may be true, but such trees and shrubs may survive for many years. Do not remove a tree simply because it has a conk. Instead, use this as a diagnostic tool. 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 help a tree in decline. Use approved cultural practices to improve vitality, including water-ing in extended drought. 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 large trees, fertilization and watering may have no benefit. Still, they may help it stay alive for years.

For more on tree decline, read Report on Plant Disease no. 641, "Decline and Dieback of Trees and Shrubs," in Extension offices or on the Web, ag. uiuc. edu/~vista/horticul.htm.

Author: Nancy Pataky


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