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Extension > Garden > Yard and Garden > Trees and shrubs > A practioner's guide to stem girdling roots of trees > Symptomology

Symptomology

Gary R. Johnson
University of Minnesota

Richard J. Hauer
Minnesota Department of Agriculture

Copyright © 2002 Regents of the University of Minnesota. All rights reserved.

Signs and Symptoms

Signs and symptoms are terms used to describe physical evidence of the causal agent(s) (signs) and plant reactions to the presence of the causal agent (symptoms). Signs are generally much less common than symptoms.

Roots that encircle or grow tangentially to the stem of a tree and cause bark and wood tissue compression are the sole signs of SGR problems (Figure 2). Roots crossing roots and causing compression are not known to be associated with plant health problems (Figure 3). Tree roots are commonly observed growing over other roots and root grafting frequently occurs (Figure 4). However, if root grafting doesn't occur and roots girdle roots, the potential effect on the total tree transport system is presumed to be minimal.

Fig. 3 - Roots crossing roots are not documented to be a problem. They often are embedded or grafted together.

The symptoms associated with the presence of problem-causing SGRs are many and often inconclusive, in that many are common and characteristic of several different causal agents. For instance:

Fig. 4 - These crossed roots have formed a graft union (arrow). Neither root is harmed.

Leaf tip and margin scorching is a common, visible symptom of physiological stresses caused by SGRs. However, this might also indicate a soil water extreme (droughty or flooded), root death from a variety of causes, bacterial- and fungal-induced leaf scorch, cambial death from cold temperatures, herbicide drift, or nutrient disorders.

(Figure 6) Severe (greater than 50% of individual leaf surfaces) and chronic leaf scorch is commonly associated not only with SGR problems, but also with drought, flooding, vascular wilt diseases, drying winds, and deicing salt accumulations in the soil.

Fig. 6 - More than 50% of the leaves on this sugar maple with SGRs had greater than 50% of the leaf surface exhibiting scorch symptoms.

A less obvious, more insidious symptom associated with SGR problems is stunting of the foliage, annual twig growth rate, and d.b.h. However, root damage/loss, nutrient imbalances, chronic drought, and poor soil water percolation often yield the same symptoms.

(Figure 8) As stress from SGRs continues, excessive and/or localized (one-sided) twig and/or branch dieback following normal winters often becomes more common. However, this could also indicate that the tree was less cold hardy than others (either genetically or due to reduced vitality for any number of reasons), or that an unusual spring weather pattern occurred (alternating warm and cold after dormancy was broken).

Fig. 8 - This sugar maple's health was severely impacted by extensive SGRs. The tree suffered significant dieback and cambial death following a normal winter.

(Figure 9) Trees, in particular deciduous trees, normally flare or expand near the ground. Trees suffering acute stem compression from SGRs often lack trunk flares, appearing more like utility poles. However, many conifers normally lack obvious trunk flares. Also, excessive and unnatural depths of soil over root systems will disguise trunk flares.

(Figure 10) As tree vitality and root systems decline, trees often lose their stability and begin to lean. However, leaning might also be due to wind, root cutting, or perched water tables.

Fig. 9 - Most trees exhibit a characteristic flaring of the stem near the ground line, such as this healthy oak.

Fig. 10 - Only three trees in this large commercial landscape exhibited an abnormal lean. All had 85% or more of tehir stem circumferences compressed by SGRs.

(Figure 11) An alarmingly high number of trees lost during windstorms break off at compression points in their stems caused by SGRs, often several inches below ground. Above the point of breakage, the stem looks like a pinched balloon. This might be confused with breaks at points of graft incompatibility or areas girdled and compressed by synthetic materials (e.g., wires, synthetic ropes).

Manifestation of Symptoms

Symptoms of physiological stress from SGRs rarely show up immediately and rapidly. Most commonly, several years pass before stem compression becomes extensive enough to become symptomatic. In studies at the University of Minnesota, above-ground symptoms became obvious 12 to 20 years after planting (Johnson 1999).

SGRs are most likely primary stressors (like extended drought or defoliation) rather than primary "killers" (like vascular wilt diseases or lightning). Primary stressors greatly weaken a tree's ability to grow and function normally, combat destructive agents such as disease pathogens or insect pests, and recover from damage. Tree vitality eventually becomes so reduced that other, relatively minor, stresses result in major, long-term damage to the tree. In other words, SGRs contribute to tree health decline and premature death. If all other stresses-for instance, drought, deicing salt accumulations, and defoliation from diseases or insects-are kept to a minimum or avoided, it is entirely possible that many trees can live many years with the stresses SGRs inflict on them.

There are instances when the effects of SGRs can be dramatic and acute (e.g., tree failure during severe weather, especially loading from snow, ice, or winds). A tree is only as strong and stable as its weakest point, and compressed stem areas are commonly weak points. High winds and loading from snow and ice often result in breaks at the point of stem compression.

Fig. 11 - This hackberry failed during a thunderstorm with high winds. It broke several inches below ground where almost 100% of the stem circumference was compressed by SGRs.

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