Forest Health > Insects and Diseases publication
Injury and Stress
Trees are like people. When people are tired and run down, they are often more susceptible to various diseases. Likewise, trees subjected to environmental stresses or injuries are usually more vulnerable to damage caused by insects and diseases. The concept of STRESS, as it relates to insects and diseases affecting trees, is vitally important. Natural and man-made stresses are major contributors to insect and disease problems in Florida. Knowledge of how susceptibility is affected by stress or, on the other hand, how pests interact with tree under stress, is a key element in the reduction of losses to stress-related pests.
Stress in trees is induced by a variety of factors and may be chronic (recurrent or long lasting) or acute (sudden and intense). Mineral nutrient imbalances and extended droughts are examples of factors including chronic stress. Untimely and severe freezes, lightning damage, floods, and construction damage are examples of acute stress. Most causes of stress are capable of inducing either chronic or acute stress; the distinctions being those of degree and duration. Table I below provides a summary of some common causes of injury and stress in trees. Examination of the table will reveal that both insects, and organisms that cause disease (pathogens) are included as causes. While we're currently emphasizing that many insects and diseases become problems as a result of stress or injury, it is equally true that insects and diseases are also causes of stress and/or injury.
| TABLE
1. Some Common Causes of Injury or Stress in Trees |
|
Environmental |
Human-Controlled |
Animal |
Plant |
Mineral
nutrient
deficiencies
Frost
or freeze
Sun
scorch
Drought
Lightning
Wind
Hail
Fire
Flood
Saltwater spray or intrusion |
Industrial
wastes
Air
pollution
Soil
compaction
Mechanical
injuries
Construction
damage
Changes
in soil levels
Changes
in soil water
status
Improper
planting
Improper
pruning
Excessive
fertilization
Misuse
of herbicides
Excessive
irrigation |
Nematodes
Insects
Mites
Millipedes
Slugs
Snails
Birds
Rabbits
Mice
Rats
Dogs
Livestock
Squirrels
Deer
Humans |
Viruses
Mycoplasmas
Bacteria
Fungi
Mistletoe
Algae
Moss
Weeds
Strangling vines |
Stress on Trees
 Stress on trees can often be a subtle thing. Tree trunks and branches damaged (stressed) by construction activities are usually obvious (breakage and debarking). Much more difficult to recognize and define are, for example, stresses resulting from soil compaction, changes in soil water drainage patterns, or air pollution. Nonetheless, these factors are just as real, and in many cases result in more damage than their visible counterparts.
A tree under stress often does not exhibit any immediately recognizable external symptoms. Internally, however, a variety of physiological changes are usually taking place. For example, photosynthesis, the process by which the tree manufacturers its food (carbohydrates, etc.) may be upset. The tree begins to starve, rapidly depleting necessary food reserves. Adequate water and nutrients are usually not supplied by root systems of stressed trees and thus fall to dangerously low levels. Growth regulating chemicals (hormones) within the tree become unbalanced. Although stressed trees may go completely unnoticed, the net result of these internal physiological changes is a tree with growth and metabolism operating at less than peak efficiency - a preferred target for many insect and disease pests.
If stress is severe (acute) or prolonged (chronic), the tree eventually exhibits various external symptoms. For example, the tree's growth rate is reduced and stunting may occur. New leaves may be smaller, fewer, and more yellow (chlorotic) in color than normal. Older leaves as well may become discolored, and fall from the tree prematurely. Branch dieback begins, giving the crown of the tree a ragged or staghead (much like deer antlers) appearance. These symptoms are outward expressions of internal, physiological imbalances. The tree has entered a vicious cycle, the STRESS CYCLE (Fig. 2A), and begins a progressive decline, usually terminating in death.
Many insects and disease will not seriously affect a tree until it has reached some point in the stress cycle. Unfortunately, insect pests, as well as certain tree pathogens (especially fungi), are actually better at recognizing trees under stress than most tree enthusiasts. As a result, stressed trees may go completely unnoticed until they are "suddenly" damaged or killed by insects or diseases. Some insects are actually attracted to stressed trees by chemical signals (odors) released into the air by trees in response to stress. Most trees are usually exposed constantly to one degree or another to many ordinarily non-aggressive tree pathogens (especially fungi). It is only when trees are seriously injured or stressed that they lose their natural ability to ward off infections by these organisms and fall victim to the attack. Once established in a stressed tree, insect and disease pests often give the stress cycle another turn full circle. See STRESS CYCLE (Fig. 2B).
A tree under continued stress can only become more stressed. Finally, it dies - in a period of days or several years, depending upon a variety of circumstances - unless the stress cycle is broken. See STRESS CYCLE (Fig. 2C). Whatever the homeowner or forest manager can do to avoid or minimize the causes of stress in trees will aid in the reduction of losses to insects and disease. PREVENTION OF STRESS by avoiding unnecessary injuries to trees during construction, road-building, or timber harvesting operations is probably the single most effective method of controlling stress-related pests. Sometimes, of course, stress cannot be prevented as in the case of untimely hailstorms, freezes or floods.
One thing can be said for sure. In Florida, as well as across the nation, WHAT CAN BE DONE IS ALL TOO OFTEN NOT BEING DONE! Lawnmowers are still banging into trees, as are trucks at construction sites; bulldozers are tearing root systems apart and burying others to the point of suffocation; road and parking lots are still being built over tree roots; roots of trees are still being severed to make way for sewers, television cables, and building foundations; and on and on and on . . . urbanization is taking its toll!
When trees are unknowingly or wantonly treated in such a manner, the end result is inevitable - death and destruction caused by insects and diseases.
In most cases, damage sustained by trees during construction or similar activities is irreparable. It's anybody's guess whether or not damaged trees will succumb or survive. One thing is certain, PREVENTION OF STRESS IS IMPOSSIBLE AFTER THE DAMAGE IS DONE.
BACK Bulletin No. 196-A | Printed October, 1983 | Contact the Forest Health Section |
