Insects and Diseases
Insects and diseases pose two of the most serious threats to your tree's health. As soon as you notice any abnormality in your tree's appearance, you must begin a careful analysis to determine the problem and then select the proper treatment.
The Nature of Tree Health Problems
Stress to trees may be classified into two broad categories: acute and chronic. Acute stress, which can be caused by such things as improper pesticide sprays or untimely frosts or freezes, occurs suddenly and causes almost immediate damage. Chronic stress takes a longer time to affect plant health and may be a result of nutritional imbalance, improper soil pH, incorrect light intensity or other factors. To make matters worse, disease-causing organisms and insects commonly attack an already stressed tree, adding further damage to the existing symptoms. Provided that symptoms are recognized early, there is usually enough time to correct imbalances caused by chronic stresses, making them easier to deal with than acute stresses. Often by the time an acute stress is apparent, the damage has already been done.
Plant Diseases
Whether on trees, shrubs or other plants, plant diseases require three things to develop:
These three factors are subject to a great deal of variation, depending upon the particular plant and pathogen involved. For example, genetic variability causes plants to respond differently to the pathogen, just as it controls height variations, autumn color and susceptibility to insect attack. Weather and soil conditions also have different effects on different plants.
Diseases can be classified into two broad categories: 1) those caused by infectious or living agents and 2) those caused by non-infectious or non-living agents.
Examples of infectious agents include fungi, viruses and bacteria. Non-infectious diseases, which account for 70 to 90 percent of all plant problems in urban areas, can be caused by nutrient deficiencies, temperature extremes, pollutants and fluctuations in moisture. Factors of non-infectious diseases often produce symptoms similar to those caused by infectious diseases; therefore, it is essential to distinguish between the two in order to give proper treatment.
Correct diagnosis of plant disease requires careful examination of the situation and systematic elimination of possibilities by following a few important steps.
1. Accurately identify the plant. Because infectious pathogens are mostly plant-specific, this information can quickly limit the number of suspected causes.
2. Look for a pattern of abnormality. This can often provide key information regarding the cause of the problem. For example, if the affected plants are restricted to a walkway, road or fence, the disorder could be a result of wood preservatives, de-icing salts or other harsh chemicals.
3. Carefully examine the land. Factors to observe include land
contour, history of the property, number of species affected, and percentage of injured
plants in the area. Land contour can be a significant contributor to plant disease because
low or poorly drained areas are ideal for the development of root rot induced by a variety
of water molds. The history of the property and adjacent land may reveal many problems
such as herbicides applied to agricultural lands or sanitary landfills whose gas can drift
several hundred feet and damage plants.
The number of species affected may also help to distinguish between infectious pathogens that are more plant-specific as compared to chemical or environmental factors that affect many different species. Most living pathogens take a relatively long time to spread throughout an area, so if a large percentage of plants become diseased virtually overnight, a pathogen is probably not involved.
4. Examine the roots. Note their color-brown or black roots may signal problems. Brown roots often indicate dry soil conditions or the presence of toxic chemicals. Black roots usually reflect overly wet soil (lack of proper drainage) or the presence of root-rotting organisms.
5. Check the trunk and branches. Examine the trunk thoroughly for wounds, as they provide entrances for cankers and wood-rotting organisms. Such wounds maybe caused by weather, fire, lawn mowers, rodents, and a variety of other environmental and mechanical factors. Remove branch stubs and prune out cankered limbs.
6. Note the Position and appearance of affected leaves. Dead leaves at the top of the tree are usually the result of environmental or mechanical root stress. Twisted or curled leaves may indicate viral infection, insect feeding or exposure to herbicides. The size and color of the foliage (leaves) also may tell a great deal about the plant's condition.
7. Think about current and past management practices. Sometimes a plant's current problem is actually a result of something that happened to it much earlier. Changes in grade, the use of pesticides or nearby construction, work may all contribute to tree problems.
Insect Problems
Insects can cause considerable damage to trees and shrubs. It is important to remember, however, that some insects are beneficial rather than destructive; they may help with pollination or act as predators of more harmful species.
Insects may be divided into three categories according to their method of feeding: chewing, sucking and boring. Insects from each-group have characteristic patterns of damage that will help you determine the culprit and the proper, way to get rid of it.
Chewing Insects eat plant tissue such as leaves, flowers, buds and
twigs. Indications of damage by these insects are uneven or broken margins on the leaves
or other affected plant parts. A few examples from this large insect category are beetles
and their larvae (grubs), webworms, bagworms, and larvae of moths, and sawflies
(caterpillars).
Because these insects ingest plant material, they may be killed by spraying plants with as to mach poison either during active feeding or before insects appear. Such poisons normally take effect 24 to 48 hours after application. These chemicals are also typically poisonous to humans, which means that they must never be used when any possibility of children chewing or eating sprayed leaves or fruits exists.
Sucking Insects Insert their mouth parts into the tissues of leaves,
twigs, branches, flowers or fruit and then suck out the plant's juices. Some typical
examples of sucking insects are aphids, mealy bugs, thrips and leafhoppers. Damage caused
by these pests is often indicated by discoloration, drooping, wilting and general lack of
vigor in the affected plant.
Because sucking insects do not eat the outer surfaces of plants, they are not affected by stomach poisons. Contact poisons, which directly hit an insect's body and kill by burning, asphyxiation or paralysis, must be used. These poisons must be applied when insects are present.
Boring insects are characterized by the tunnels they make in the wood just under the bark of a tree as they eat through it. Because each kind has its own style and tunnel pattern, borers may be Identified by their work even after they have left the scene. One example of borers is the termite (usually not serious in trees).
Trees infested with borer, typically display a thinness of crown and a gradual or sudden decline In vigor. Conclusive symptoms are circular holes in the trunk or branches with frass (semi-digested wood which borers produce) and sometimes dripping sap, which forms a dark stain along the bark. Borer holes are deep, Irregularly located and usually made at an angle, indicating tunnels underneath. Depending on what part of the tree borers eat, they can kill the tree.
When present, borers may be directly attacked. When their tunnels are open, they may be impaled on a piece of wire. If they cannot be reached mechanically, they may be asphyxiated by poisonous gas or liquid.