Reports on Plant Diseases

Symptoms

Symptoms of decline and dieback are often subtle, slow in developing, and usually uniform throughout the crown. A tree or shrub in the dieback stage, however, may have localized symptoms such as apparently healthy twigs and branches adjacent to dead or dying twigs and branches. Dieback usually begins in the top of a plant and progresses downward, but it may start on the lower branches, especially with conifers.

General symptoms of decline and dieback may include pale green or yellow leaves, delayed spring flush of growth, scorching of the leaf margins, small leaves, reduced twig and stem growth, early leaf drop, premature fall coloration, and, as the disease complex worsens, thinning of foliage in the crown, dieback of twigs and branches, and production of suckers on the branches and trunk (Figure 1).

Leaf scorch, a yellow to brown discoloration of the leaf margins and tip, is commonly a part of the decline and dieback syndrome, however, lack of adequate soil moisture, resulting in less water reaching the leaf tips and margins can also cause scorch. Abnormally large seed crops, sometimes associated with decline, is a normal response to certain weather conditions. In some tree species, heavy seed production occurs normally every few years.
Premature fall coloration, delayed spring flush, decrease in twig growth, and early leaf drop are typical symptoms of maple, oak, ash, honeylocust, birch and sweetgum decline and dieback, and the conditions usually become progressively worse each year with the leaves becoming smaller in size and fewer in number.

Figure 2. Site factors that can lead to tree decline and dieback (Purdue University photo).

Trees and shrubs are long-lived and over a period of years are subject to attack by a variety of insects and diseases (Figure 3), extremely high or low temperatures–especially harmful is a rapid drop in temperature following a period of mild weather in the fall or spring–(Figure 4), great fluctuations in soil moisture during long-term weather cycles, mechanical damage to roots from construction (Figure 5) or livestock, and numerous other environmental effects (Figure 2).

These stress factors alone or in various combinations can reduce leaf and shoot growth (that is, initiate decline) and lead to dieback, although decline and dieback rarely result from a single stress factor. The combination of human impact on the local environment and natural climatic changes provides a multiplying effect that is more serious than any single factor. Usually a tree or shrub is first injured or damaged by disease (Figure 3), insect attack, or adverse soil or air environmental conditions (Figure 2). The damaged or weakened plant is then subject to attack by one or more secondary organisms or agents. For example, trees and shrubs weakened by drought or neglect are more susceptible to attack by borer-type insects and canker diseases than healthy, vigorous plants. Severe defoliation by leaf-eating insects, diseases, herbicides, hail, or wind at critical times of plant development also may initiate decline and dieback. If a defoliated plant develops new leaves late in the year, and if these leaves are, in turn, killed by an early frost, the plant will be low in food reserves and more subject to winter injury. Weakened trees also are subject to invasion by various fungi. Armillaria root rot, for example, commonly attacks and kills the roots of weakened trees. Many other fungi attack the lower trunk and buttress roots of weakened trees.

Of the many stress factors that can initiate the beginning of decline and dieback, those that weaken or damage the root system are perhaps the most threatening. In most instances of decline and dieback the deterioration of the root system or the blockage of normal root functions occurs before any symptoms are visible in the crown. The root system is especially vulnerable to changes in the soil environment. Soil compaction, changes in the soil drainage pattern, excessive soil moisture (from rain or poor drainage) or lack of water (from prolonged drought), the removal or addition of soil over the root system (Figures 1 and 5), soil compaction, and chemical injury from excess deicing salt, pesticide (Figure 7), or fertilizer all can weaken the root system of trees and shrubs. An excess or deficiency of water, in particular, can lead to permanent root damage. Ash, birch, honeylocust, maple, oak, and sweetgum trees are particularly sensitive to an excess or deficiency of water.

Trees and shrubs planted im-properly or in unfavorable locations will also be stressed by poor root growth and development. Planting trees and shrubs too deeply or incorrectly (Figure 6) or in sites with poor drainage, mineral deficiencies or imbalances, a soil reaction (pH) that is too alkaline, poor soil type, or soil compaction should be avoided. Paved sidewalks, driveways, streets, building foundations, patios, septic tanks, and other obstructions can greatly restrict the growing space for proper root development. If a balance between the crown and root system cannot be maintained, the tree or shrub will be weakened, and decline and dieback may develop a few years after transplanting.

Because so many factors can cause decline and dieback, the primary causes are listed below in the approximate order of general frequency:

1. Poor soil structure and drainage (important when the soil is predominantly clay)

2. Herbicide injury to foliage, roots, or other parts (Figures 3 and 7).

3. Poor transplanting procedure and lack of proper maintenance after transplanting (Figure 6).

4. Construction damage–cutting and removal of roots (Figure 5).

5. Significant damage to trunk or major limbs (mechanical injury from lawn mowers, vandalism, vehicles, squirrels and other rodents, livestock, etc.) (Figure 2).

6. Repeated defoliation by insects or diseases, especially such leaf disorders as scorch, anthracnose , rust, and leaf spot or needle blight (Figure 3).

7. An extended drought in combination with high temperatures and strong southerly winds

8. Vascular diseases–such as Verticillium wilt, oak wilt, or Dutch elm disease (Figure 3).

9. Soil nutrient deficiencies

10 Insect borer injury to the trunk or branches.

11. Canker disease–(Figure 3).

12. Excessive soil moisture

13. Extremely low winter temperatures or a rapid change in temperatures (Figure 4).

14. Poorly formed or girdling roots (Figures 2 and 6).

15. Soil compaction from vehicles or heavy construction equipment

16. Fungal root and trunk decays such as Armillaria root rot (Figure 3).

17. Lightning injury

18. Soil fill or removal (Figures 1 and 5).

19. Bacterial wetwood and slime flux

Figure 3. Red maple killed by rapid drop in temp after mild weather in winter.

Identification

The exact cause or causes of decline and dieback needs to be identified so that corrective steps may be taken. Accurate diagnosis is often difficult however, especially on older trees. Usually an on-site examination of the diseased tree is required to assess the influence of the environment and to inspect for foliage, branch, trunk, and root problems. Laboratory examination of diseased leaf, twig, or branch specimens may confirm that an infectious disease problem exists (Figure 3). A careful examination of the roots, trunk, and soil conditions can reveal some basic causes for decline. In some cases, a precise diagnosis can be made only by a combination of field and laboratory examinations. It is very important to consider both the site and the past care given the plant. The following steps, as well as the answers to the questions posed, may help to determine the underlying cause or causes of the decline and dieback.

1. Determine the case history of the plant and general area: Has severe and repeated defoliation by insects, disease, or another cause occurred in recent years? Has severe drought or other adverse weather factors affected the plant in recent years? Has the soil been saturated or flooded for extended periods? Has there been construction work near the tree in recent years causing trunk or root damage, soil compaction, or soil deterioration? Has there been soil or root removal? Has there been soil fill? (If unknown, observe whether the normal trunk flare is visible at the soil line. If not, determine the depth to the buttress roots.) Has the water table in the area changed? The use of a soil profile tube is essential in making many of the observations concerning soil problems.

2. Examine nearby vegetation: Is there evidence of injury to surrounding trees, flowers, shrubs, vegetables, fruits, turfgrass, or weeds that would suggest general environmental (Figure 2) or toxic symptoms? Is the tree or shrub's root system subject to salt accumulation from winter ice control along nearby sidewalks or streets? Is there a toxic sewage disposal field or gas line near the root system that may be leaking?

3. Consider chemical treatments to or near the tree or shrub: What is the history of pesticide use, particularly herbicides or "weed and feed" combinations? Was a soil sterilant or biocide used in a nearby gravel driveway or sidewalk?

4. Examine leaves for foliar diseases and insects.

5. Eliminate the possibility of a vascular disease, that is, oak wilt, Dutch elm disease, Verticillium wilt, or mimosa wilt, by considering the pattern of symptom development and by examining for internal sapwood discoloration.

6. Have a professional arborist determine the year(s) or period(s) of tree stress by examining the amount of twig growth and the width of growth rings in the wood. Also have the arborist check the pattern of annual stem elongation to determine if and when growth has slowed or stopped. (The arborist will examine the growth of annual rings over the last several years with an increment hammer or borer.)

7. Examine branches and trunk for extensive cankers that may be the cause of damage or that may be associated with an environmental or other stress.

8. Examine trunks and buttress roots for evidence of injury, for example, a sunscald, fire, mower, frost crack, or lightning injury. Look for loose bark (tap the bark and exposed roots and listen for a telltale hollow sound). Check for mushrooms or conks of wood and root decay fungi. Fungal fruiting structures are most common in spring and fall following periods of wet weather.

9. Carefully excavate the buttress roots for evidence of fungal decay, poorly formed roots, girdling roots or twine, and similar problems.