Abiotic Diseases of Woody Ornamentals


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Janette L. Jacobs
Assistant Extension Plant Pathologist

Michael A. Schnelle
Extension Horticulture Specialist

Trees and shrubs, whether in a lawn or along a street, are subject to stresses that plants in a native environment seldom encounter. Trees and shrubs in a landscape setting are often planted in disturbed soils, may not be adapted to the local climate, or may be exposed to poor cultural management practices. Plants grow best within certain ranges of the various factors that make up their environment. Such factors include temperature, soil moisture, soil nutrients, light, air and soil pollutants, relative humidity, soil structure, and soil pH. Although these factors affect all plants growing in nature, their importance is considerably greater for cultivated trees and shrubs which may be grown in areas marginally meeting the requirements for normal growth. Moreover, cultivated plants are frequently subjected to a number of cultural practices (fertilization, irrigation, spraying with pesticides, or pruning) which may adversely affect their growth.

The stresses associated with growing woody ornamentals under less than ideal conditions can result in abiotic diseases. Abiotic diseases are noninfectious diseases—they occur in the absence of pathogens and cannot be transmitted from diseased to healthy plants. Abiotic diseases are responsible for more than 85 percent of the plant disorders found in the landscape. Noninfectious diseases may affect plants in all stages of their lives, such as seed, seedling, mature plant, or fruit. The symptoms ca used by abiotic diseases vary in kind and severity with the particular environmental or cultural factor involved and with the degree of deviation of this factor from its norm. Symptoms may range from slight to severe, and affected plants may even die. The common characteristic of abiotic diseases is that they are caused by the lack or excess of a factor which supports life.

In general, abiotic diseases which occur on woody ornamentals usually do not arise from a single factor. Adverse environmental conditions and cultural practices can cause abiotic disease or predispose plants to attack by disease pathogens and insects. The number of environmental conditions and cultural practices that can cause abiotic diseases is almost unlimited, but most of them affect plants by interfering with normal physiological processes. Such interference may be the result of an excess of a toxic substance in soil or air, a lack of an essential substance (water, oxygen, or mineral elements), or a result of an extreme in the conditions supporting plant life (temperature, humidity, oxygen, or light). Some of these effects may be the result of normal conditions (for example, low temperatures) striking at an unseasonable time or abnormal conditions occurring naturally (flooding or drought). Also, the activities of people and their machines (heavy equipment, lawn mowers, and weedeaters) may lead to abiotic diseases.

The diagnosis of abiotic diseases is sometimes made easy by the presence of characteristic symptoms known to be caused by the lack or excess of a particular factor. At other times, diagnosis can be arrived at by carefully examining and analyzing: 1) the weather conditions prevailing before and during the appearance of the disease symptoms; 2) recent changes in atmospheric and soil contaminants at or near the area where the plants are growing; and 3) the cultural practices preceding the appearance of the disease symptoms. Often, however, the symptoms of abiotic diseases are too vague or nonspecific and closely resemble those caused by disease pathogens. The diagnosis of such noninfectious diseases then becomes a great deal more complicated. Unless the history of the environmental conditions and cultural practices is known, it becomes difficult to accurately diagnose the cause. Table 3-1 gives examples of nonspecific disease symptoms on woody ornamentals and their possible causes.

Abiotic plant diseases can be controlled by ensuring plants are not exposed to the extreme environmental conditions and cultural practices responsible for such disease or by supplying the plants with protection or substances that would bring these factors to levels favorable for plant growth.

This chapter describes some of the more common abiotic disease problems found on woody plants and suggests remedies to correct, minimize, or prevent these problems.

Nutrient Deficiency

Nutrients are essential for healthy plant growth. The growth of healthy plants is normally limited by the supply or availability of one or more nutrients. All nutrients except carbon, hydrogen, and oxygen are normally supplied by soil. Woody ornamentals often grow in soil which has been disturbed by construction. The remaining soil may not contain the proper nutrients necessary for plant growth, or the pH of the soil may not allow nutrients to be taken up by the plants. Likewise, leaves are removed from the soil each year so the tree or shrub has little chance to improve the soil's condition. The tree or shrub may look fine for years and then suddenly show the effects of a nutrient deficiency. A growth-limiting nutrient supply is considered to be deficient if the limitation causes abnormally slow growth, deformity, depressed yield, chlorosis (yellowing), or necrosis (dead tissue). In Oklahoma, when plants become chlorotic in the landscape, the soil pH is usually not favorable rather than an actual nutrient deficiency occurring. The most common problem is soil with a high pH (too alkaline).

Nutritional deficiencies incite varied symptoms and, in most cases, cannot be diagnosed reliably on the basis of symptoms alone. Different deficiencies may cause similar symptoms, and symptoms are often complicated by simultaneous deficiencies of two or more nutrients. Also, many symptoms of nutrient deficiency mimic those caused by other factors, such as heat, drought, waterlogged soil, chemical injury, or damage by insects or pathogens. A chemical analysis of the soil is needed for accurate diagnosis in most situations, and a positive plant response to application of the limiting nutrient(s) is often necessary to complete the diagnosis.

Since, highly alkaline soils (pH 7.6 to 8.5) are quite common in Oklahoma, micronutrient deficiencies are a problem. Normally, iron and zinc are the most common micronutrient deficiencies, but manganese deficiency may occasionally occur. Also, keep in mind that severe nitrogen or other macronutrient deficiencies can occur as well.

Iron Deficiency

Iron is most readily available to plants from soils with a pH of 6.0 or less because iron is bound in insoluble forms in alkaline or calcareous soils (soil high in lime). Iron is relatively immobile within plants; thus, symptoms of deficiency tend to appear first on young shoots that cannot obtain sufficient iron from older leaves. Iron deficiency causes interveinal chlorosis and slow growth that is at first irregular from branch to branch within a tree and is variable among trees of the same species in a given locale. Affected leaves retain chlorophyll along the veins; tend to be undersized; and, in severe cases, develop irregular browning of leaf margins. Branches with severe foliar symptoms eventually die back.

Figure 3-1
Iron deficiency symptoms on pin oak
Pin oak (Quercus palustris) are notoriously prone to iron chlorosis resulting from iron deficiency (Figure 3-1). This proneness varies within pin oaks. Studies in Oklahoma have shown that often the iron nutrient lacking in a tree is present in the soil, but is not readily available to the tree because of a high soil pH. Soil amendments such as elemental sulfur or iron sulfate to lower the pH have given promising results, but are slow-acting. Improvement has also been found with the application of foliar iron nutrients. However, when the leaves fall, most of the iron fertilizer falls too, and next year the application needs to be repeated. Therefore, foliar applications are temporary fixes and are considered cosmetic. Trunk implantation of iron salts or chelated iron capsules has also given good results in some cases. However, the relief sought must outweigh the permanent damage that occurs as a result of trunk injections. Also, the tree may wall-off the foreign substance, thus not allowing the iron to translocate. Adjusting the soil pH is the proper approach to treatment. Keep in mind, however, that at least the first growing season after soil treatment, another treatment such as a foliar spray will be necessary for rapid response.

Zinc Deficiency

Zinc is readily available to plants from soils at pH 6.0 or less. Zinc deficiency causes interveinal chlorosis or mottling, dwarfed leaves, crinkled leaf margins, and stunted or rosetted shoots. Symptoms tend to be variable within the plant and between adjacent plants. In Oklahoma, pecan trees tend to be prone to zinc deficiency. Zinc deficiency can be corrected by injection of a zinc sulfate solution or, in some cases, by simply making foliar applications of zinc sulfate.

As with most plant problems, proper care and maintenance of a plant throughout its life will lessen the likelihood of nutrient problems. When planting in poor soil, use woody plants tolerant of a wider variety of soil conditions and pH range. Your local nursery worker can help you make a proper choice of trouble-free trees and shrubs.

Leaf Scorch

Figure 3-2
Leaf scorch symptoms on oak
Often in mid- to late summer, tree or shrub leaves show a browning or drying at the outer margin or in the areas between the veins of the leaf (Figure 3-2).

The areas near the veins generally remain green. However, in extreme cases, the entire leaf may dry and fall prematurely. Scorch itself will not kill a plant, but it may weaken it to the point where insects or disease pathogens can further injure it. Scorch symptoms develop on leaves as a result of rapid water loss by evaporation (transpiration) which exceeds the rate of water absorption by the roots from soil. This can be caused by too little water in the soil or a physical restriction of the roots. Usually, scorch symptoms appear during hot, dry, windy weather. Newly transplanted trees, trees planted in poorly selected sites, trees growing along streets or sidewalks (reflective heat), and trees growing in areas where the roots are restricted or underdeveloped seem to suffer the most.

Trees and shrubs most likely to develop scorch in Oklahoma include azalea; river birch; dogwood; Japanese, Norway, red, and sugar maples; oak; ornamental pear; redbud; rhododendron; and sweetgum.

Leaf scorch is best controlled by deep watering during dry periods. If scorch is severe early in the season, a judicious pruning may help increase the supply of water to the remaining leaves and also make a cosmetic improvement for that growing season. Planting trees that are adapted to Oklahoma's climate will best help prevent leaf scorch.

Drought

Native plants in a given area are adapted to seasonal and annual variations in water supply characteristic of the local climate. Only severe drought is likely to cause noticeable injury to plants that have grown naturally on a given site. Nonadapted trees and shrubs, on the other hand, often show symptoms of severe water stress. Perhaps more importantly, a water deficiency predisposes plants to infection by pathogens, attack by insects, and injury by severe summer and winter weather extremes.

Drought damage develops in plants when the transpiration rate exceeds the rate of water absorption by the roots, as it does almost daily during the growing season. The water deficiency is normally made up at night or during periods of rain or dew formation when transpiration slows or ceases. As soil dries, however, roots fail to absorb as much water as has been lost and physiological stress develops. If this condition intensifies sufficiently, leaves lose turgor, wilt, turn yellow, and die.

Drought damage also occurs in dormant plants, especially narrow- and broad-leaved evergreens, during warm weather in winter or early spring when water evaporates from leaves and stems while the soil is cold or frozen. Roots absorb insufficient to no water from frozen soil (see the Winter Burn of Evergreens section).

Plants vary in the ability to tolerate drought. Recently transplanted plants are at greatest risk of drought damage. Trees or shrubs recently transplanted have lost many absorbing roots, so the creation of an abnormal water deficiency is unavoidable. If the root ball contains a highly porous growing medium instead of soil, a water shortage could occur even though the surrounding soil contains water sufficient for plant growth. This problem continues until roots grow beyond the root ball. Refer to the Cultural Management for Woody Plants chapter for an overview of preventive planting techniques.

Leaves of drought-stressed plants lose turgor and may droop, wilt, turn yellow, turn brown at the tips and margins, curl, or show all of these symptoms. When green leaves wilt and turn brown, the oldest leaves usually succumb first. Severely stressed deciduous species may drop all their leaves. Similarly, the oldest needles (nearest the trunk) on pine trees may turn yellow and drop. Drought symptoms on pine trees should not be confused with natural autumn shedding of older needles.

Drought damage can be prevented by deep watering during dry periods throughout the year, including winter months when temperatures remain above freezing for prolonged periods. Select plant species that tolerate drought conditions. The following plants are drought tolerant: chittamwood, lacebark elm, goldenrain tree, hackberry, fruitless mulberry, bur oak, osage orange, Bradford pear, Chinese pistache, smoketree, soapberry, and desert willow.

Winter Burn of Evergreens

Winter burn is common in Oklahoma to such plants as azalea, boxwood, holly, magnolia, rhododendron, and viburnums, but it can affect narrow-leaved evergreens and pines and deciduous species as well. Winter burn is often misdiagnosed as an infectious disease or damage from excessively cold temperatures. Winter burn is caused from desiccation, which is a type of dehydration injury. When roots are in dry or frozen soil, water lost through transpiration cannot be replenished by the roots and dehydration occurs. Water loss through transpiration is normally low during winter months, but it increases when plants are subjected to drying winds or are growing in warm sunny spots.

Symptoms of winter burn include scorching of leaf tips or outer leaf margins, complete browning of needles or browning from the needle tips downward, or death of terminal buds and/ or twigs.

Several means of eliminating or minimizing winter burn may be used. Avoid planting broad-leaved evergreens in areas of high wind exposure. Deep water plants during dry periods throughout winter months when temperatures remain above freezing for prolonged periods. Erect physical windbreaks. Burlap "walls" can help cut down wind and subsequent moisture loss to evergreen shrubs and small trees. Antitranspirants of various types are available, but have shown limited success un der Oklahoma's climatic conditions.

Injury by Freeze or Frost

Woody plants undergo seasonal changes in their ability to tolerate low temperature. In autumn, perennial plants become acclimated to withstand low temperature. The degree of cold acclimation varies during winter in relation to ambient temperature. The hardiest trees or shrubs, fully acclimated, can withstand temperatures much lower than ever occur where they grow. The majority of woody plants of the temperate zone become acclimated to withstand minimum temperatures between -4°and -40°F. As temperatures rise in late winter and early spring, plants deacclimate until, by the time growth begins, they can no longer tolerate more than a few degrees below freezing.

Southwest Injury

Southwest injury is a local injury that develops on the south and southwest sides of the trunk or on the upper surfaces of limbs exposed to sun. The temperature of the sun-warmed sides of limbs or trunks may exceed 68°F in late winter when the air temperature and the temperature of shaded bark barely exceed 32°F. This heating causes deacclimation, which is followed by lethal freezing when the temperature drops at night. Damaged bark and cambium dry out, crack, separate from the wood, and eventually fall away, exposing dead sapwood. Young trees with thin, smooth bark are most susceptible to this type of injury. Southwest injury has been diagnosed on trunks of flowering cherry, maple, callery pear, weeping willow, and many kinds of fruit trees and on limbs of ginkgo, red oak, and Japanese pagoda tree.

Figure 3-3
Winter freeze injury on Bradford pear
Winter Freeze Injury

Injury to plants by freezing during dormancy occurs when unusually warm weather in autumn retards acclimation, and/ or warm weather in winter or early spring induce partial deacclimation. Most damage by freezing during winter follows untimely deacclimation during temporary warm weather. Often after a period of unusual warmth, the temperature drops rapidly to a level normal or subnormal for the season and severe freeze damage can occur to plants that are marginally hardy in a given region.

The most common external symptoms caused by winter freezing are dieback, foliar browning, sunscald, and bark splitting on branches or the trunk (Figure 3-3). Dieback of twigs and branches, and foliar browning in evergreens, commonly follow freeze injury when winter temperatures arrive suddenly after warm autumn weather (Figure 3-4). Plants which survive may produce new branch systems from dormant or adventitious buds. Major freeze injury that kills the sapwood or cambium leads to cankers, dieback, and often wilting and death during the next growing season, because water lost by transpiration cannot be sufficiently replaced by conduction through damaged wood.

Figure 3-4
Winter freeze injury on pine
Sapwood exposed by bark splitting or branch dieback can soon be invaded by decay fungi. In trees prone to bacterial wetwood, elms for example, this condition becomes more widespread within trees after damage by freezing. Also, freezing insufficient to cause evident stress in plants may interfere with defense against opportunistic fungal pathogens that cause cankers, dieback, sapwood decay, or root rot. Some of the common diseases for which predisposition is important include: Cytospora canker on poplar, willow, and cottonwood; dieback of sycamore and oak caused by Botryosphaeria; and Sphaeropsis (formerly Diplodia) tip blight of pine.

Secondary insects often join opportunistic fungi in the attack on stressed trees. Stressed conifers, for example, can be attacked by bark beetles and blue-stain fungi. The two-lined chestnut borer and Hypoxylon fungal species collaborate in causing death of stressed oaks.

Frost Injury

Frost injury during the growing season is possible because plants lack any cold acclimation at that time. Leaves and stems damaged or killed by spring frost are usually small and succulent at the time of injury. Damaged leaves often have jagged open spaces which are similar to feeding holes by chewing insects. Leaves that have been killed first appear water-soaked and soon become shriveled and reddish brown to dark brown or nearly black, depending on the species. Dead leaves or shoots break off or abscise during the ensuing several weeks. New shoots and leaves begin to grow from dormant or adventitious buds almost immediately and soon mask the early season damage.

Figure 3-5
Hail damage
Hail Damage

In Oklahoma, injuries to woody plants by hail commonly occur. Hail stones lacerate leaves, defoliate branches, remove twigs, bruise or break the bark of twigs and small branches, and kill small trees. Bruises and wounds tend to be elliptically shaped and vary in length from a few millimeters to 10 cm or more (Figure 3-5). By uniting, severe hail wounds may kill all the bark on one side of a stem. All occur on the upper sides of branches and on the side of the tree facing the storm. Bruised bark can crack after the storm as a result of drying and mechanical stress from the callus growth at edges of the injured area.

Figure 3-6
Ice damage
Ice Damage

Ice is most destructive when a heavy glaze forms on plants during freezing rain. The weight of a glaze more than about 1 cm thick breaks twigs, branches, and trunks or can even uproot trees (Figure 3-6). The amount of damage increases if the wind rises before the ice melts.

Lightning Injury

Lightning is most important as a cause of forest fires, but it also causes significant damage to landscape trees in the absence of fire. Struck trees often have a strip of bark and sapwood blown off the trunk, leaving a continuous or intermittent rough groove which follows the grain of wood (Figure 3-7). Trees that are struck but not killed are likely to be disfigured by death of limbs. In addition, wounds and destroyed parts provide entry for borers and fungi that decay wood. Conifers weakened by lightning strikes may be attacked and killed by bark beetles.
Figure 3-7
Lightning damage

Lightning rods are sometimes installed on large valuable trees. Copper cables are lead down the trunk and through the soil to grounding rods driven into soil beyond the branch spread of the tree. However, the degree of protection from such rods is questionable.

Damage by Air Pollutants

Plant disorders caused by air pollutants vary with concentration of pollutant, duration of exposure, natural sensitivity of the plants, and environmental conditions affecting the plants before and during exposure. Visible injuries by air pollutants are classed as acute or chronic. Acute injury, associated with brief episodes of greater than normal air pollution, results from rapid absorption of enough toxicant to kill portions of leaves, causing characteristic markings or symptoms. Chronic injury results from repeated absorption of a pollutant in amounts that do not kill tissues, but are sufficient to cause cumulative physiological disturbance. Symptoms include chlorosis (yellowing) and premature foliar senescence (death), reduced growth, and, in some cases, progressive decline in plant health.

The major classes of phytotoxic air pollutants, in descending order of direct damage caused, are oxidants (ozone, oxides of nitrogen, and peroxyacl nitrates), sulfur dioxide, and fluorides (hydrogen fluoride and silicon tetrafluoride). Ammonia, chlorine, and hydrogen chloride cause occasional damage to plants. Fortunately, air pollutants are rarely found in sufficient concentrations in Oklahoma to inflict significant damage on landscape plants.

Ozone

Ozone probably causes more injury to plants than any other air pollutant in the United States. Ozone commonly builds up to phytotoxic levels in the atmospheres of urban areas during warm, sunny weather. It may cause damage to plants far from the source of its precursors as masses of polluted air move overland. Many species and varieties of plants are sensitive to ozone.

Response of plants to ozone is dependent on various environmental factors. The stage of plant growth, nutrition, light, relative humidity, temperature, and various otherfactors can determine the response to a given ozone dosage. External symptoms from ozone damage include suppressed plant growth; formation of small dotlike, pigmented (purple or red) lesions on leaves; bleaching of leaf surfaces; leaf necrosis (death of living tissue); and leaf chlorosis. Various disorders may produce symptoms similar to those produced by ozone. Certain sucking insects, such as leaf hoppers or mites, produce injuries which could be confused with ozone injury. Certain viral diseases or damage from herbicide produce patterns on leaves which are similar to ozone injury. Also, moisture stress caused by hot, dry wind can cause necrosis which may resemble ozone injury.

The following plants are tolerant to ozone: alder; eastern arborvitae; green and white ash; river birch; bald cypress; flowering dogwood; hybrid elms; littleleaf linden; black locust; honey locust; mimosa; bur, northern red, and Shumard oaks; Japanese pagoda tree; peach; callery pear; Japanese black and red pines; spirea; viburnum; and yew.

Sulfur Dioxide

Sulfur dioxide is emitted to the atmosphere during the combustion of many fuels, especially coal and petroleum. Damage to plants by sulfur dioxide is usually found in areas adjacent or close to the source.

Acute foliar injury by sulfur dioxide is indicated by bleached or pigmented (tan to reddish brown or dark brown, depending on species) necrotic interveinal areas on broad-leaved plants and chlorotic spots and bands or brown tips on needles of conifers. Chronic injury causes chlorosis, premature senescence (death) of leaves, and suppressed growth.

Different species of plants vary widely in their sensitivity to sulfur dioxide. The following plants are tolerant to sulfur dioxide: arborvitae; common boxwood; eastern red cedar; bald cypress; flowering dogwood; ginkgo; Oregon grape; English ivy; juniper; Amur and silver maples; live, northern red, and white oaks; Japanese pagoda tree; Austrian and mugo pines; London plane; spirea; and yew.

Fluorides

Fluoride pollutants affect the health of plants near sources of emission. Most fluoride pollutants are released during the manufacture of various products for which raw materials are mined from earth. Plant injury by fluorides is most common and severe near sites where phosphate fertilizer and aluminum are produced because these processes employ fluoride-rich materials.

Acute foliar symptoms by fluoride are expressed by interveinal necrosis on broad leaves or tip burn of conifer needles. Plant tissues that accumulate an injurious amount first turn yellow, then some shade of tan to brown or reddish brown, often with a narrow, darker band at the boundary of living tissue. Slow accumulation of fluoride over days or weeks leads to chronic symptoms of chlorosis at leaf tips and margins. This is the more common situation because pollution control devices and tall smoke stacks have greatly diminished the atmospheric fluoride concentration at ground level near many pollution sources.

Plant species and individual plants within species vary widely in tolerance to fluoride. The following woody plants are tolerant to fluoride pollutants: alder, arborvitae, basswood, dogwood, American and Chinese elms, firethorn (pyracantha), sweet gum, juniper, oak, Russian olive, common pear, London plane, flowering plum, sycamore, tree-of-heaven, and willow.

Damage by Misapplied Pesticides

Those who apply pesticides to plants or soil in which plants grow rely on the principle of selective toxicity. The pesticide is intended to suppress or kill specific plants or plant pests, while causing little or no damage to nontarget plants or pests. Environmental contamination and injury to nontarget plants can occur occasionally when all normal precautions are taken. However, such contamination and injury are more common when pesticides are mishandled or applied under improper conditions.

Herbicide Injury

Herbicides are normally separated into two general categories in regards to their intended purpose—selective and nonselective herbicides. Selective herbicides are used to kill established broad-leaved weeds in turf or landscape plantings. Well-known herbicides in this group include the phenoxyacetic acids (2,4-D; MCPA; MCPP; and related compounds) and the benzoic acid derivative dicamba. These herbicides act as plant hormones that disrupt normal growth processes. Selective herbicides have the potential to become nonselective when applied at rates higher than specified or under improper environmental conditions or to plants not specified on product labels. Herbicides in this group may reach and enter nontarget plants by as many as three modes. The most common is absorption of herbicide from droplets of spray that drift away from the site of application. A second mode is absorption of herbicide in the gaseous phase from drifting vapor after the evaporation of spray droplets or of liquid on sprayed surfaces. This occurs with highly volatile herbicides in warm weather. The third mode is through soil, where the herbicide is absorbed by roots. The phenoxy herbicide 2,4-D is the best known example of a chemical that may teach nontarget plants by any or all three modes. Dicamba is more likely to cause injury as the result of uptake by roots.

The second group of herbicides includes chemicals intended to prevent emergence of weed seedlings (preemergence herbicides) or to kill all vegetation to which they are applied (soil sterilants). Some of these herbicides persist in soil for a year or more. Simazine is a much used preemergence herbicide which sometimes causes unwanted residual effects after application to landscape soils. Herbicides such as glyphosate will kill all vegetation, but do not persist in the soil. The use of herbicides with short-term residual is ideal in the landscape, since plants may be safely introduced (planted) into a treated area.

Figure 3-8
Herbicide injury on Bradford pear
Soil sterilants sometimes cause trouble for nontarget trees or shrubs when either sprayed on target plants or applied to areas adjacent to landscapes. These herbicides are commonly used to control vegetation along highways, railroads, fences, power lines, and similar places. Never use such herbicides (soil sterilants) in a landscape setting.

Symptoms of injury by herbicides vary with the type of chemical and are not diagnostic, except for the atypical growth caused by hormone-type herbicides. Symptoms similar to those caused by herbicides can be caused by some diseases, insects, insufficient or excess water or heat, deficiencies of certain nutrients, and other kinds of misapplied pesticides.

Hormone-type herbicides, especially dicamba, are taken up by tree or shrub roots growing in the area of the treated grass. Dicamba can cause drastic growth suppression, leaf cupping, bending and sometimes coiling of shoot tips, yellowing of new growth, bud failure, browning or blackening of foliage, defoliation, and sometimes death (Figure 3-8). Hormone-type herbicides are translocated to growing points and cause multiple deformities in new leaves and shoots (Figure 3-9). Symptoms develop several days to several weeks after exposure or may appear in the spring following an autumn exposure. Nontarget plants usually receive sublethal doses and outgrow the symptoms within one to two years. Symptoms include cupped leaves; abnormally prominent veins; wavy, frilled, or curled leaf margins; tough or leathery leaves; partial failure of chlorophyll development; delayed bud break in the spring; and abnormal purple coloration of normally green stems.
Figure 3-9
Phenoxy herbicide injury on pine

Preemergence herbicides and nonselective soil sterilants, tend to halt growth and cause chlorosis (yellowing) of new and old leaves. If the dose is sufficient, they cause foliar browning, leaf cast, and dieback of twigs and branches. The herbicides simazine and atrazine cause marginal and interveinal chlorosis (yellowing) of broad-leaved plants starting at needle bases of conifers. Trees or shrubs severely injured by these herbicides are less likely to recover than are plants injured by hormone-type herbicides.

Figure 3-10
Tree damage caused by soil sterilant
Herbicides generally cause damage on plants treated or inadvertently exposed during growth in warm weather. Water shortage or heat stress may enhance toxic effects. Dormant plants or those in a resting phase are less sensitive. Herbicides are commonly applied to sites that contain ornamental trees and shrubs. Careful attention should be given to the prevention of drift to nontarget plants. There are many herbicides that should never be applied under the dripline of established trees or shrubs. Avoid using soil sterilants around ornamental plantings. Never apply long-lasting soil sterilants under new pavement or sidewalks if trees or shrubs are growing or to be planted nearby. Tree roots from nearby species will eventually grow into the site and translocate the herbicide throughout the plant. These trees will be severely injured or killed as a direct result of the soil sterilant applied (Figure 3-10). Purchase herbicides from nurseries or garden centers which are staffed by qualified individuals who can recommend the appropriate chemical control when necessary. To avoid drift, apply herbicides on calm days. When possible, use a coarse droplet spray applied at low pressure. Strive to use herbicides early in the morning when winds are likely to be calm. Excessive temperatures can also be avoided in the morning hours. Intense heat can cause phenoxy-type herbicides to volatilize and injure nearby nontarget plants. Also, avoid applying herbicides at their highest labeled rates in soils low in organic matter. Less herbicide is required for proper weed control in soils low in organic matter.

Phenoxy herbicides and dicamba are the weed killers that most commonly injure nontarget trees and shrubs in horticultural landscapes. The following plants are sensitive to phenoxy herbicides: apple, birch, cottonwood, dogwood, Siberian elm, forsythia, grapevine, hackberry, honey locust, Amur maple, mimosa, London plane, redbud, rose, serviceberry, sumac, sycamore, tree-of-heaven, tulip tree, walnut, willow, wisteria, and yellowwood.

The following plants are sensitive to dicamba: apple, eastern arborvitae, barberry, basswood, birch, catalpa, cottonwood, sweet gum, hackberry, hawthorn, juniper, lilac, black locust, honey locust, pin and white oaks, shortleaf pine, redbud, serviceberry, sycamore, tree-of-heaven, tulip tree, walnut, and yew.

Insecticide or Fungicide Injury

Plant injury by insecticides and fungicides is infrequent compared to that by herbicides. The active ingredients in modern insecticides and fungicides seldom cause visible symptoms unless deliberately applied at rates higher than specified or under improper environmental conditions or to plants not specified on product labels. The likelihood of pesticide injury is greatest for plants stressed by heat (temperatures above 86° to 90°F) or water shortage, or for plants sprayed under poor drying conditions such that spray liquid remains on plant surfaces. Plants sensitive to particular pesticides are usually identified on the labels.

Symptoms of plant injury by insecticides and fungicides are not usually diagnostic, and therefore cannot reliably be interpreted unless the history and treatment of the plant are known. Visible symptoms include yellow to brown leaf spots; chlorosis of the leaf tips, margins, or interveinal areas; general chlorosis; browning of leaf margins or interveinal area; stunted shoots; and abnormal crinkling or curling of leaves. Foliar yellowing or browning is often followed by premature leaf drop.

Inorganic pesticides such as lime-sulfur, and copper fungicides, including Bordeaux mixture, can cause injury to many kinds of plants if not applied at the proper rate and under the right environmental conditions. To prevent plant injury by pesticides, always read and follow the label directions. Choose correct pesticides which least affect the plants in the surrounding environment.

Natural Gas Injury

In Oklahoma, natural gas is commonly transported through underground pipelines which often run near the root systems of landscape plants. When a leak occurs, natural gas gradually fills up the air spaces in the surrounding soil, displacing oxygen as it does so. The displacement of oxygen leads to the development of anaerobic (absence of free oxygen) soil conditions and to the eventual production of hydrogen sulfide by bacteria in the soil. Hydrogen sulfide disrupts root respirati on and nutrient absorption leading to the death of plants.

Symptoms of natural gas injury are similar to symptoms caused by other disorders—be certain that there is a gas line in the vicinity. Look for a linear pattern of declining or dying plants of all types in the vicinity of the suspected leak. Symptoms range from mildly affected plants to those quickly killed, depending upon the severity of the leak and the sensitivity of the plant species. Soil in the suspected area may be darkened and smell sour. Also, if all the roots in the area are dead or unhealthy, this too is a good indication.

Once the leak has been located, the natural gas line should be repaired. Aeration of the affected soil replaces oxygen. Prune out the dead or dying parts of plants that are still alive. Plants could fully recover once the gas leak is corrected.

Grade Changes and Construction/ Oxygen Deficiency

Cultivation

Many shrubs and tree species decline from chronic movement or cultivation of even the top few inches of soil. The majority of roots are in the top few inches of the soil. Therefore, this should be kept in mind when annual flowers or spring flowering bulbs are desired around or underneath established woody ornamentals. Also, take into consideration that roots grow beyond the dripline of a tree.

Spring flowering bulbs should be planted at the time when the tree or shrub is planted or insert bulbs carefully when planting in later years. Always plant winter-hardy bulbs that can remain in place year-round. This will minimize disruption of the plant's root system. After planting annuals or bulbs, mulch to keep weeds in check which otherwise will have to be hoed, resulting in disruption of tree or shrub roots. If not mulched, cultivate lightly, just to keep the weeds out, or hand pull weeds when practical. One of the best solutions is to place a perennial ground cover which, once established, will choke out potential weeds. However, additional irrigation and fertilizer may be needed for young trees or shrubs with competition from the ground covers.

Soil Grade Change
Figure 3-11
Excessive fill

Anytime the soil grade is lowered or raised even a few inches. existina shrubs and trees become predisposed to various stresses. Raising the grade is particularly damaging to woody plants. Normally, the plant is forced to grow a new root system up higher where oxygen relations are best. Unfortunately, these adventitious root systems are not always formed quickly enough and the tree or shrub dies. Common causes of tree and shrub decline from grade changes are construction and raised flower beds around established trees or shrubs (Figure 3-11).

Make necessary soil changes as far away from the woody plants as possible. Damage can occur far outside the dripline.

Barriers

Construction such as asphalt paving can reduce soil oxygen levels from 18 percent to as low as three percent in some instances. Besides pavement, plastic barriers or black plastic, which have been widely used under various mulch materials, have created problems by reducing gaseous exchange. One improvement is the advent of numerous weed barriers (landscape fabric) which "breathe."

Secondary Effects of Oxygen Starvation. Sublethal stresses from oxygen deprivation can lead to numerous secondary problems. Disease and insect-related problems resulting from oxygen induced stresses can decrease leaf production needed for proper root growth. These stresses create a cyclical effect on shoot and root growth, leading to decline and possibly death of the plant. In addition, water and mineral relationships are adversely affected by reduced permeability of roots to water. Hormonal products may also be altered in roots growing under low oxygen conditions. Lastly, excessive moisture resulting in low oxygen may severely hamper effective mycorrhizal colonization. Some tree species need mycorrhizae for effective water and nutrient uptake and utilization.

In many commercial urban areas, the most common cause of trees dying is the pouring of cement around the existing tree trunks. This can be commonly seen in parking lots and other such areas. The concrete acts as a grade change and prevents the tree's roots from functioning properly

Waterlogged Soils

Roots in flooded or waterlogged soils often die of oxygen deficiency. Damage occurs not only to plants on obviously wet sites, but also to those in planting holes along city streets and in landscapes where soil drainage is impeded by high clay content. Most trees and shrubs cannot grow for long periods in waterlogged soil, and some die if flooded for only a few days during the growing season.

Figure 3-12
Plant death caused by waterlogged soil
External symptoms of injury include downward bending of leaf petioles, stem swelling, chlorosis, red or purple pigmentation in leaves, browning of leaf margins, twig dieback, death of roots, wilting, leaf drop, and death of the entire plant (Figure 3-12).

Plants with roots injured by waterlogged soil may subsequently suffer drought stress or death when, after the soil drains, the root system is unable to meet transpirational demands of the top. This commonly occurs in trees planted in holes in concrete landscapes (parking lots). Plants stressed or injured by waterlogging also become abnormally susceptible to soil-borne pathogens. Phytophthora species cause root rot most often in soils that are periodically waterlogged.

The following tree species tolerate waterlogged or flooded soils: birch, buttonbush, bald cypress, deciduous holly, red maple, native pecan, sweet gum, and willow. These tolerant tree species could be planted in soils with poor drainage or areas that tend to flood, but then recede quickly. Also, organic amendments should be incorporated into soils with high clay content to improve the drainage. Do not, however, just amend the planting hole because that will intensify the problem. Refer to the Cultural Management for Woody Plants chapter for information on soil amendments.

Girdling Roots
Figure 3-13
Root girdling

Trees are often stressed or killed by their own roots when they circle around the main trunk and cause girdling (Figure 3-13). Often, strangulation or root girdling leads to restriction of carbohydrates to the root system, ultimately leading to root starvation. Also, when roots are stressed, they transport less water and nutrients to above-ground portions of the tree, leading to decline in the above-ground portion of the plant as well. Usually, girdling roots slowly weaken trees over a several year period, where decline can be observed to become steadily worse each growing season. Soil may have to be removed around the trunk's base to find the roots.

Girdling roots are usually found on trees that were grown in nurseries, but not on those growing naturally in an area. Containerized trees are particularly susceptible to circling roots, which ultimately girdle the plant. If circling roots are not corrected at a young age, they have to be removed with a mallet and chisel or axe later. Refer to the Cultural Management for Woody Plants chapter for additional information on preventing root damage and planting.

Figure 3-14
Weedeater damage
Trunk Girdling

Young plants are particularly susceptible to bumps and scrapes from mowers and weedeaters (Figure 3-14). Trees can be killed directly by trunk girdling, which prevents water uptake from the soil, or by allowing entry points for diseases and/or insects.

Always keep a three foot wide area free of weeds and turf around new trees. Not only does this prevent mower or weedeater damage, but also curtails competition for water and nutrients. Tree guards may be placed loosely around the base of trunks to help protect trees.

Sapsucker Damage

The sapsucker is a bird that feeds on tree sap, the inner bark of trees, and the insects that get caught in the sap flowing from wounds on the trunk. The most common trees attacked by the sapsucker are apple, birch, larch, magnolia, sugar maple, and Austrian and Scotch pine. Sapsuckers peck holes in the bark of trees, causing trees to "bleed" sap. Trees are often repeatedly attacked and, when heavily injured, they may die. The injury also permits insects and diseases to enter the tree. Do not confuse sapsucker damage with borer invasion. Sapsucker feeding damage can be identified by evenly spaced rows of holes through the bark on the trunk. These holes are arranged characteristically in horizontal rings and are sometimes aligned in vertical rows. Preventing damage caused by sapsuckers is difficult and may not be justified in most cases.

Selected References

Derr, J. F., and B.L. Appleton. 1989. Herbicide Injury to Trees and Shrubs: A Pictorial Guide to Symptom Diagnosis.
Blue Crab Press, Virginia Beach.

Jacobson, J. S., and A.C. Hill (eds.). 1970. Recognition of air pollution injury to vegetation: A pictorial atlas.
Air Pollution Control Assoc., Pittsburgh, Pa.

Levitt, J. 1972. Responses of Plants to Environmental Stresses.
Academic Press, New York.

Sinclair, W. A., H.H. Lyon, and W.T. Johnson. 1987. Diseases of Trees and Shrubs.
Cornell University Press. 124 Roberts Place, Ithaca. N.Y. 14850.


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