In This Section |
Common Disorders Of Broad-Leaved Trees
Table of Contents
IntroductionDeciduous trees are an integral part of many landscapes. Trees function as windbreaks, boundaries for property lines, and also provide privacy, shade and aesthetic value to the home or cottage. Occasionally, trees are planted in less than ideal sites or the conditions around established trees are modified in a detrimental way. If adverse conditions persist, the health of such trees may be at risk. When trees are under stress, they become more susceptible to invasion by insect and disease pests. Pest problems are quite often symptoms of underlying health problems. For example, root diseases are often a symptom of poor soil aeration (e.g. from overwatering). It is important to recognize and identify the cause of stress in trees so that with early implementation of remedial measures, tree vigour can be improved. | Top of Page | Diagnosing the ProblemThe first task in identifying disorders of broad-leaved trees is to determine whether the stress is biotic (living) or abiotic (non-living) in nature. Biotic stresses are caused by insects, mites, disease (i.e. bacteria, fungi, and viruses), animals or other living organisms. Abiotic stresses are caused by non-living factors and often involve excess or deficiency of water (flooding or drought), nutrients, chemicals (salts or pesticides), heat or cold (frost or ice).Biotic StressesWhen looking for damage caused by insects, examine the affected tree carefully for signs of insect infestation. Affected areas of the tree may contain insects or signs of their activity. For example, many wood-boring insects create holes in the bark and sawdust during their tunnelling activity. Some insects, such as the Gypsy moth, leave empty cocoons behind after emerging as adults. Other indications of insects include egg cases, webbing, feces or honeydew. Honeydew is a sticky secretion produced by sucking insects such as aphids, leafhoppers and scales. Scale insects appear as fleshy bumps along the undersides of leaves or twigs and are often noticed by the presence of sticky honeydew. Mites (arachnids) are tiny spiders that can also cause damage, often leaving tiny egg cases and webs around infested foliage. Take a close look at the leaves and examine them for things like bronzing, stippling, curling and chewing damage. A 10x powered hand lens helps with insect detection and identification. When checking for disease, examine twigs for sunken areas (cankers) or raised, blister-like structures (fungal fruiting bodies). Affected twigs can be cut off to examine the inner wood (vascular tissue) for brownish discoloration. Healthy wood colour ranges from greenish-white to creamy-yellow. Foliar disease symptoms often include discoloration that range from yellow to brown to black. Discoloured spots and blotches, discoloured veins and pin-head size black or brown fruiting bodies along leaf and twig surfaces may be evident. (Note: some insects produce galls that resemble fungal structures.) Large fruiting bodies, either mushrooms or woody protruding shelf-like structures, can often be found at the base of large older trees. These structures indicate that the interior or the roots of the trees are rotting. Animals can damage trees by feeding directly on the tree or by foraging for insects in the bark or wood. Squirrels can prune new growth from elm, maple and oak during spring foraging. A telltale sign of this activity is large bunches of healthy leaves littering the ground. Deer browsing and antler rubbing can also cause significant damage, resulting in torn plants and shredded bark. Many animals including mice, porcupines and some birds remove bark to feed on the tender underlying cambium layer (especially in winter). This damage is often noticed on thin-barked trees at ground level or on the main trunk. When a section of bark is removed in a ring around the tree this damage is referred to as "girdling" injury. Animal girdling injury can be recognized by uneven tearing of the bark in the damaged area. Teeth marks and a general sparseness or dieback of the crown above the injured area are often a sign of girdling. Girdling is also the term used when a vine, root, rope or cable encircles another root, stem or trunk and slowly constricts it over time.Abiotic StressesThese types of stresses are usually more difficult to recognize and correct than biotic stresses. When assessing what is affecting the growth of a tree, it is important to not only examine the damaged portion of the tree, but the rest of the tree as well. Check the surrounding vicinity and examine other plant species to see if they are showing similar symptoms. Try to establish if the stress affecting the tree has been occurring for some time or if it appeared suddenly. Be sure to assess the soil, local weather conditions, and any recent activities that have occurred around the tree (e.g., construction, pesticide application, grading). It is a good idea to dig a hole around the root system of smaller trees and shrubs and check for adequate/excessive moisture and good structure (i.e. is it difficult to break apart?). Four general factors can be examined when assessing what is affecting tree growth. These are soil, hardiness, environmental stresses and human activities. | Top of Page | SoilsThe quality of soil in which a tree grows is one of the most important factors affecting tree health. Soil conditions, including the amount of organic matter, pH (acidity/alkalinity), nutritional status, porosity and aeration all have a direct affect on tree health.DrainageThe survival and growth of tree roots depends on soil drainage. Soil drainage affects the amount of oxygen to the roots, and is necessary for normal root growth and nutrient uptake. Soil drainage can be affected by many factors including texture (clay, silt or sand), mineral composition, amount of organic matter and proximity to the water table. Roots growing in a poorly drained soil are deficient in oxygen and have limited nutrient uptake. If soil saturation is continuous, root hairs die and the tree becomes an easy target for root-rot organisms. The tree then begins to wilt (because of the reduced number of water-conducting roots), and starts losing leaves and small branches. Trees growing in gravel or sandy soils that do not retain adequate soil moisture may also show symptoms of drought during dry spells or periods of excessively hot weather. Lack of moisture is also a problem where soils are severely compacted, as the water tends to run off the surface instead of percolating down through the soil. If the water table drops because of excavations, additional wells or high water usage, drought symptoms (leaves wilting, brown margins and veins) are possible.pH (Reaction)Soil pH is a measure of acidity or alkalinity of a soil. A soil pH reading above 7.0 is alkaline or basic and below 7.0 is acid. Most plants grow well in soils with pH values of 6.0-7.5. Most of soils in Southern Ontario are neutral to alkaline (7.0 and above). If the soil is too alkaline, it may influence the plant's ability to take up some nutrients (e.g. iron and manganese). To help lower alkalinity, sulphur can be applied to the soil. To increase the alkalinity of a soil, dolomitic limestone can be applied.Iron (Fe) and Manganese (Mn) DeficienciesDeficiencies of iron and manganese may occur in trees that are growing in alkaline soils. Trees that commonly exhibit these deficiencies include red maples, red and pin oaks, tulip trees, rhododendrons and azaleas. Symptomatic leaves are chlorotic (yellow) with contrasting green veins. | Top of Page | HardinessThe ability of a tree to withstand cold temperature is called hardiness. Hardiness varies with tree species, and each species is assigned a numerical hardiness rating. In Ontario there are 7 hardiness zones. Each zone is divided into 2 subcategories (a and b). See Figure 1 for an illustration of the Plant Hardiness Zones of Ontario. These zones are based on a number of factors including minimum temperature during the winter, the number of frost-free days, summer rainfall and wind conditions. Areas in the north are given a lower hardiness rating or number. This rating generally increases as one continues south. It is important to know your property's hardiness zone, as well as the hardiness rating of the tree to be planted. A tree that has a higher hardiness rating than the area it is planted in may not survive. If such a tree does survive, it is more likely to be affected by environmental stresses, and it may sustain injury in the form of branch dieback, severe trunk cracks, desiccation of foliage (as seen in broad-leaved evergreens such as holly and Oregon grape) and root freezing. Such stresses leave a tree more susceptible to attack from insects and disease. Hardiness ratings are meant as guidelines only. Many plants can be grown in an area beyond their hardiness rating but this is usually due to a warmer microclimate (such as a south facing, protected area such as a patio). The hardiness of a particular tree also varies with the season and with cultural practices (such as burlap windbreaks). Cold hardiness increases as the autumn progresses and decreases over the spring to a very low level in the summer. Many plants can be severely damaged by extreme temperature fluctuations (e.g. early fall frosts or late spring frosts). Many nursery and garden centre catalogues list a hardiness rating along with a description of the tree. Hardiness zones of many popular trees can be found in reputable gardening references. | Top of Page | Environmental StressesEnvironmental stresses refer to stresses that have been caused by weather conditions or other natural factors. Some of these are:Winter InjuryWinter injury is perhaps one of the most difficult disorders to diagnose due to the different forms of injury that can occur (listed below). Although repeated freezing and thawing causes damage to plant tissues, it is the formation of ice crystals within the cells of buds, sapwood and cambium that causes the real damage. When ice crystals form in a plant cell, the cell membrane is ruptured and the contents are lost. If many cells are ruptured, tissue death results. Often the symptoms of winter injury are not expressed until the following spring or early summer when it can be confused with other disorders. Winter injury may be caused by the following factors:
Lightning InjuryTall trees growing in exposed locations such as open fields, hill tops and along fencerows are more likely to be struck by lightning than those trees growing in groups or low-lying areas. However, all trees are susceptible to lightning damage. The severity of lightning damage varies depending on the directness of the strike and the strength of the charge. Typical symptoms of lightning damage include large strips of bark and inner wood being blown out, or large portions of a tree suddenly turning black after a recent electrical storm. As lightning has an enormous amount of electrical energy, this energy is grounded through the tree into the soil. This energy sometimes kills an area of cambium that results in adventitious root growth (air roots). Occasionally a large clump of soil and roots are disturbed where the electrical energy left the tree. To minimize stress caused by lightning strikes, remedial measures are recommended. Remove long strips of burnt bark with a clean sharp knife so that callus tissue can repair the damage. Fertilizing with a blend that is moderate in phosphorous, but low in nitrogen can encourage root growth.Wind DamageThe damage usually associated with strong winds is broken and torn branches (similar damage can also be caused by lightning). Shallowly rooted trees are also susceptible to strong winds and complete uprooting of these trees can occur. Weak-wooded, fast growing trees such as willows and poplars, are especially vulnerable to high winds, since they have large crowns with dense foliage that can easily "catch" the wind. Branch and trunk breakage is more likely to occur on trees infected with wood decay. High winds can also desiccate, damage or tear off leaves. Affected leaves appear tattered with large missing portions, and occasionally the margins are scorched. These symptoms are more severe in drought stressed trees.HailHard driven hail can cause serious injury to a tree especially in young succulent tissues. Hail damage is recognized by severely shredded leaves and a large number of "pock" marks on twigs and branches that later heal, leaving many small callused scars. Severe callusing can result in a branch becoming partially girdled and eventually dieback occurs. These wounds may in turn provide entrances for insects and disease causing organisms.Freezing Rain, Ice and Wet SnowLarge amounts of ice and wet snow that accumulate after a winter storm can be heavy enough to cause branches to snap. Small trees that still have a few leaves attached can easily be broken from the weight of snow and ice. This injury can be prevented by properly staking small trees, and removing snow and ice from larger specimen trees (within reason) with a broom.Competition from Other Plant SpeciesThe fine root hairs, which absorb water and nutrients, are found in the upper 30 cm of soil and well beyond the spread of the crown. It is in this zone that the roots of turf and other plant species are also competing for food and water. If a tree does not get enough nutrients it begins to show deficiency symptoms in the leaves. Affected leaves may appear pale green in the case of nitrogen deficiency or purple-red in the case of phosphorous or potassium deficiency. A soil test is often the most commonly-used method of testing for soil nutrient deficiencies. A foliar nutrient analysis is a more effective means of uncovering a nutrient deficiency (but a sample from a neighbouring healthy specimen is required for comparison). When nutrient and pH levels are normal, misapplied pesticides or other stresses may be causing the discoloration. DroughtTrees may suffer from both short-term and long-term drought stress. The leaves of trees experiencing short-term drought stress tend to wilt during the hottest part of the day, and later the leaves develop margins that are brown and brittle. This damage is more severe if strong breezes are present. Long-term drought stress shows short-term symptoms as well as the development of premature fall colour and defoliation in midsummer. Trees growing in clay soil may suffer longer periods of drought because sun baked clay soil particles tend to swell-up at the surface when wetted, blocking further moisture from reaching the roots. Aeration of the area under the dripline may help prevent this problem. Since trees are perennial plants, they are subject to winter drought as well. In the fall, water the soil around the tree well so the roots have a good "reservoir" of water to resume growth in the following months. | Top of Page | Human ActivitiesEvery year many trees are killed or damaged indirectly by human activities. In this section stresses are listed along with common activities that contribute to them.Transplanting StressThere are a number of stresses that affect trees before, during and after transplanting. Some of these are as follows:
Improper WateringRoot systems from containers or B&B (balled and bulapped) material are quite small. Until they become established, they have a very limited ability to take up water. Overzealous watering Many landscape plants require supplemental watering in the year following transplanting. Root systems from containers or B&B (balled and bulapped) material are small. Until they become established, they have a very limited ability to take up water. Overzealous watering actually drowns the root zone, reducing the amount of oxygen available for root growth and causing the roots to rot. In new housing developments, the likelihood of overwatering is even greater since the soils are often heavily compacted and of poor quality below 6 inches. Try watering woody material every 7-10 days. Leave the water on long enough to moisten the upper few inches of the soil. A long, gentle watering cycle allows for proper soil infiltration, encouraging the development of healthy roots at a reasonable depth. Short, frequent irrigation cycles do not allow for proper soil infiltration and cause roots to form at the soil surface, making them more susceptible to drought stress in high summer. Run irrigation systems in the cool evening or (even better) in the early morning hours, to limit the amount of loss by evaporation. Try to avoid getting water on the foliage. Leaf wetness increases the infection of diseases, creating more problems on a plant that is already under stress. Also, water droplets act as a lens on the foliage and can cause foliar burn through the action of the sun's rays. Direct the watering downward where roots grow.Improper MaintenanceApplication of large amounts of fertilizer at any time of the year stimulates excessive vegetative growth (sometimes inhibiting flowering) and reduces the plant's natural ability to resist stress. Fertilizer applications late in the season (coupled with unusually-warm autumn temperatures) may result in the failure of branches and bark to harden-off sufficiently before winter. Trees that have not had sufficient time to harden-off are generally more susceptible to winter injury. Pruning in early autumn may stimulate lateral bud break and succulent, vegetative growth into the autumn months (especially during an unusually-warm fall). These specimens may not have had enough time to harden off before winter. Improper pruning techniques that leave large stubs and torn bark leaves a wound vulnerable to pathogens of environmental stresses. Girdling StressesGirdling or strangulation of a tree trunk can be caused when wire fencing, nylon or steel cable has been tied around the tree for anchorage. If the wire is left around the tree for a number of years, the bark slowly grows around the cable. The cable gradually restricts the flow of water and nutrients in the cambium (the regenerative tissue under the bark). Similar damage may also occur when guy-wires become too tight. Guy-wires are a short-term, preventative solution to wind throw. Check these wires every year until they are removed. Partial girdling of the trunk (especially in young or thin-barked trees) can result when weed-eaters and lawn-mowers are used in close proximity to a tree.Site AlterationsIncreasing the soil level by more than 8 cm around an established tree reduces the oxygen and moisture supply to the roots. Eventually decline occurs. If soil has to be added because of landscaping needs, the tree may be saved by placing a radiating grid of drainage tile and gravel around the tree. When the fill is less than 45 cm deep, drainage tile is adequate. However, when the fill is over 45 cm, put a layer of gravel 20-30 cm deep along with the drainage tile.Construction StressTrenching and excavation activities within the root zone of trees damage roots, causing a reduction in the uptake of moisture and nutrients. This eventually causes leaf scorch and dieback in the crown. Trees with many roots severed or injured by excavation equipment may also set large amounts of seed from time to time. Although some trees naturally set heavy crops in alternate years, heavy fruit formation may also be a stress-related response to physical damage on the roots or trunk. Examples include maples, ash, birch and elm.Using heavy machinery or installing driveways, side walks or patios over the root zone can cause soil compaction. Compaction of the upper few inches of soil, especially in a clay soil, results in oxygen and water depletion in the root zone. Trees growing in compacted soil show drought stress.Salt DamageTrees growing along a street or side walk may be stressed by deicing salt spray and by deicing salt build-up in the soil. Damage increases with closer proximity to the road and is more severe on the foliage facing the road. The foliage of roadside evergreens may take on a "burned" appearance in early spring due to salt spray. The frequent use of deicing salts causes the accumulation of salts in surrounding soils. The adjacent trees are subjected to a greater degree of moisture stress or physiological drought. In general, high salt levels can cause root burning and decreased root function, resulting in symptoms of drought and nutrient stress in the crown. Deciduous trees with salt damage have unopened buds, twig dieback and tufted growth on the exposed side. If salt damage is suspected, take a soil sample and test for salt levels (E.C. or electrical conductivity) by an accredited laboratory. Fertilizers and rotting organic matter are other sources of soluble salts.Pesticide DamageThe severity of pesticide damage varies with the chemical involved, its concentration, duration of exposure and climactic conditions during that exposure. Herbicides applied to a lawn or garden may affect a nearby tree. Symptoms may include discoloured leaves, twig dieback, reduced growth and distortion of leaves or twigs. Herbicides applied to patios or driveways may be absorbed into tree roots and affect growth. These herbicides can persist for more than one growing season. Application of pesticides to trees when temperatures are above 250C may cause leaf scorch or other types of injury. Avoid pesticide applications when wind speed is greater than 11 km/hour. The insecticide carbaryl, when applied within 3 weeks of flowering, causes fruit thinning. Dimethoate (a systemic insecticide) applied to the same area of the trunk, year after year, leaves a dark ring and girdling injury may occur. This problem may often be seen on birch trees treated for birch leafminer in this manner. Consult pesticide product labels for specific application and use guidelines. Other StressesGas leaking from underground pipes can cause root damage. Natural gas does not normally cause direct injury to trees, but may cause damage by displacing soil oxygen needed for root growth. Chlorinated water from swimming pools or hot tubs may also damage nearby tree roots. | Top of Page | Getting Professional Help For Your TreesIf you require further assistance in diagnosing the problem on your tree, you may want to consult a professional arborist or speak with the staff at your local nursery or garden centre. Samples can also be mailed or brought in person to: The Pest Diagnostic Clinic
Figure 1. Plant Hardiness Zones of Ontario, 2002 | Top of Page | The author would like to thank the following individuals for their editorial comments for the 2002 revision of this Factsheet: Dr. Tom Hsiang, Ron Dutton and Michael Celetti. This Factsheet was originally written by R. Hamersma, Horticultural Research Institute of Ontario, Vineland Station. Toll Free: 1-877-424-1300 Local: (519) 826-4047 E-mail: ag.info.omafra@ontario.ca |
||||||||||||||
This site is maintained
by the Government of Ontario
Queen's
Printer for Ontario
Last Modified:
