Deciduous Tree Diseases

PP-697 (Revised), November 1995

R.W. Stack, Professor of Plant Pathology
H.A. Lamey, Extension Plant Pathologist

Parasitic Diseases of Widespread Occurrence

Many tree problems are caused by the action of one or more living organisms. Organisms which cause disease are called pathogens. The most common tree pathogens are fungi, a large and diverse group of plants lacking chlorophyll which derive their nourishment by parasitizing green plants, thereby causing disease. Most fungi are microscopic, but a few, especially the wood-rotting fungi, produce large, often colorful, fruiting bodies as mushrooms or conks. Some tree diseases are also caused by bacteria and viruses.

Some types of pathogens infect many different tree species with similar disease symptoms. Other pathogens attack only a specific tree or only certain cultivars of a tree. Diseases of the first type will be considered in this section. Diseases unique to specific trees will be considered under the tree name in the second section on Parasitic Diseases of Specific Trees.

Powdery Mildew

Powdery mildew is a common foliage disease on nearly all kinds of trees. Powdery mildew appears as a white growth on the surface of leaves. The powdery appearance comes from millions of tiny spores of the causal fungus, which are spread in air currents to cause new infections. Tiny black dots found in the powdery patches are another type of spore body by which the fungus survives the winter. Trees most commonly affected by powdery mildew are linden, crabapple, catalpa and chokecherry, but almost any tree or shrub, including alpine current and rose, may be infected at some time or other. Powdery mildew is especially common on lilac. Because the usually dry climate of North Dakota is unfavorable for the humidity-loving powdery mildew fungi, the disease is seldom a severe problem here (Figure 1).

Figure 1. Powdery mildew on leaves of hop hornbeam. (17KB b&w photo)

Control: Occasional or late-season infections are seldom important enough to warrant chemical control. If necessary spray with wettable sulfur at 2 to 4 pounds per 100 gallons (1 to 2 tablespoons/gal.) unless specified otherwise on the label. Sulfur may injure tender foliage, especially in hot weather. Do not use sulfur on walnuts, as injury may occur. Funginex can also be sprayed on lilac at a rate of 1 tablespoon per gallon.

Sooty Mold

Sooty mold, as the name implies, is a black growth on the surface of leaves, especially those in shade. The sooty mold fungi are not plant parasites but grow on the sweet honeydew deposited by various insects, especially aphids. Although unsightly, it seldom damages the tree. Sooty mold may occur on any tree but is most common on boxelder, elm, linden, and maple.

Control: Control depends on reducing the numbers of the honeydew secreting insects in the tree. Honeydew on leaves of small trees can be reduced by directing a stream of water over the foliage at frequent intervals.

Verticillium Wilt

Verticillium wilt, caused by the soil-borne fungus Verticillium alboatrum, causes light colored leaves with a dull appearance in early summer. The leaves then begin to drop without wilting, starting at the bases of affected branches and progressing upward. The symptoms may involve only one side of the tree or they may involve the entire tree. In severe cases, and on highly susceptible trees such as sugar maple, entire branches may wilt and die. Dry leaves remain attached to these branches. Stunting occurs in trees that have been affected for several seasons. When an affected branch is cut off, a ring of discolored wood can be seen, with the color varying from gray to light or dark brown to green. If the bark is peeled back, discolored streaks can be seen in the wood (Figure 2). On elm, streaking caused by Verticillium resembles that from Dutch elm disease. A laboratory test is needed to be sure which organism is present.

Figure 2. Streaking in wood by Verticillium wilt. Bark has been removed from this branch to show the characteristic streaking of sapwood caused by Verticillium wilt. (9KB b&w photo)

The Verticillium fungus invades through the root system. Many tree species are susceptible, but the danger is greatest in a highly susceptible tree such as maple, catalpa, elm or stone fruit. The fungus attacks over 300 species of cultivated plants.

There is no good control for verticillium wilt. Sometimes a tree will recover if affected branches are pruned out and proper fertilizing and watering are provided to help promote vigorous growth. If a tree must be removed because of verticillium wilt, do not plant another susceptible tree in the same spot, since the Verticillium fungus survives a number of years in the soil. Plantings on land formerly in potatoes may be particularly likely to get verticillium wilt.

The following trees have been reported as resistant to verticillium wilt: Birch (Betula), Hornbeam (Carpinus), Hawthorn (Crataegus), Honey Locust (Gleditsia), Flowering Crabapple (Malus), Willow (Salix), Mountain-ash (Sorbus), Pine (Pinus), Spruce (Picea).

Canker

The term "canker" is used to describe a killed area in the bark on the branch or trunk of an infected tree. More than 20 species of fungi that cause canker diseases are found in North Dakota. These fungi commonly invade wounded or injured bark tissues to form a canker and subsequently produce reproductive structures called fruiting bodies. Spores from these fruiting bodies serve as inoculum for new infections. Cankers may be "superficial," forming on the outer portion of the bark and doing little damage, or "cambial" when extended into the cambium, where the bark sloughs off and sapwood is exposed allowing subsequent decay to develop in woody stem tissues.

Annual cankers are active for one growing season, and infected tissues are sloughed off or healed over by callus (Figure 17). New cankers may form, but existing cankers cease to develop.

Perennial cankers are most conspicuous and destructive. There are two types. In target cankers (Figure 14) a succession of callus tissue repeatedly forms on canker margins over a period of years. Diffuse cankers are generally oval in shape and either sunken or swollen, little or no callus is formed, and stems, twigs and branches are girdled.

Infections of living trees by canker fungi are influenced by site, age, tree vigor, environmental conditions and cultural practices. Stress by drought, lack of fertility, insect defoliation, or herbicide damage predisposes trees to canker. In general, cankers are more prevalent on young trees, on suppressed low vigor trees, or on trees at poor sites. Cankers most often occur on trees predisposed to infection by environmental stresses such as herbicide drift, defoliation by insects or chemicals or root damage.

Control: A sound control program for canker diseases must be based on knowledge of the causal organism, the host tree, the cultural and environmental conditions under which the trees are grown, and available control methods. Control involves both disease prevention and treatment. While homeowners may be able to justify control measures, they may not be economical in most windbreak situations in North Dakota.

Disease prevention involves growing vigorous trees to prevent entrance of pathogens into the bark. Wounds are essential for establishment of most canker infections. The best preventive measure is to avoid unnecessary wounds, especially in the immediate area of active cankers containing fungus spores. Establish new plantings on good sites, use vigorous planting stock, fertilize trees to promote growth, and control weeds for several years after planting. Improve vigor of landscape trees by deep watering or trickle irrigation, especially during dry summer months, and maintain good drainage. Protect trees from wounds or injuries. Sunscald control measures may also be implemented. See winter damage under Non-Parasitic Disorders.

It is difficult to cure infected trees by pruning diseased branches, since pruning frequently causes increased exposure and sun injury to remaining branches. In the case of landscape or severely storm damaged trees, dead, dying, or severely cankered branches may be pruned from infected trees during winter or early spring, but not at other times.

Leafspots

Localized infections of leaves ("leafspots") on many trees are caused by a variety of fungi and some bacteria. Positive identification usually requires laboratory diagnosis. Leafspot diseases are seldom of sufficient importance in North Dakota to warrant control measures. The few exceptions which may require control are noted under specific diseases for that tree species.

Leafspot fungi infect leaves by spores which are dispersed by air currents or splashing rain. Infection usually requires moisture, such as dew, fog, or rain. Leafspots are more likely to occur on trees growing in shade, crowded together or in low spots or frost pockets. The foliage on such trees tends to remain wet longer following rain or dew.

Leafspots increase in frequency late in the summer as leaves begin to senesce (Figure 3). Buildup of leafspots on autumn leaves generally does not affect the health of the tree and is the beginning of the natural process of decay by which the leaf is returned to the soil.

Figure 3. Fungal leafspot on green ash. Compare the irregular distribution to the regular pattern of environmental scorch shown in Figure l8 and Figure 19. (17KB b&w photo)

Control: Good cultural practices usually give sufficient control. To minimize leafspot problems, avoid crowding plantings too closely. Thin out branches to open up the tree crown, but don't top or dehorn. Rake up leaves in fall and bury or compost them. Plant different kinds of trees in mixture. Fertilize trees in spring with a complete fertilizer. Water trees deeply during dry spells.

When fungicides are used, they must be applied prior to buildup of disease to effectively control leafspotting fungi. If severe leafspotting and/or defoliation occurs for several years, chemical control may be necessary, but the causal agent should be identified first (submit samples to the NDSU Plant Diagnostic Laboratory through your county agent). Timing of protectant fungicide sprays is critical and varies for different fungi. Correct timing is a key to effective chemical control.

Heart Rot

Heart rot in living trees is caused by fungi which have the ability to decay wood. These fungi gain entrance to the wood of the tree through wounds, branch stubs, etc., which expose the bare wood. The fruiting bodies, mushrooms or "conks," are common on trunks of decaying trees. Production of fruiting bodies on a living tree is a sign of extensive decay in the stem at that point. A useful rule of thumb might be that a cubic foot of wood has been decayed for each conk produced. Heart rot fungi do not invade living wood of healthy trees. As long as the tree is growing vigorously the rot will be confined to a small central core of the trunk and the structural integrity of the tree will be maintained. If the tree is weakened for any other reason or fresh wood exposed by severe pruning or storm damage, then the decay fungi can advance to more and more wood (Figure 4). When this happens the tree may become unsafe and the risk of wind or storm breakage greatly increases.

Figure 4. Cottonwood branch with heart rot. The entire discolored center of the stem is decayed. The decay started from an untrimmed branch stub. (30KB b&w photo)

Control: Avoid pruning wounds which expose large areas of wood. Shape trees when young so major branch removal will not be necessary later. Remove broken branch stubs following storm damage. Keep trees growing vigorously. Have trees suspected of having heart rot or hollows checked by a trained arborist to determine if sufficient live wood is present for structural safety. Your local city or state forester can recommend a reputable professional. Check such trees every few years to be sure new growth is maintaining sound structure. Large trunks and main branches with extensive decay may have little sound wood to support the tree (Figure 5).

Figure 5. Cross section of severely decayed boxelder. Decay has progressed to the point where part of the center is hollow. The concentric rings of discolored wood show the advance of the decay fungi. Only the thin ring of white wood around the outside of this tree was structurally sound. (24KB b&w photo)

Parasitic Diseases of Specific Trees

(Listed alphabetically by tree common name)

Non-Parasitic Disorders

Trees deteriorate and die for many reasons. As with people, trees may die from hunger, thirst, old age, toxic chemicals, or improper care. Disorders of this sort are termed non-parasitic or abiotic to indicate that the cause is not some living agent or pathogen. Abiotic disorders are seldom the cause of death of a tree but, all too often, they pave the way for some pathogen which could not attack a healthy tree directly. Non-parasitic disorders of trees most commonly encountered in North Dakota are scorch (desiccation injury), winter damage, and root injury.

Leaf Scorch

The symptom known as leaf scorch results when the roots are unable to supply enough water to replace that used by the leaves. For this reason scorch is most commonly seen following dry spells, drought, or on windy sites. The combination of environmental conditions which favor scorch occurs nearly every summer in North Dakota. By the end of the season many trees show leaf scorch symptoms.

The most common pattern of leaf scorch is a drying out and browning of the leaf margins (Figure 18). The symptom is often more severe on outer, more exposed leaves, especially on the sunward and windward sides of the tree.

Figure l8. Leaf scorch on silver maple. The dark areas on margins and tips of the leaves are killed by hot dry winds. (14KB b&w photo)

Where hot drying winds occur for several days during the period of active tree growth, more severe scorch symptoms may appear. The outer parts of leaves or whole leaves dry up. Following such an episode new leaves will often form if moisture supplies to the roots are adequate.

Leaf scorch symptoms may also appear following damage or disease in the roots (see root disturbance below) or injury to the trunk. Scorch symptoms may also occur from salt damage, especially from deicing salt used on roads, sidewalks or driveways near the tree.

Elm, maple, linden, and walnut appear to be the species most commonly affected by scorch. Large-leaved trees such as catalpa or buckeye may also be more susceptible. Some leaf scorch on susceptible trees by the end of summer must be considered normal in North Dakota (Figure 19).

Figure 19. Leaf scorch on linden. Drying of leaf margins and between veins is common on many trees by late summer. (16KB b&w photo)

Control: In exposed areas, plant trees in groups rather than singly. Plant scorch susceptible trees in protected locations. Water young trees as needed and water deeply. Fertilize trees of low vigor. Do not plant shallow-rooted trees on droughty or excessively wet sites.

Winter Damage

Winter damage may take several forms on deciduous trees: frost cracks, bud or shoot death and sunscald.

Frost Cracks
Frost cracks develop from internal stresses in the tree trunk. Frequently these cracks open up in coldest weather and close again when the weather warms up. Ash and maple are especially prone to frost cracking. Frequently, cracks open in the same place year after year resulting in a callus ridge ("frost rib") down the side of the tree. As long as the tree is otherwise healthy, frost ribs do little harm, but if the tree also has heart rot or is hollow they may constitute a serious structural weakness. No control is known for frost cracking.

Bud or Shoot Death
Bud or shoot death (freezing injury) occurs if temperatures get too low in winter or from frosts in early fall or late spring. Prolonged warm spells during the winter will cause this condition, particularly in those trees in which the rest period can be broken by low temperatures. It usually is aggravated if a warm period is followed by an extremely cold one. Buds or entire shoots may be killed. Buds may be injured sufficiently to allow entry of disease-causing fungi. Flower buds are more cold tender than leaf buds and may be killed while leaves are unaffected. This type of winter injury is most likely when non-adapted tree species or marginally hardy cultivars are planted in North Dakota. Occasionally, very unusual weather may cause injury even to otherwise hardy native trees.

Symptoms first become evident in spring when plants fail to flower or to leaf out. When new growth finally begins, shoots arise from older parts of the branches. Winter-killed twigs may be quickly colonized by weakly parasitic fungi. This can give a false impression that a parasitic disease is the cause. Excessive fertilizer levels or failure to reduce irrigation frequency in late summer will delay hardening off, which makes shoots more susceptible to injury.

Control: Do not use high nitrogen fertilizer (lawn fertilizer) under trees in late summer or early fall. Reduce or stop irrigation as fall approaches. Plant trees known to be hardy in North Dakota.

Sunscald or Sunburn
Sunscald or sunburn occurs in late winter when the sun is bright – especially if new snow is on the ground. Sunscald usually occurs on the south or southwest side of a tree. Thin bark on the tree is warmed enough to become active. When temperatures fall sharply at night, the inner living bark is killed. Thin-barked trees such as maple are most susceptible, especially when young.

Control: Sunscald on newly planted trees may be prevented by wrapping the trunks with burlap, kraft paper or special tree wraps. Trunks may also be shaded on the south and west side with boards or screens. Tree paints (white-wash) have also been utilized to reduce sunscald damage. Plant trees of marginal hardiness only in protected locations. Avoid frost pockets. Plant only hardy species or cultivars (see NDSU Extension Bulletin No. 13, Trees and Shrubs for North Dakota).

Air Pollution Injury

Although the levels of air pollutants such as sulfur dioxide (SO2) or ozone (O3) are generally low in North Dakota, locally high levels of SO2 may occur downwind from power stations. Vegetation injury by SO2 is similar to scorch (see previous section) in that parts of the leaves are killed, usually at the margins or between the veins. Pale killed spots may be found on the surface of the leaf as well.

The most common plant-damaging air pollutant in North Dakota is drift from herbicides. Low levels of volatile herbicides such as 2,4-D are carried many miles and are sufficient to damage sensitive species such as boxelder and Siberian elm, even in cities. Herbicidal injury is usually evident as deformation of young leaves and shoots. Leaves which develop before or after the exposure will be normal, so that affected foliage clothes only the middle part of the shoot (Figure 20). Higher dosages of herbicide will kill foliage or even shoots. This latter type of damage is common in field windbreaks.

Figure 20. Herbicide injury to Siberian elm. Symptoms typical of 2,4-D injury.
(A) Branches showing different severities of leaf distortion. Leaves which grew out before exposure are normal. (24KB b&w photo)
(B) Extreme distortion resulting from a high level of herbicide.
(24KB b&w photo)

Control: Minor deformity of sensitive species is uncontrollable as long as phenoxy herbicides remain in widespread agricultural use. Usually no permanent damage is done from low concentrations as the trees grow out of the symptoms. To prevent severe damage, avoid spraying trees when applying herbicides. In home yards, do not apply lawn herbicides within the dripline of trees. Do not spray trees with the same sprayer used to apply herbicides.

Root Disturbance

One of the most common and yet least recognized causes of tree problems is root disturbance, often associated with some sort of construction activity. This may be laying of pipe or cable (Figure 21A), street widening, placing a new building in an older site or turning residential areas into parking lots.

Because young trees — and even moderately-sized ones — can be successfully transplanted, people sometimes think that root damage is of no consequence. An established tree has a wide, spreading root system which may extend as much as two to three times beyond the width of the canopy. Loss of a substantial part of this root system will result in decline of the tree, unless corrective measures are taken. Old or very large trees seldom have sufficient regenerative capacity to survive major root damage, although it may take such a tree several years to die (Figure 21B). Home builders or contractors frequently try to preserve trees by fencing off an area around them. This protects the trunk from damage but is of little help to the root system, which extends out well beyond the drip line of the tree crown. Excavation, even at a distance of l0 feet from the trunk of a large tree, may destroy 30 percent of the root system. It does not take major soil disturbance to cause damage. Cutting a narrow ditch for laying utility cable effectively disconnects the entire root system on that side of the tree.

Figure 21. Root injury from construction damage.
(A) Major disturbance of adjacent trees. (40KB b&w photo)
(B) The tree on the left has lost more than half of its root system.
(42KB b&w photo)

Changes in grade will also damage tree root systems by upsetting the balance of air and water the roots need to survive. Soil compaction from heavy vehicles can kill roots. When tree roots are damaged by any of these causes, the tree begins to decline. The first symptoms may be leaf scorch. Later, twigs and shoots die back, then whole branches may die. Death of the tree may follow.

Trees vary in their ability to tolerate root damage. The American elm is one of the most tolerant, and it may take several years to die following severe root disturbance. Some other trees, such as oak and cottonwood, are extremely sensitive to any root disturbance and large trees of those species may die quickly from major damage.

Oaks vs. People

Bur oaks grow native along river valleys in North Dakota. These wooded areas are often sought as prime home building sites. On such sites, the oaks frequently begin to decline shortly after new construction. Even so minor a disturbance as changing underbrush into lawn, or water drainage changes from installation of a septic system may send oaks into a slow, irreversible decline. Very careful planning and management are necessary if oak woodland is to be preserved around a new home. Many prospective home buyers may find the needed protection is too costly or forces unacceptable design requirements.

Control: Protect tree root systems, not just the trunk, during construction. Prevent heavy equipment or traffic from travelling near trees. If this is unavoidable, aerate the soil immediately following such activity. Do not allow utility trenching near trees.

Insist upon written guarantees by contractors working near valuable trees. If major root damage to large trees is unavoidable, consider removing the tree and replanting. Often the savings by this procedure will more than pay for specimen size replacements. Do not pave over tree root systems. Roots need air to live!

If tree roots are damaged, careful pruning to balance top and roots can sometimes help the tree survive. Obtain the help of a professional arborist. Your city forester may recommend someone or offer direct assistance.

Reference to commercial products or trade names is supplied with the understanding that no discrimination is intended and no endorsement by the NDSU Extension Service is implied.

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[ Parasitic Diseases of Specific Trees ]
(Listed alphabetically by tree common name)

PP-697 (Revised), November 1995