Deciduous Tree Diseases (continued)

PP-697 (Revised), November 1995

Hop Hornbeam or Ironwood (Ostrya sp.)

The hop hornbeam is a North Dakota native which deserves to be planted more widely. It has few disease problems. Occasionally leafspots, mildew or scorch may be of minor importance.

Horse Chestnut

See Buckeye.

Ironwood

See Hop Hornbeam.

Kentucky Coffee Tree (Gymnocladus sp.)

Few diseases have been reported on this tree. Several leafspots (see "leafspots" Parasitic Diseases of Widespread Occurrence) are known to occur, but none is considered serious. This is one of the most disease-free trees suitable for planting in North Dakota. Winter injury (see "Winter injury" Non-Parasitic Disorders) may be a problem, although some large trees are found in eastern North Dakota. Trees grown from reliable seed sources should be winter hardy.

Linden or Basswood (Tilia sp.)

Leafspots
Several leafspots and a leaf blight or anthracnose have been reported on linden but none is serious in North Dakota. See "leafspots" under Parasitic Diseases.

Powdery Mildew, Sooty Mold
See Powdery Mildew and Sooty Mold under Parasitic Diseases of Widespread Occurrence.

Herbicide Injury
Linden is less sensitive to 2,4-D than boxelder or Siberian elm but may suffer damage if sprayed directly. Leaf curling or distortion is the usual symptom.

Linden is quite sensitive to injury from dicamba (Banvel), an herbicide often included in lawn fertilizer mixtures. Symptoms of dicamba injury are leaf-cupping or leaf distortion, including leaves that are thicker and darker green than normal. Because this chemical is in the soil, the effects may persist for a long period.

Control: Do not use fertilizers containing dicamba around or under linden trees.

Scorch
Lindens are quite susceptible to scorch on the leaf margins (Figure 19). See "scorch" under Non-Parasitic Disorders.

Verticillium Wilt
Linden is susceptible to verticillium wilt although reports from North Dakota are rare. See "Verticillium Wilt" under Parasitic Diseases of Widespread Occurrence.

Winter Injury
Lindens, especially young trees, may be subject to both sunscald and freezing damage. See "Winter Injury Disorders. Such injuries may provide infection sites for canker-causing " under Non-Parasitic fungi.

Cankers
Canker fungi – especially Cytospora sp. – are sometimes associated with dying branches or with young trees which have died. Many of these canker-causing fungi do not attack healthy established trees but can attack following injuries, sunscald, etc. – especially on newly transplanted or weakened trees.

Control: The best control for cankers is to prevent conditions which favor their development. Avoid wounds or injuries. Protect young trees from winter injury. Plant only hardy cultivars. Water and fertilize young trees to promote vigorous growth. Prune off cankered branches and destroy them by burying or burning. Sterilize tools between cuts. Examine nursery stock carefully before purchase. Avoid buying trees with wounds or abrasions on bark.

Maple (Acer spp.) (Silver, A. saccharinum; sugar, A. saccharum; Norway, A. platanoides; red, A. rubrum; Amur, A. ginnala; mountain, A. spicatum; boxelder, A. negundo)

The maples are a large diverse group of trees, only a few of which are grown in North Dakota.

Winter Injury
The sugar, silver, and Amur maples and the boxelder are winter hardy in North Dakota; even so, young trees may be damaged by sunscald if not protected. Many cultivars of Norway maple are of questionable hardiness and may suffer severely from winter injury, especially when heavily fertilized and watered. See "winter injury" under Non-Parasitic Disorders.

Poor Growth, Chlorosis, Dieback of red and sugar maples. Although their native range includes adjacent northern Minnesota and Canada, these species often do not succeed when planted in North Dakota. Trees or saplings collected from the northwest extremity of the native range may be better adaped to the Plains environment. This non-specific disorder appears to be a root/soil problem. Neither species likes alkaline soils. There may also be problems with the ability of the essential symbiotic root-fungus associations called mycorrhizae to form in North Dakota soils.

Herbicide Injury (Boxelder blight)
Boxelder is one of the most sensitive of all trees to phenoxy-type herbicides such as 2,4-D. Low level exposure produces leaf distortion while higher levels kill foliage. Such trees have a scorched appearance as if burned by flame. These extreme symptoms are common in remaining hedgerows alongside grain fields, but most of the boxelders in shelter plantings have succumbed to this problem. See "pollution injury" under Non-Parasitic Disorders.

Leafspots
Several leafspot fungi attack maples (Figure 13). See "leafspots" under Parasitic Diseases of Widespread Occurrence. The ocellate leaf gall insect causes a circular spot on maple leaves which is often confused with a fungal leafspot.

Figure 13. A leafspot of boxelder caused by Cristulariella pyramidalis. The large white circular spots show concentric zones. (19KB b&w photo)

Leaf Galls
Several species of mite cause leaf galls on maple which may be confused with a disease. These galls may look like spines, or may be woolly or felty patches on the leaf surface and of different colors, often bright red, (see NDSU Extension Circular E-296, Insects of Trees and Shrubs, for description and controls).

Powdery Mildew
See Powdery Mildew under Parasitic Diseases of Widespread Occurrence.

Sooty Mold
Sooty mold is especially common on boxelder. See Sooty Mold under Parasitic Diseases of Widespread Occurrence.

Verticillium Wilt
Maples are especially susceptible to verticillium wilt. See Verticillium Wilt under Parasitic Diseases of Widespread Occurrence.

Chlorosis
The symptom called chlorosis is a yellowish coloration of the foliage. The leaves may turn pale yellow-green to bright yellow, depending on the tree species and the severity of the problem. The leaves may be uniformly yellowish or, more often, the veins are green while the area between is yellow. Severe chlorosis may lead to scorch on leaf margins.

Chlorosis is usually caused by lack of some nutrient, especially iron or manganese, in the leaves. This deficiency may be due to lack of fertility, but may also result because the element, while present in the soil, is unavailable to the roots. This condition is especially common in alkaline soils such as occur over most of North Dakota.

Birches and maples seem especially prone to chlorosis, and should be avoided if high soil pH problems are known to occur. Chlorosis is extremely common on silver maples. Some trees may show severe chlorosis year after year and yet make good growth otherwise. Individual trees vary in appearance of chlorosis even when grown under identical conditions. No reason for this variability is known, other than genetic variation.

Control: Foliar applications of iron chelate or other iron-containing compounds will sometimes temporarily correct the chlorosis. Where practical, adjustment of the soil pH and proper application of micronutrient fertilizer may offer the best long-term solution. Micronutrient capsules implanted in the trunk may provide a remedy lasting for several seasons but mixed results have been reported from this therapy.

Cankers
The fungus Eutypella parasitica attacks maples and boxelder causing large perennial cankers on trunks or main branches. On sugar and Norway maple, eutypella cankers are elongate with conspicuous ridges of callus tissue around the sides (Figure 14). On sugar maple these cankers seldom girdle the stem but the enlarged, distorted growth around the canker and subsequent exposure of wood to decay fungi may structurally weaken the tree. On Norway maple, eutypella stem cankers can completely encircle the trunk causing top dieback. Cankers on Norway maple are often associated with frost crack or winter sunburn of the lower trunk.

Figure 14. Eutypella canker on a sugar maple stem. This perennial canker causes the distortion shown here. Concentric rings on the canker face indicate successive years of growth. (19KB b&w photo)

On boxelder, eutypella cankers tend to be sunken and rounded, often nearly circular, while on Norway maple cankers are very elongate, especially when associated with cracks in the stem. In contrast to sugar maple, where bark on the canker face sloughs off, bark on faces of eutypella cakers on Boxelder and Norway maple remains attached.

Control: Protect stem wounds and pruning stubs. Small cankers may be carefully excised. When selecting those trees to leave on a new building lot, reject any trees with large cankers. When purchasing Norway maple nursery stock, examine the trunk carefully for canker infection.

Wood Decay and Heart Rots
The wood of maples, especially silver maple and boxelder, is very decay susceptible. Any pruning wound or injury which exposes the wood is an avenue for entry of decay fungi. Avoid major surgery whenever possible as large wound faces heal slowly and readily admit decay fungi. See "heart rots" under Parasitic Diseases of Widespread Occurrence.

The bright red coloration often seen in the wood of boxelder is caused by a fungus, Fusarium lateritium, which infects the wood. This fungus does not affect the soundness of the wood, but its presence often indicates decay fungi are also present (Figure 5).

Wetwood
See under Elm.

Mayday Tree (Prunus sp.)

See under cherry.

Mountain Ash (Sorbus sp.)

The mountain ash is a member of the rose family and closely related to apple and pear. Many of the same diseases such as fireblight and cankers occur on both mountain ash and apple. See under apple.

Oak (Quercus sp.)

Bur oak (Q. macrocarpa) is the common native oak in North Dakota.

Anthracnose
Anthracnose, caused by the fungus Gnomonia veneta, is a serious disease of oaks, including the bur oak. The twiggy habit of bur oak is largely due to repeated infection by anthracnose.

The oak anthracnose fungus overwinters in fallen leaves, infected shoots, and in buds. The disease shows several different symptoms depending on the source and time of infection. There are three phases of this disease, twig blight, shoot blight and leaf blight. In the twig blight stage, buds are killed before they can begin to grow. Later, shoots may grow out from lower down on the branch. When this occurs repeatedly, "witches' brooms" or clusters of dead twigs may occur at ends of branches (Figure l5). In the shoot blight phase, new shoots are killed while they are expanding. Often blighted shoots appear scorched as if by fire. In the leaf blight phase of anthracnose, leaves may be distorted and partially killed at the tips or along veins. (Compare the leaf blight phase of ash anthracnose, Figure 6).

Figure 15. "Witches' brooms" on bur oak caused by oak anthracnose. Repeated killing of new shoots produces the twiggy branching shown here. (15KB b&w photo)

If infection occurs after leaves have expanded, circular to irregular brown spots may develop on the leaf. These spots may enlarge or coalesce in wet weather, blighting large areas of the leaf. Often all these phases of anthracnose are present together on different branches of the same tree.

Weather conditions in spring, when new growth is occurring, are the most important factors determining the severity of oak anthracnose. Mean daily temperatures between 50 and 57 F during shoot growth are most favorable for oak anthracnose infection. Cool, wet, late springs favor anthracnose development.

The anthracnose fungus is almost always present, even though weather may be unfavorable for serious disease development. For this reason a serious epidemic can occur even though symptoms have been virtually absent for several years.

Control: Repeated anthracnose attacks seldom permanently damage established bur oaks; in fact, the anthracnose-caused, gnarled, twiggy growth is sometimes considered picturesque and characteristic of the trees.

Young or recently transplanted trees and trees in nurseries may need protection from anthracnose. Spray protectant fungicides at properly timed intervals to prevent anthracnose infection. A dormant spray of lime sulfur at 1 gallon (30 percent) per 10 gallons (1.5 cups per gallon) or copper Bordeaux at label rate may help prevent bud blight. Do not apply lime sulfur after bud break.

Rake up and destroy (burn, bury, or compost) leaves in the fall. Broomed or cankered twigs may be pruned out. Fertilize and water to promote vigorous growth.

Oak Leaf Blister
This disease may go unnoticed unless a large number of leaves are severely infected or leaves begin to fall prematurely. The initial symptom is a slight yellowing of infected leaf tissue, followed by the formation of circular raised blisters on the leaves. Blisters form when infected cells are stimulated to enlarge, while surrounding noninfected cells remain rigid. Blisters are usually less than an inch in diameter, and the lower surface will appear gray as the fungus develops in the leaf tissue. Multiple infections cause a single leaf to become distorted. Premature defoliation may occur in early fall. The fungus survives on bud scales and in bark crevices.

Control: Particularly valuable ornamental trees may warrant treatment with a fungicide, which must be applied as a dormant spray to be effective. Use lime-sulfur at 10 tablespoons per gallon in the fall or in spring before bud break. Fungicides are not effective after leaves begin to develop because infection has already occurred. Collecting and disposing of infected leaves has no effect on the disease.

Leafspots
Several fungi cause spotting of oak leaves. Few are serious in North Dakota. See "leafspots" under Parasitic Diseases of Widespread Occurrence. Note: compare the leafspot phase of anthracnose.

Powdery Mildew
See Powdery Mildew under Parasitic Diseases of Widespread Occurrence.

Sooty Mold
Common on bur oak. See Sooty Mold under Parasitic Diseases of Widespread Occurrence.

Scorch
See Scorch under Non-Parasitic Disorders.

Oak Wilt
Oak wilt is an important and serious disease of oaks in Wisconsin, Iowa and southeastern Minnesota. The nearest known locations of oak wilt are in Minnesota in counties around St. Cloud. Oak wilt has never been found in North Dakota or in the northwestern part of Minnesota.

All oaks can be infected by the oak wilt fungus but they vary in susceptibility. The red oaks are most susceptible and rapidly killed. White oaks are moderately resistant while bur oak is intermediate.

A drought-induced decline of oak in the Red River Valley and adjacent parts of northern Minnesota sometimes appears similar to oak wilt symptoms. A laboratory diagnosis is necessary to confirm the presence of oak wilt. Suspect trees should have samples taken as for Dutch elm disease (see NDSU Extension Circular PP-324, Dutch Elm Disease) and submitted to the appropriate state laboratory for diagnosis.

Decline
A slow decline or gradual deterioration of bur oaks is a common problem in newly builtup areas where the oaks were native. At first, leaves may be scorched or fewer than before; later, twigs and then branches die and eventually the tree becomes unsightly and is removed.

Although disease or insects may play a role in the final death of such trees, the real cause of the decline is root disturbance. Oaks are particularly sensitive to even small disturbances of their root systems. Even slight changes in drainage caused by installation of driveways near existing trees, installations of septic systems, etc., may send oaks into decline. Although care can prolong their demise, once large oaks begin to decline, the process is irreversible. Extraordinary measures to preserve native oaks near new construction are impractical or incompatible with modern construction methods. See "root disturbances" under Non-Parasitic Disorders.

Shoestring Root Rot
The oak root rot fungus Armillaria mellea often attacks roots of oak, especially trees which are stressed by drought or defoliation. This fungus fruits in the fall producing clusters of tan or yellowish mushrooms at the base of the infected tree. The color of these mushrooms gives the fungus its other common name, "honey mushroom." Many people consider this mushroom a delicacy and collect it in large quantities to eat.

Oak trees attacked by Armillaria often show a decline similar to that described above. In addition to the mushrooms in the fall, presence of this fungus can be determined by examination of the base of the tree. Black "shoestrings" or rhizomorphs of the fungus will be seen under the bark. The wood will be decayed with a whitish rot. Trees with substantial decay of the root collar may be easily blown down in windstorms.

Control: The fungus may survive many years in pieces of wood (trunk or roots) buried in soil. When removing a diseased tree, dig out butt and larger roots. Do not replant another tree in the same location.

Pear

See under apple.

Poplar (Populus spp.) (aspen, cottonwood, hybrid poplars)

Septoria Leaf Spot
This disease is caused by Septoria musiva and occurs on native and hybrid poplars commonly grown in windbreaks and used in landscape plantings. Symptoms vary according to time of infection, hosts, texture and age of leaves. Four types of leaf spot symptoms occur: 1) Small flecks with angular margins; 2) white or silvery spots; 3) brown circular leaf spots with brown or yellow margins; 4) irregular shaped large tan spots in center with dark brown margins (Figure 16). Premature defoliation occurs on highly susceptible trees. With successive years this disease may predispose a tree to other disease pathogens.

Figure 16. Septoria leaf blight on balsam poplar. Both small and large spots are present. (20KB b&w photo)

Control: Plant tolerant or resistant clones. Sanitation in the field – burial or removal of leaves and stems – can reduce primary infections in the spring. In severe cases in landscape plantings, shortly after July 1 apply Daconil 2787 Flowable at two-thirds tablespoon per gallon (2 pints per 100 gallons) or Daconil Ultrex at 1.4 pounds per 100 gallons every seven to 14 days through the remainder of the growing season.

Marssonina Leaf Spot
This is a disease problem of native poplars, aspen, and hybrid poplars. It may severely defoliate susceptible trees well before normal leaf drop. The disease is caused by fungi in the genus Marssonina. Dark brown flecks, often with yellow margins, appear on leaves within a few weeks after leaves emerge in spring. Diseased leaves on affected trees appear smaller than normal, turn yellow-bronze, and are shed prematurely. The fungus moves progressively upward in the crown. If viewed from a distance the diseased leaves appear bronzed. On more established plantings and in native stands repeated outbreaks result in branch dieback and predispose trees to secondary pests and low temperature injury.

Control: Plant poplars resistant to or tolerant of marssonina leafspot. Remove dead and infected twigs from diseased trees. Rake up and destroy fallen leaves during the growing season. For ornamentals apply Ortho Multi-Purpose Fungicide at three-fourths tablespoon per gallon of water, or Daconil 2787 at two-thirds tablespoon per gallon (2 pints per 100 gallons) or Daconil Ultrex at 1.4 pounds per 100 gallons at 7 to 14-day intervals starting July 1 of the growing season.

Poplars may be affected by other leafspots, scorch, powdery mildew or wetwood, all covered under "Parasitic Diseases of Widespread Occurrence" except scorch, which is covered under "Non-Parasitic Disorders."

Shoot Blight
Young shoots and leaves shrivel and turn black. The fungus Venturia populina and related fungi infect young succulent shoots and cause death. The disease is favored by moisture. It is more severe in wet weather and in shaded locations or where dew is frequent. Although temporarily disfiguring, shoot blight seldom damages established trees unless it occurs for several years in succession.

Control: Prune out affected shoots. Young trees and trees in nurseries may require fungicidal protection. Apply three sprays at l0-day intervals, beginning at bud break. Spray a fixed copper fungicide according to label directions.

Rust
Poplar rust is caused by the fungus Melampsora medusae and related species. Rust generally appears after midsummer. First symptoms are small yellow pustules on the lower leaf surface. These contain the yellow to orange uredospores by which the fungus spreads from poplar to poplar. Later in the season the leaf appears covered by orange to brown waxy crusts which are the overwintering stage of the rust.

Spores produced in spring from these overwintering stages on fallen leaves can infect nearby larch trees, if present. Later, spores produced on infected larch branches reinfect poplars. Since larches are absent over most of North Dakota, mid-to-late summer infections probably arise by wind-borne spores from adjacent areas.

Control: Rust may become a problem in North Dakota under certain moist environmental conditions. Early defoliation by rust may weaken young or newly-planted trees and can be a serious problem in nurseries.

Removal of fallen leaves will not control rust as the infectious spores are wind-borne, often from great distances.

Protectant fungicide sprays may be used where protection of young trees or nursery stock is needed. Use Funginex at 1 tablespoon per gallon at seven to l0-day intervals as needed.

Cankers
The single type of disease causing more problems on poplars than any other in North Dakota is cankering caused by parasitic fungi. Fungi in the genera Cytospora, Phomopsis, Septoria, and Dothichiza cause cankers on poplars. These fungi kill areas of the bark on branches and main stems. If the killed area (canker) is large enough or if several cankers grow together, the stem may be girdled and killed. Spores which spread the canker fungi are produced in tiny pimple-like fruiting bodies in bark of cankers. In wet weather they ooze from the bark and may be splashed about by wind and rain. The canker fungi infect healthy stems when the spores land on wounds in the bark. Even minute cracks or scars may be sufficient to let in canker fungi.

Bark on cankers is sunken and discolored. Foul-smelling sap often oozes from cankers. The pimple-like dark fruiting bodies form on the affected bark. Later, the bark may loosen and split or fall off, exposing the wood. If healing is taking place, the canker will be surrounded by raised ridges of callus tissue which gradually grow together to heal the canker. Cankers may be any shape, but are often diamond-shaped or elongate (Figure 17).

Figure 17. Annual canker on young cottonwood stem. The original diamond-shaped canker is evidenced by the torn bark. This canker is healing, as can be seen by the vigorous callus rolls at the sides.
(21KB b&w photo)

Control: Certain poplar clones, particularly the Lombardy poplar (P. nigra `Italica'), are extremely susceptible to cankers and should not be planted. Drought stress greatly increases the susceptibility of poplars to cankers. Keep trees well watered and promote good growth with proper fertilization.

Prune out cankered branches during dry weather and destroy by burying or burning. Avoid wounding trees. Treat pruning cuts with a fungicidal wound dressing. Protect young stems from sunscald in winter. Septoria canker may be controlled by utilizing septoria leaf spot recommendations. See "canker" under Parasitic Diseases of Widespread Occurrence for additional information.

Russian-Olive (Elaeagnus angustifolia)

Scorch, Herbicide Injury, Winter Injury
See Scorch, Herbicide Injury and Winter Injury under Non-Parasitic Disorders.

Leafspots, Verticillum Wilt
See Leafspots and Verticillum Wilt under Parasitic Diseases of Widespread Occurrence.

Rust
A leaf rust may damage leaves and cause some defoliation in certain seasons. No control is generally needed.

Stem Cankers
Several canker fungi attack Russian olive. These include Phomopsis elaeagni, Botryodiplodia theobromae, and Tubercularia ulmea.

These fungi invade the bark through wounds. Bark on cankers is at first sunken and may later dry up and split. Fruiting bodies of the causal fungus may appear on the bark. Cankers may increase in size until they girdle a branch or stem which then dies. If uncontrolled, cankering may gradually kill branch by branch until the tree dies or loses its usefulness and must be removed.

See "canker" under Parasitic Diseases of Widespread Occurrence for control.

Walnut (Juglans nigra) and Butternut (J. cinerea)

Leafspots
Several leafspot fungi attack walnut. See Leafspots under Parasitic Diseases of Widespread Occurrence.

Anthracnose, Leaf Blight
See "anthracnose" under Ash.

Scorch
See Scorch under Non-Parasitic Disorders.

Winter Injury
Trees from different seed sources vary in hardiness. Plant only hardy sources. Understock used on named cultivars may affect hardiness. Butternut is of questionable hardiness in North Dakota, although some large specimens exist in the eastern part of the state. See "winter injury" under Non-Parasitic Disorders.

Willow (Salix spp.)

Several species of willows, both native and introduced, are widely planted in North Dakota. Willows are moisture-loving trees and planting on sites with adequate soil moisture will prevent many problems with scorch, cankers, etc.

Leafspots, Scorch, Winter Injury and Wetwood
See under Parasitic Diseases of Widespread Occurrence for leafspots and wetwood and under Non-Parasitic Disorders for scorch and winter injury.

Rust
Willow rust is caused by species of Melampsora, similar to the poplar rust fungus. See "rust" under Poplar for description of symptoms and control. Funginex is not labeled for rust control in willows.

Shoot Blight or Scab
Willow shoot blight, caused by the fungus Venturia saliciperda, is closely related to the fungus causing shoot blight of poplars. See "shoot blight" of Poplar for description of symptoms and control.

Cankers
Several fungi cause stem cankers on willow. The most important willow canker fungus in North Dakota is Cytospora chrysosperma. Cytospora cankers cause death of shoots and some larger branches. See "canker" under Parasitic Diseases of Widespread Occurrence for further description and control.

Heart Rot
Wood of willow is very decay susceptible and several wood-decaying fungi attack wood of living trees. Among the most important is Trametes trogii. See "heart rots" under Parasitic Diseases of Widespread Occurrence for control. Because willow is a weak-wooded tree and often develops hollow trunks, trees with any sign of decay fungi (mushrooms or conks) should be examined carefully for safety, if the trees are in yards, play areas, along streets, etc.

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[Parasitic Diseases of Widespread Occurrence]
[Non-Parasitic Disorders]
[Parasitic Diseases of Specific Trees (trees A-Ha)]

PP-697 (Revised), November 1995