Deciduous Tree Diseases (continued)

PP-697 (Revised), November 1995

Parasitic Diseases of Specific Trees

Apple (Malus spp.)

The most common and severe diseases of apple are fireblight, scab and rust. Short descriptions follow. For more complete information on apple diseases, see Circular PP-454, Diseases of Apples and Other Pome Fruits, available from your county agent.

Fireblight
Fireblight is a common disease of apples, crabapples and mountain ash. Shoots become curled at the tip and blackened as if scorched by fire. Blossoms may also wilt, fruiting spurs may turn black and be killed, and suckers may be curled and blackened.

Fireblight is caused by a bacterium that invades the water conducting tissues of the tree. It overwinters in cankers on larger branches. These often form around a diseased sucker or fruiting spur. The bark on cankers is slightly sunken and discolored.

Control. Prune out diseased cankers in late winter. Sterilize pruning tools with 10% bleach, full strength Pine Sol, Lysol (use the type with the red label), or denatured ethyl alcohol (eg. shellac thinner). Bleach and Pine Sol are corrosive; be sure to wash and oil pruning tools after using these products. Streptomycin should be applied every four to five days at blossom time to apples if daytime temperatures exceed 65 degrees Fahrenheit.

Scab
Scab is common on apples and crabapples following wet weather. Feathery olive brown spots develop on the leaves. Later, these may become scabby in appearance. Severe scab can result in leaves yellowing and premature leaf drop. Fruits may also be infected; if the infection occurs before the fruits are full sized, the fruits will be distorted.

The apple scab fungus survives the winter on fallen apple leaves. Infection is favored by rainy weather.

Control. Rake and destroy fallen apple and crabapple leaves in the fall. To control apple scab infections during the summer, captan or benomyl plus captan fungicides can be used on apples, and chlorothalonil can be used on ornamental crabapples.

Rust
Rust produces yellow orange spots on leaves of apples, crabapples, hawthorn and juneberry. Tiny cup-like structures develop on the undersides of leaves. Juniper is the alternate host. On juniper the rust forms galls on the branches and twigs. Wet spring weather favors the formation of gelatinous horns on the galls. These horns release spores. Spores from juniper infect apples and related hosts; spores from apples and related hosts infect juniper.

Control. Pick galls off juniper in the spring to break the fungus life cycle. Don't mix junipers and susceptible crabapples in the same planting. Sulfur fungicide may be used on apples and chlorothalonil may be used on ornamental crabapple. Other products are available to commercial orchardists.

Apricot

See under Cherry.

Ash (Green, Black, Manchurian, `Marshall,' `Summit') (Fraxinus spp.)

Anthracnose
Anthracnose is the most common malady of green ash in North Dakota. Ash anthracnose is caused by the fungus Gnomoniella fraxini. It infects buds, leaves and sometimes twigs. Spring infection of buds or expanding leaves causes necrosis and distortion of leaves, the most typical symptom of ash anthracnose. Often such leaves have irregular brown blotches associated with leaf veins (Figure 6). Whole shoots may be stunted and the leaves deformed or killed. Infections occurring later, after leaves have expanded, cause small brown circular lesions which may later expand or coalesce.

Figure 6. Ash anthracnose. Leaves show distortion and killed areas typical of ash anthracnose. (21KB b&w photo)

In some years anthracnose is associated with extensive defoliation of ash trees in May or June. The exact conditions under which defoliation occurs are not known, nor are the reasons why it occurs in some years and not others.

The ash anthracnose fungus develops best at temperatures of 60 to 70 F. Wet weather in May favors anthracnose development.

Control: Rake up and destroy (burn, bury or compost) leaves in autumn to reduce the disease carryover. Prune out cankered branches and remove twiggy growth to promote air movement within the crown. Fertilize trees in spring to promote vigorous growth.

Where trees are attacked by anthracnose repeatedly, protectant sprays may be desired. Timing of fungicide application is critical if it is to do its job of protecting new growth. Three applications are best. Apply the first spray just as buds are beginning to swell but before bud break. Apply the second spray when buds show green tips and the third when leaves are half grown. The first spray should be lime sulfur; the other two sprays should be Daconil 2787 Flowable at 2 pints per 100 gallons of spray solution (two-thirds tablespoon per gallon) or Daconil Ultrex at 1.4 pounds per 100 gallons.

Scorch
See Scorch under Non-Parasitic Disorders.

Rust
Ash rust, caused by Puccinia sparganioides, is more of a curiosity than a real problem. Ash rust attracts attention because of the bright orange color of the fungus fruiting structures and the deformities which it causes on petioles and leaves. This rust alternates its hosts. The orange spores produced on ash do not reinfect ash but rather infect a wild grass (cordgrass, Spartina spp.). The spores produced in fall on the grass are responsible for infection of ash leaves the following spring. Since cordgrass often grows in wet areas or ditches, ash trees in the vicinity may show more severe rust.

Control: No control is needed on established trees.

Heart Rots
Several wood-rotting fungi attack living green ash trees. The most common is Fomes fraxinophilus. Two other fungi, Polyporus sulfureus and Phellinus punctatus, also cause heart rots in living ash trees in North Dakota. Heart rots are much more severe in mature and overmature trees in native woodlands. When purchasing a building lot or a home in a naturally wooded area, examine ash trees carefully for evidence of heart rot — especially that caused by Fomes fraxinophilus (Figure 7). Infected trees on such sites often have serious structural defects and will need to be removed for safety. See "heart rot" under Parasitic Diseases of Widespread Occurrence at the beginning of this circular for management.

Figure 7. Conk of Fomes fraxinophilus. The cause of ash heart rot, this fungus is widespread, especially on native trees. Presence of conks indicates extensive decay of the wood. (15KB b&w photo)

Leafspots
Several fungi cause leafspots on ash. None merit chemical control under ordinary circumstances, except in nurseries. See "leafspots" under Parasitic Diseases of Widespread Occurrence at the beginning of this circular.

Ash Yellows
Ash Yellows is a new name for a problem that has been around for many years. For more than 50 years foresters have described a slow decline of ash trees. Although environmental factors may cause a slow decline, ash yellows can also cause such a decline in individual trees. The disease, caused by a phytoplasma-like organism, is common in southern Wisconsin and Minnesota, has been observed in southeastern North Dakota, and one infected tree has been observed in the Badlands of southwestern North Dakota.

Infected trees usually show slower growth, about 70 percent of normal. They may also show decline, dieback and occasionally may develop dense clusters of short, twiggy branches known as witches' brooms.

Epicormic sprouts near the base of the tree occur in a small portion of infected trees. When present, these sprouts are diagnostic. Clusters of spindly twigs develop in the epicormic sprouts. The leaves in these sprouts are light green or develop chlorosis between the veins. Dieback of these twigs often occurs in the winter. The leaves may be simple or, if compound, the number of leaflets may be reduced. The sprouts often are only about 15-18 inches long, although in some cases they may be several feet long.

Since only a small portion of trees show epicormic sprouts or witches' brooms and many trees show no obvious symptoms, a laboratory test is required to confirm presence of ash yellows. Several laboratory tests have been used. One of these, called the DAPI test, is quick and reliable, especially if used on a root sample. This test is being used for diagnosis of the disease in Minnesota and North Dakota. Information on this test is available from your state's plant diagnostic laboratory.

Little is known about the means of spread of this disease, although insect transmission is likely. Infected green ash trees which lack severe symptoms will likely survive for many years if given good culture, watering and fertilization. The disease on green ash is generally far less severe than on the highly susceptible white ash of the eastern United States.

Aspen

See under Poplar.

Basswood

See under Linden.

Birch (Betula sp.)

Two birches are commonly planted in North Dakota. One is the paper birch (B. papyrifera), native in Minnesota and in some forested areas of North Dakota. The other is the introduced European white birch (B. alba) and its cultivated forms (cutleaf weeping birch). North Dakota conditions are not the best for growth and longevity of birches. Even where paper birch occurs in North Dakota as a native, its development and lifespan here are less than the optimum for the species.

Leafspots
Several fungi cause leafspots or blotches on birch. None merit chemical control. See "leafspots" under Parasitic Diseases of Widespread Occurrence at the beginning of this circular.

Chlorosis
Chlorosis or leaf yellowing is a common malady of birches in North Dakota. See "chlorosis" under Maple.

Scorch
Birches are susceptible to leaf scorch, which may appear on leaf margins or at the tips (see "scorch" under Non-Parasitic Disorders). Scorch on birch may be associated with wetwood or with decline (see "decline" below).

Herbicide Injury
Birches are very sensitive to herbicides applied to soil, especially dicamba (Banvel). This chemical may be used in some lawn fertilizer preparations. Heavy application of dicamba may cause rapid, complete defoliation. Such defoliated trees may not die and may leaf out normally the next year if the chemical is not used again. Milder effects of herbicides may be leaf distortion, chlorosis or scorch.

Birches may also show effects of phenoxy herbicide drift. See "herbicide injury" under Non-Parasitic Disorders.

Heart Rots
The wood of birch is very decay-susceptible. Several fungi cause rot in living trees (Figure 8). The most common are Fomes fomentarius, Daedalia spp., and Polyporus squamosus. See "heart rots" under Parasitic Diseases for symptoms and control. Because birch wood is so decay susceptible, it is very important to avoid large wounds such as are caused by removal of major branches. Do not attempt to head back large birch trees. Birch trees showing dieback and having conks or mushrooms of decay fungi on the trunks probably are safety hazards and should be removed. Consult your city forester or a trained commercial arborist.

Figure 8. Fomes fomentarius conk on white birch. Sometimes called "horse's hoof" or "false tinder conk." Presence of several conks indicates extensive decay. (27KB b&w photo)

Decline
"Decline" is a term used by pathologists and arborists to describe a gradual dying of trees where no single cause is obvious. Birch decline has been reported in the woodland areas of the Great Lakes states in native birch stands. A similar type of gradual dying has been observed in older birch trees in North Dakota cities.

Although the exact cause of birch decline is not known, it appears to be set off by drought, possibly coupled with root damage or other stress factors. Trees under stress conditions may become prey to pests or diseases not affecting healthy trees. Declining birch trees often have canker fungi associated with dying branches, but it seems likely that the fungi are only attacking the already weakened tree.

The bronze birch borer is attracted to weakened birch trees, and once it becomes established it often kills the tree. Identification and control of bronze birch borer is described in NDSU Extension Circular E-296, Insect Pests of Trees and Shrubs. One important source of bronze birch borer infestation in eastern North Dakota may be birch firewood imported from adjacent forest areas where natural borer populations are high.

Boxelder

See under Maple.

Buckeye (and Horse-chestnut) (Aesculus sp.)

Scorch
By late summer some leaf scorch must be considered usual on buckeye in North Dakota. See "scorch" under Non-Parasitic Disorders. Symptoms of verticillium wilt may resemble scorch. Scorch is sometimes confused with leaf blotch.

Leaf Blotch
Buckeye leaf blotch or leaf blight is caused by the fungus Guignardia aesculi (Figure 9). The blotches start as small irregular reddish-brown spots with yellow margins. These later enlarge and coalesce. Premature leaf drop may also occur. Minute black fruiting bodies of the fungus are found in the brown blotches.

Figure 9. Guignardia leaf blotch. This is common on both horse chestnut and buckeye. (20KB b&w photo)

Control: Gather and dispose of fallen leaves by burying, composting, or burning. In most seasons in North Dakota, late season appearance of leaf blotches is not important and adequate control will be obtained by removal of fallen leaves. Leaf blotch may be confused with scorch.

Powdery Mildew
Two species of powdery mildew sometimes infect buckeyes. (See "powdery mildew" under Parasitic Diseases).

Winter Injury
The true horse-chestnut (A. hippocastanum) of the eastern and southern U.S. is beyond the range of hardiness in North Dakota. If planted here it may suffer severe winter killing each year — if it survives at all. The tree planted in North Dakota as buckeye (A. glabra) is hardy and also more variable. Many hardy individuals are found growing well in North Dakota. Even so, freezing injury may occur in some winters (see "winter damage" under Non-Parasitic Disorders). Winter-killed shoots should be pruned back only after new growth has started. Because most "Ohio buckeyes" are really of hybrid parentage, some individual trees may be winter-injured more or less often than others.

Verticillium Wilt
Buckeye is susceptible to verticillium wilt. Mild symptoms may resemble scorch. More severe symptoms may result in death of one or more branches. (See "verticillium wilt" under Parasitic Diseases of Widespread Occurrence).

Butternut

See under Walnut.

Catalpa (Catalpa sp.)

In milder climates, the catalpa is a showy ornamental tree. Most of North Dakota is at the margin of catalpa's hardiness range, although some trees survive well. Winter injury, especially dieback of shoots and buds, is the most common problem with the tree in our area. Catalpa is susceptible to leafspots, powdery mildew, and verticillium wilt, all of which are discussed under Parasitic Diseases of Widespread Occurrence.

Cherry (Prunus spp.)

The most common and severe diseases of cherry and chokecherry are black knot, plum pockets and X disease. Short descriptions follow. For more complete information on diseases of cherries and chokecherries, see Circular PP-689, Disease Control in Cherries, Plums, and Other Stone Fruits, available from your county agent.

Black Knot
Black knot is easily recognized by the hard black swellings or "knots" on smaller branches. These knots eventually girdle and kill branches. New knots develop a felty olive green appearance about blossom time. This is due to the release of spores that start new infections in wet weather.

Control. Prune all developing knots at blossom time, cutting 4-5 inches below the knot. Annual pruning may be required for several years to achieve control. Best control is achieved by pruning combined with use of a fungicide. Lime sulfur may be applied as a dormant spray before bud-break. Once the buds break, use captan plus benomyl.

Plum Pockets
Plum pockets infects cherry, chokecherry, wild plum, some domestic plum hybrids, sand cherry, Nanking cherry and wild black cherry. The fruits become hollow and bladder-like and are greatly enlarged. Shoots may be deformed and curled on some stone fruits and a leaf curl and witch's broom may develop on others.

Several related Taphrina species infect stone fruits. Spores are dispersed in early spring and become lodged in bud scales and other small crevices and remain dormant until bud break the following spring.

Control. Use a delayed dormant spray just before bud break. Lime sulfur can be applied in spring once the temperatures are above freezing, or Bordeaux mixture can be applied. Lime sulfur can also be applied in the fall after leaf drop.

X Disease
X disease is currently epidemic throughout the northern Great Plains. Many native and planted groves of chokecherries have been wiped out. X disease is caused by a phytoplasma which becomes systemic in the plant. The disease is spread by leafhoppers and takes several years to develop. First symptoms are early fall color. In subsequent seasons plants show bronzing or reddening of leaves in mid summer. Shoot dieback occurs and eventually the plants die. There is no cure. Do not replant chokecherries near affected plants.

Cottonwood

See under Poplars.

Crabapple

See under Apple.

Elm (Ulmus spp.) (American or white elm, red elm, Siberian or so-called "Chinese" elm)

Despite many problems with disease and injury, the elms, especially American and Siberian elms, are by far the most commonly grown trees in North Dakota.

Dutch Elm Disease
This lethal disease of American elm was first found in North Dakota in l969. At the present time it has spread throughout North Dakota. Because of its importance, Dutch elm disease (DED) is the subject of a special NDSU Extension Circular, PP-324, Dutch Elm Disease. Several other diseases of American elm cause symptoms which may resemble DED. These are verticillium wilt, native elm wilt, botryodiplodia canker, and wetwood.

Wilt
Several diseases may cause wilting of foliage on elms. Wilted foliage usually dries up quickly under dry North Dakota conditions. Wilting may be a symptom of Dutch elm disease. In this case the foliage almost always turns yellow before wilting and turns brown after wilting. Symptoms of DED progress down the tree. Wilted brown leaves hang on the tree (See NDSU Extension Circular PP324, Dutch Elm Disease).

Verticillium Wilt
Verticillium wilt causes a wilting very similar to Dutch elm disease but leaves do not always yellow before wilting. Often a whole branch or side of the tree may wilt suddenly and leaves may be shed from wilted branches. Laboratory diagnosis is the only sure way to distinguish DED and verticillium wilt (see "verticillium wilt" under Parasitic Diseases of Widespread Occurrence).

Native Elm Wilt or Dothiorella Wilt
Native elm wilt or Dothiorella wilt, like the two other wilts above, is caused by a fungus which grows in the vascular, water-conducting tissues of the tree. The native elm wilt fungus infects through wounds in leaves or branches. It may cause leaves to wilt or turn yellow, cause shoots to die back or form cankers in branches. Major branches or entire trees are seldom killed outright.

Control: Prune out and destroy infected branches, cutting at least a foot below affected areas. Fertilize trees of low vigor. Prevent insect injuries.

Wetwood
Wetwood is a bacterial infection in the heart of the tree. It is evidenced by discoloration of the wood and sap oozing from wounds or cracks in the bark. Virtually every elm tree is infected with wetwood; usually it is more of a nuisance than a real problem. Occasionally, individual young elms may be severely injured by wetwood. Leaves may wilt and yellow, and branches may die back. Usually these symptoms appear during or following a drought. Affected trees may recover in following seasons with adequate rainfall.

The external symptom of wetwood, an oozing of sap, is termed "slime flux." It is associated with branch stubs, pruning wounds or other stem injuries. The slime flux on the outside of the tree results when internal pressure in the wood, caused by growth of the bacteria, forces out the sap. This bacteria-laden sap supports the growth of yeasts and molds which produce the foul odor and texture on the flux area (Figure l0).

Figure 10. Wetwood slime flux on American elm. The flux is oozing from an old branch stub. (36KB b&w photo)

Wetwood sap is toxic to living plant tissue. Wounds with wetwood seldom heal completely and often become larger due to the toxic flux. Such openings may properly be termed wetwood cankers.

Wetwood is most often noticed on elm (both American and Siberian). It is also very common on cottonwood and on willows. Wetwood occurs to a lesser extent or with less obvious symptoms on birches and maples.

Severe wetwood may cause foliar symptoms and dieback of branches. On American elm and cottonwood, foliage often shows characteristic toxicity symptoms of chlorosis and scorch; ends of affected branches may die back. These symptoms seem to be more pronounced during or following dry seasons and may disappear completely in succeeding wet years.

There is no cure for wetwood. The objectionable flux may be directed away from the trunk with a tube inserted into the bark so that the flux drips to the ground instead of running down the trunk. Dealing with fluxing by use of drain tubes inserted into the wood has recently been questioned, as the drilling of drain holes may actually aid the spread of the bacteria inside the tree.

Leafspots
While several fungi cause leafspots on elms, only one is of importance. This is Stegophora (Gnomonia) ulmea, the cause of elm black leafspot (Figure 11). Infected leaves show small grayish, irregular spots on the upper leaf surface. Often these are surrounded by a yellow halo. Later the spots become black and thickened. If infection is heavy, leaves may turn yellow and fall prematurely. Elm black leafspot infects all species of elm. It is often more severe on leaves in shade and lower in the crown. Individual trees vary in susceptibility to elm black leafspot, so some trees may be heavily infected while others nearby are not.

Figure 11. Elm black leafspot. The small spots are covered by dark, raised structures (stromata) of the causal fungus. Spots are often (but not always) surrounded by a yellow ring. (23KB b&w photo)

Control: Rake up leaves in fall and burn, compost, or bury them since the fungus survives the winter in fallen leaves. This will usually give adequate control under North Dakota conditions. If elm black leafspot is severe or occurs repeatedly, fungicidal sprays may be used. If sprays are needed, use a fixed copper fungicide or Fore fungicide at 1.5 pounds or 1.2 quarts per 100 gallons of water. Apply three sprays, the first as leaf buds open, the others at 10-day intervals. Fungicidal control depends on protecting leaves before infection, hence timing is important.

Scorch
Leaf scorch is common on elm in North Dakota. (See "scorch" under Non-Parasitic Disorders). Scorch-like symptoms may indicate a root problem (see "root disturbance" under Non-Parasitic Disorders) or a wilt disease.

Herbicide Injury
Elms, especially Siberian elm, are sensitive to phenoxy herbicide drift and fumes, as from 2,4-D. The response in elm is curling and discoloration of newly developing leaves (Figure 20). Trees need not be sprayed directly to be affected. Individual elm trees vary in their sensitivity to 2,4-D. Symptoms may be evident on some trees in a row and not on others. Elms may appear to suffer no permanent injury from low levels of phenoxy herbicide exposure. Later growth appears normal and trees survive for many years. (See "pollution" under Non-Parasitic Disorders). Research in South Dakota has shown that phenoxy herbicide increases the susceptibility of Siberian elm to canker fungi.

Elm Yellows
Elm yellows (formerly called phloem necrosis), is a lethal disease of American elms. Once thought to be a virus disease, yellows is caused by phytoplasma — a tiny relative of the bacteria. The disease is transmitted by leafhoppers from tree to tree. Scattered reports of elm yellows have been noted in North Dakota and in adjacent states. Symptoms include decline and early fall color. Inner bark on infected trees has a wintergreen odor. There is no cure. Trees with elm yellows will gradually die. They never recover. Infected trees should be removed because as they become weaker they will act as "brood trees" for elm bark beetles.

Cankers
Several fungi cause cankers on elms in North Dakota. The most common are Botryodiplodia hypodermia and Tubercularia ulmea. Tubercularia cankers occur mostly on Siberian elm, while Botryodiplodia attacks both Siberian and American elm.

Botryodiplodia canker develops on the branches of American elm in natural woodlands and in landscape plantings and on Siberian elm in all situations. Infected bark becomes reddish-brown to brownish-black, watersoaked and very soft. Cambial and sapwood tissues immediately beneath infected bark become dark reddish-brown, and a distinctive line of demarcation separates healthy and infected bark. Siberian elm foliage on stems girdled by cankers initially becomes chlorotic, then wilts and dies. Adventitious sprouts frequently develop below girdling cankers and produce a witches' broom appearance. Eventually, Siberan elms may be killed. On American elm, Botryodiplodia is usually restricted to branch tips and weak shaded branches. The leaves on cankered branches turn bright yellow in mid-summer but generally do not wilt. The bright yellow "flags" caused by this canker are sometimes confused with Dutch elm disease. Cankered branches usually die back gradually from the tip. Botryodiplodia infects through wounds over a wide temperature range.

Tubercularia ulmea can develop on trunks, branches, and twigs of affected trees, especially Siberian elm. Dead branches with dead leaves still attached can indicate the presence of the canker. The surface of infected bark is reddish-brown and becomes brown to black as it dies and dries out. T. ulmea readily colonizes dead or broken branches, thus its presence does not necessarily mean it killed the branch or tree.

Control: See under Parasitic Diseases of Widespread Occurrence — Cankers.

Hackberry (Celtis sp.)

Leafspots
See Leafspots under Parasitic Diseases of Widespread Occurrence.

Scorch
See Leaf Scorch under Non-Parasitic Disorders.

Powdery Mildew
See under Parasitic Diseases of Widespread Occurrence for mildew on leaves and also "witches' broom" below.

Witches' Broom
Many people recognize hackberry by the conspicuous witches' brooms on the branches (Figure 12). These bunched twiggy growths occur as a result of a dual infection by a tiny gall mite and the powdery mildew fungus. The galls do not harm the tree but may be unsightly (or picturesque, depending on your taste).

Figure 12. Witches' broom on common hackberry. Several brooms of different sizes are shown along with remnants of old ones that have died. (21KB b&w photo)

Control: There is no practical way to control the brooming. If brooms are objectionable they can be pruned off.

Nipple Gall
Not a disease. Galls formed on underside of hackberry leaves are caused by the jumping louse. See Insect Pests of Trees and Shrubs (NDSU Extension Circular E-296).

Honey-Locust (Gleditsia triacanthos)

Winter Injury
The honeylocust and its thornless clones (`Moraine,' `Imperial,' `Skyline,' `Sunburst') are of marginal hardiness in North Dakota. Slow growing forms may be more hardy. Avoid high fertility to minimize winter injury. (See "winter injury" under Non-Parasitic Disorders).

Leafspots
Several fungal leafspots may occasionally appear on honey-locust but are of little consequence. (See "leafspots" under Parasitic Diseases of Widespread Occurrence).

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

Cankers
May be caused by Tubercularia, Nectria and Thyronectria. These cankers have caused serious problems in states where honey-locust was widely planted following the loss of elms to Dutch elm disease. Cankers occur on twigs, large branches or even the main stem. Both young and older trees are affected. Cankers appear as elongate (sometimes a foot or more), sunken, dark areas in the bark. The killed bark soon dries out and cracks, exposing the wood. Cankers continue to enlarge and may girdle the stem. Sunken dark-red cankers may also form at branch crotches or at pruning wounds. Cankers may be more common in periods of drought and on trees in stressful situations such as parking lots.

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 the hardiest cultivars, such as Imperial' and Skyline.' 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. See Cankers under Parasitic Diseases of Widespread Occurrence for more information.

Hawthorn (Crataegus spp.)

Although hawthorn is not generally considered a fruit tree, the disease problems of hawthorn are those of its closest relative, the apple, and are covered in a companion NDSU Extension Circular PP-454, Diseases of Apples and Other Pome Fruits." The most common problems of hawthorn are rust, fireblight and leafspots. See under Apple.

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PP-697 (Revised), November 1995