PP697 (Revised) 1\ III III 1\ 311~[llill~I~!rlilllr~lil 0109 00838 111111~11~lllll 8350

Deciduous Tree Diseases

R.W. Stack H.A. Lamey Professor of Plant Pathology Extension Plant Pathologist North Dakota State University North Dakota State University

NDSU LIBRARIES

111111111111111111111111111111111111111I i 1111 I11111111111111111111111111111111111111111111111111111111I111I11 ill· 1 II i III i 1111111111111111111111I IIIIIIIIIIIIIIII!I III I! 111111111111111111111111111111111111111111111111111111III!IIIIIIIIIIIII ,11111 NDSU EXTENSION SERVICE NOVEMBER 1995 North Dakota State University, Fargo, NO 58105 ~.... ble of Contents

Parasitic Diseases of Widespread Occurrence 8 Parasitic Diseases of Specific Trees (Listed alphabetically tree common name) 27 Non-Parasitic Disorders

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. 2 Parasitic Diseases of Widespread Occurrence

any tree problems are caused by the Powdery Mildew M action of one or more living organisms. • Powdery mildew is a common foliage disease on Organisms which cause disease are called patho­ nearly all kinds of trees. Powdery mildew appears gens. The most common tree pathogens are as a white growth on the surface of leaves. The powdery appearance comes from millions of tiny fungi, a large and diverse group of plants lacking spores of the causal fungus, which are spread in chlorophyll which derive their nourishment by air currents to cause new infections. Tiny black parasitizing green plants, thereby causing dots found in the powdery patches are another disease. Most fungi are microscopic, but a few, type of spore body by which the fungus survives especially the wood-rotting fungi, produce large, the winter. Trees most commonly affected by powdery mildew are linden, crabapple, catalpa and often colorful, fruiting bodies as mushrooms or chokecherry, but almost any tree or shrub, includ­ conks. Some tree diseases are also caused by ing alpine current and rose, may be infected at bacteria and viruses. some time or other. Powdery mildew is especially Some types of pathogens infect many differ­ common on lilac. Because the usually dry climate of North Dakota is unfavorable for the humidity­ ent tree species with similar disease symptoms. loving powdery mildew fungi, the disease is seldom Other pathogens attack only a specific tree or a severe problem here (Figure 1). only certain cultivars of a tree. Diseases of the Control: Occasional or late-season infections first type will be considered in this section. are seldom important enough to warrant chemical control. If necessary spray with wettable sulfur at Diseases unique to specific trees will be consid­ 2 to 4 pounds per 100 gallons (1 to 2 tablespoons/ ered under the tree name in the second section gal.) unless specified otherwise on the label. on Parasitic Diseases of Specific Trees. 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.

Figure 1. Powdery mildew on leaves of hop hornbeam.

3 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 Sooty Mold seen, with the color varying from gray to light or • Sooty mold, as the name implies, is a black dark brown to green. If the bark is peeled back, growth on the surface of leaves, especially those discolored streaks can be seen in the wood (Figure in shade. The sooty mold fungi are not plant 2). On elm, streaking caused by Verticillium re­ parasites but grow on the sweet honeydew depos­ sembles that from Dutch elm disease. A laboratory ited by various insects, especially aphids. Although test is needed to be sure which organism is unsightly, it seldom damages the tree. Sooty mold present. may occur on any tree but is most common on The Verticillium fungus invades through the boxelder, elm, linden, and maple. root system. Many tree species are susceptible, Control: Control depends on reducing the but the danger is greatest in a highly susceptible numbers of the honeydew secreting insects in tree such as maple, catalpa, elm or stone fruit. the tree. Honeydew on leaves of small trees can be The fungus attacks over 300 species of cultivated reduced by directing a stream of water over the plants. foliage at frequent intervals. There is no good control for verticillium wilt. Sometimes a tree will recover if affected branches are pruned out and proper fertilizing and watering Verticillium Wilt are provided to help promote vigorous growth. If a tree must be removed because of verticillium • Verticillium wilt, caused by the soil-borne wilt, do not plant another susceptible tree in the fungus Verticillium albo-atrum, causes light col­ same spot, since the Verticillium fungus survives ored leaves with a dull appearance in early sum­ a number of years in the soil. Plantings on land mer. The leaves then begin to drop without wilting, formerly in potatoes may be particularly likely to starting at the bases of affected branches and get verticillium wilt. progressing upward. The symptoms may involve The following trees have been reported as only one side of the tree or they may involve the resistant to verticillium wilt: entire tree. In severe cases, and on highly suscep­ Birch Betula tible trees such as sugar maple, entire branches Hornbeam Carpinus may wilt and die. Dry leaves remain attached to Hawthorn Crataegus Honey Locust Gleditsia Flowering Crabapple Malus Willow Salix Mountain-ash Sorbus Pine Pinus Spruce Picea

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.

4 Canker drought, lack of fertility, insect defoliation, or • The term "canker" is used to describe a killed herbicide damage predisposes trees to canker. area in the bark on the branch or trunk of an In general, cankers are more prevalent on young infected tree. More than 20 species of fungi that trees, on suppressed low vigor trees, or on trees at cause canker diseases are found in North Dakota. poor sites. Cankers most often occur on trees pre­ These fungi commonly invade wounded or injured disposed to infection by environmental stresses bark tissues to form a canker and subsequently such as herbicide drift, defoliation by insects or produce reproductive structures called fruiting chemicals or root damage. bodies. Spores from these fruiting bodies serve as Control: A sound control program for canker inoculum for new infections. Cankers may be diseases must be based on knowledge of the causal "superficial," forming on the outer portion of the organism, the host tree, the cultural and environ­ bark and doing little damage, or "cambial" when mental conditions under which the trees are extended into the cambium, where the bark grown, and available control methods. Control sloughs off and sapwood is exposed allowing involves both disease prevention and treatment. subsequent decay to develop in woody stem tissues. While homeowners may be able to justify control Annual cankers are active for one growing measures, they may not be economical in most season, and infected tissues are sloughed off or windbreak situations in North Dakota. healed over by callus (Figure 17, page 25). New Disease prevention involves growing vigorous cankers may form, but existing cankers cease to trees to prevent entrance of pathogens into the develop. bark. Wounds are essential for establishment of Perennial cankers are most conspicuous and most canker infections. The best preventive mea­ destructive. There are two types. In target cankers sure is to avoid unnecessary wounds, especially in (Figure 14, page 20) a succession of callus tissue the immediate area of active cankers containing repeatedly forms on canker margins over a period fungus spores. Establish new plantings on good of years. Diffuse cankers are generally oval in sites, use vigorous planting stock, fertilize trees shape and either sunken or swollen, little or no to promote growth, and control weeds for several callus is formed, and stems, twigs and branches years after planting. Improve vigor of landscape are girdled. trees by deep watering or trickle irrigation, espe­ Infections of living trees by canker fungi are cially during dry summer months, and maintain influenced by site, age, tree vigor, environmental good drainage. Protect trees from wounds or conditions and cultural practices. Stress by 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.

5 Leafspots • Localized infections of leaves ("leafspots") on When fungicides are used, they must be many trees are caused by a variety of fungi and applied prior to buildup of disease to effectively some bacteria. Positive identification usually control leafspotting fungi. If severe leafs potting requires laboratory diagnosis. Leafspot diseases and/or defoliation occurs for several years, chemi­ are seldom of sufficient importance in North cal control may be necessary, but the causal agent Dakota to warrant control measures. The few should be identified first (submit samples to the exceptions which may require control are noted NDSU Plant Diagnostic Laboratory through under specific diseases for that tree species. your county agent). Timing of protectant fungicide Leafspot fungi infect leaves by spores which sprays is critical and varies for different fungi. are dispersed by air currents or splashing rain. Correct timing is a key to effective chemical Infection usually requires moisture, such as dew, control. 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 Heart Rot tends to remain wet longer following rain or dew. • Heart rot in living trees is caused by fungi which Leafspots increase in frequency late in the have the ability to decay wood. These fungi gain summer as leaves begin to senesce (Figure 3). entrance to the wood of the tree through wounds, Buildup of leafspots on autumn leaves generally branch stubs, etc., which expose the bare wood. does not affect the health of the tree and is the The fruiting bodies, mushrooms or "conks," are beginning of the natural process of decay by which common on trunks of decaying trees. Production the leaf is returned to the soil. of fruiting bodies on a living tree is a sign of exten­ Control: Good cultural practices usually give sive decay in the stem at that point. A useful rule sufficient control. To minimize leafspot problems, of thumb might be that a cubic foot of wood has avoid crowding plantings too closely. Thin out been decayed for each conk produced. Heart rot branches to open up the tree crown, but don't top fungi do not invade living wood of healthy trees. As or dehorn. Rake up leaves in fall and bury or long as the tree is growing vigorously the rot will compost them. Plant different kinds of trees in be confined to a small central core of the trunk and mixture. Fertilize trees in spring with a complete the structural integrity of the tree will be main- fertilizer. Water trees deeply during dry spells. tained. If the tree is weakened for any other reason or fresh wood exposed by severe pruning or storm dam­ age, then the decay fungi can advance to more and more wood Figure 3. Fungal (Figure 4). When this leafspot on green ash. Compare the irregular happens the tree may distribution to the become unsafe and the regular pattern risk of wind or storm of environmental scorch shown in breakage greatly Figures 18 and 19. increases.

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

Figure 4. Cottonwood branch with heart rot. The entire discolored center of the stem is decayed. The decay started from an untrimmed branch stub.

Figure S. 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.

7 Parasitic Diseases of Specific Trees

Apple (Malus spp.) The most common and severe diseases of develop on the leaves. Later, these may become apple are fireblight, scab and rust. Short descrip­ scabby in appearance. Severe scab can result in tions follow. For more complete information on leaves yellowing and premature leaf drop. Fruits apple diseases, see Circular PP-454, Diseases of may also be infected; if the infection occurs before Apples and Other Pome Fruits, available from your the fruits are full sized, the fruits will be distorted. county agent. The apple scab fungus survives the winter on Fireblight is a common disease of apples, fallen apple leaves. Infection is favored by rainy crabapples and mountain ash. Shoots become weather. curled at the tip and blackened as if scorched by Control. Rake and destroy fallen apple and fire. Blossoms may also wilt, fruiting spurs may crabapple leaves in the fall. To control apple scab turn black and be killed, and suckers may be infections during the summer, captan or benomyl curled and blackened. plus captan fungicides can be used on apples, and Fireblight is caused by a bacterium that chlorothalonil can be used on ornamental invades the water conducting tissues of the tree. crabapples. It overwinters in cankers on larger branches. Rust produces yellow orange spots on leaves These often form around a diseased sucker or of apples, crabapples, hawthorn and juneberry. fruiting spur. The bark on cankers is slightly Tiny cup-like structures develop on the undersides sunken and discolored. of leaves. Juniper is the alternate host. On juniper Control. Prune out diseased cankers in late the rust forms galls on the branches and twigs. winter. Sterilize pruning tools with 10% bleach, Wet spring weather favors the formation of gel­ full strength Pine Sol, Lysol (use the type with the atinous horns on the galls. These horns release red label), or denatured ethyl alcohol (eg. shellac spores. Spores from juniper infect apples and thinner). Bleach and Pine Sol are corrosive; be sure related hosts; spores from apples and related to wash and oil pruning tools after using these hosts infect juniper. products. Streptomycin should be applied every Control. Pick galls off juniper in the spring to four to five days at blossom time to apples if day­ break the fungus life cycle. Don't mix junipers and time temperatures exceed 65 degrees Fahrenheit. susceptible crabapples in the same planting. Sulfur Scab is common on apples and crabapples fungicide may be used on apples and chlorothalonil following wet weather. Feathery olive brown spots may be used on ornamental crabapple. Other products are available to commercial orchardists.

Apricot • See under Cherry.

8 Ash (Green, Black, Manchurian, 'Marshall,' do its job of protecting new growth. Three applica­ 'Summit') (Fraxinus spp.) tions are best. Apply the first spray just as buds Anthracnose is the most common malady are beginning to swell but before bud break. Apply of green ash in North Dakota. Ash anthracnose the second spray when buds show green tips and is caused by the fungus Gnomoniella fraxini. It the third when leaves are half grown. The first infects buds, leaves and sometimes twigs. Spring spray should be lime sulfur; the other two sprays infection of buds or expanding leaves causes should be Daconil 2787 Flowable at 2 pints per necrosis and distortion of leaves, the most typical 100 gallons of spray solution (two-thirds table­ symptom of ash anthracnose. Often such leaves spoon per gallon) or Daconil Ultrex at 1.4 pounds have irregular brown blotches associated with leaf per 100 gallons. veins (Figure 6). Whole shoots may be stunted and Scorch. See under Non-Parasitic Disorders. the leaves deformed or killed. Infections occurring Rust. Ash rust, caused by Puccini a spar­ later, after leaves have expanded, cause small ganioides, is more of a curiosity than a real prob­ brown circular lesions which may later expand lem. Ash rust attracts attention because of the or coalesce. bright orange color of the fungus fruiting struc­ In some years anthracnose is associated with tures and the deformities which it causes on extensive defoliation of ash trees in Mayor June. petioles and leaves. This rust alternates its hosts. The exact conditions under which defoliation The orange spores produced on ash do not reinfect occurs are not known, nor are the reasons why ash but rather infect a wild grass (cordgrass, it occurs in some years and not others. Spartina spp.). The spores produced in fall on the The ash anthracnose fungus develops best at grass are responsible for infection of ash leaves temperatures of 60 to 70 F. Wet weather in May the following spring. Since cordgrass often grows favors anthracnose development. in wet areas or ditches, ash trees in the vicinity Control: Rake up and destroy (burn, bury or may show more severe rust. compost) leaves in autumn to reduce the disease Control: No control is needed on established carryover. Prune out cankered branches and trees. remove twiggy growth to promote air movement Heart Rots. Several wood-rotting fungi within the crown. Fertilize trees in spring to attack living green ash trees. The most common is promote vigorous growth. Fomes fraxinophilus. Two other fungi, Polyporus Where trees are attacked by anthracnose sulfureus and Phellinus punctatus, also cause repeatedly, protectant sprays may be desired. heart rots in living ash trees in North Dakota. Timing of fungicide application is critical if it is to 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 Figure 6. Ash anthracnose. Leaves show distortion and killed areas typical of ash anthracnose.

9 Figure 7. Conk of Fames fraxinophilus. The cause of ash heart rot, this fungus is widespread, especially on native trees. Presence of conks indicates extensive decay of the wood.

by Fomes fraxinophilus (Figure 7). Infected trees simple or, if compound, the number of leaflets may on such sites often have serious structural defects be reduced. The sprouts often are only about 15-18 and will need to be removed for safety. See "heart inches long, although in some cases they may be rot" under Parasitic Diseases of Widespread Occur­ several feet long. rence at the beginning of this circular for manage­ Since only a small portion of trees show ment. epicormic sprouts or witches' brooms and many Leafspots. Several fungi cause leafspots trees show no obvious symptoms, a laboratory test on ash. None merit chemical control under ordi­ is required to confirm presence of ash yellows. nary circumstances, except in nurseries. See Several laboratory tests have been used. One of "leafspots" under Parasitic Diseases of Widespread these, called the DAFI test, is quick and reliable, Occurrence at the beginning of this circular. especially if used on a root sample. This test is Ash Yellows is a new name for a problem being used for diagnosis of the disease in Minne­ that has been around for many years. For more sota and North Dakota. Information on this test than 50 years foresters have described a slow is available from your state's plant diagnostic decline of ash trees. Although environmental laboratory. factors may cause a slow decline, ash yellows can Little is known about the means of spread of also cause such a decline in individual trees. The this disease, although insect transmission is likely. disease, caused by a phytoplasma-like organism, Infected green ash trees which lack severe symp­ is common in southern Wisconsin and Minnesota, toms will likely survive for many years if given has been observed in southeastern North Dakota, good culture, watering and fertilization. The and one infected tree has been observed in the disease on green ash is generally far less severe Badlands of southwestern North Dakota. than on the highly susceptible white ash of the Infected trees usually show slower growth, eastern United States. about 70 percent of normal. They may also show decline, die back and occasionally may develop dense clusters of short, twiggy branches known Aspen as witches' brooms. • See under Poplar. Epicormic sprouts near the base of the tree occur in a small portion of infected trees. When present, these sprouts are diagnostic. Clusters of Basswood spindly twigs develop in the epicormic sprouts. The • See under Linden. 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 10 Birch (Betula sp.) Two birches are commonly planted in North not used again. Milder effects of herbicides may Dakota. One is the paper birch (B. papyrifera), be leaf distortion, chlorosis or scorch. native in Minnesota and in some forested areas of Birches may also show effects of phenoxy North Dakota. The other is the introduced Euro­ herbicide drift. See "herbicide injury" under Non­ pean white birch (B. alba) and its cultivated forms Parasitic Disorders. (cutleaf weeping birch). North Dakota conditions Heart Rots. The wood of birch is very are not the best for growth and longevity of decay-susceptible. Several fungi cause rot in living birches. Even where paper birch occurs in North trees (Figure 8). The most common are Fomes Dakota as a native, its development and lifespan fomentarius, Daedalia spp., and Polyporus here are less than the optimum for the species. squamosus. See "heart rots" under Parasitic Dis­ Leafspots. Several fungi cause leafspots eases for symptoms and control. Because birch or blotches on birch. None merit chemical control. wood is so decay susceptible, it is very important to See "leafspots" under Parasitic Diseases of Wide­ avoid large wounds such as are caused by removal spread Occurrence at the beginning of this circular. of major branches. Do not attempt to head back Chlorosis. Chlorosis or leaf yellowing is a large birch trees. Birch trees showing dieback and common malady of birches in North Dakota. See having conks or mushrooms of decay fungi on the "chlorosis" under Maple. trunks probably are safety hazards and should be Scorch. Birches are susceptible to leaf removed. Consult your city forester or a trained scorch, which may appear on leaf margins or at the commercial arborist. tips (see "scorch" under Non-Parasitic Disorders). Decline. "Decline" is a term used by pa­ Scorch on birch may be associated with wetwood thologists and arborists to describe a gradual dying or with decline (see "decline" below). of trees where no single cause is obvious. Birch Herbicide Injury. Birches are very decline has been reported in the woodland areas sensitive to herbicides applied to soil, especially of the Great Lakes states in native birch stands. dicamba (Banvel). This chemical may be used in A similar type of gradual dying has been observed some lawn fertilizer preparations. Heavy applica­ in older birch trees in North Dakota cities. tion of dicamba may cause rapid, complete defolia­ Although the exact cause of birch decline is tion. Such defoliated trees may not die and may not known, it appears to be set off by drought, leaf out normally the next year if the chemical is 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 Figure 8. dying branches, but it Fornes fomentarius conk seems likely that the on white birch. fungi are only attack­ Sometimes called "horse's hoof" or ing the already weak­ "false tinder conk." ened tree. Presence of several conks indicates extensive decay.

11 The bronze birch borer is attracted to weak­ sons in North Dakota, late season appearance of ened birch trees, and once it becomes established it leaf blotches is not important and adequate control often kills the tree. Identification and control of will be obtained by removal of fallen leaves. Leaf bronze birch borer is described in NDSU Extension blotch may be confused with scorch. Circular E-296, Insect Pests of Trees and Shrubs. Powdery Mildew. Two species of pow­ One important source of bronze birch borer infesta­ dery mildew sometimes infect buckeyes. (See tion in eastern North Dakota may be birch fire­ "powdery mildew" under Parasitic Diseases). wood imported from adjacent forest areas where Winter Injury. The true horse-chestnut natural borer populations are high. (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 Boxelder killing each year - if it survi yes at all. The tree • See under Maple. planted in North Dakota as buckeye (A. glabra) is hardy and also more variable. Many hardy indi­ viduals are found growing well in North Dakota. Buckeye (and Horse-chestnut) Even so, freezing injury may occur in some winters (Aesculus sp.) (see "winter damage" under Non-Parasitic Disor­ ders). Winter-killed shoots should be pruned back Scorch. By late summer some leaf scorch only after new growth has started. Because most must be considered usual on buckeye in North "Ohio buckeyes" are really of hybrid parentage, Dakota. See "scorch" under Non-Parasitic Disor­ some individual trees may be winter-injured more ders. Symptoms of verticillium wilt may resemble or less often than others. scorch. Scorch is sometimes confused with leaf Verticillium Wilt. Buckeye is susceptible blotch. to verticillium wilt. Mild symptoms may resemble Leaf Blotch. Buckeye leaf blotch or leaf scorch. More severe symptoms may result in blight is caused by the fungus Guignardia aesculi death of one or more branches. (See "verticillium (Figure 9). The blotches start as small irregular wilt" under Parasitic Diseases of Widespread reddish-brown spots with yellow margins. These Occurrence). later enlarge and coalesce. Premature leaf drop may also occur. Minute black fruiting bodies of the fungus are found in the brown blotches. Butternut Control: Gather and dispose of fallen leaves • See under Walnut. by burying, composting, or burning. In most sea-

Figure 9. Guignardia leaf blotch. This is common on both horse chestnut and buckeye.

12 Catalpa (Catalpa sp.) In milder climates, the catalpa is a showy Several related Taphrina species infect stone ornamental tree. Most of North Dakota is at the fruits. Spores are dispersed in early spring and margin of catalpa's hardiness range, although become lodged in bud scales and other small some trees survive well. Winter injury, especially crevices and remain dormant until bud break the die back of shoots and buds, is the most common following spring. problem with the tree in our area. Catalpa is Control. Use a delayed dormant spray just susceptible to leafspots, powdery mildew, and before bud break. Lime sulfur can be applied in verticillium wilt, all of which are discussed under spring once the temperatures are above freezing, Parasitic Diseases of Widespread Occurrence. or Bordeaux mixture can be applied. Lime sulfur can also be applied in the fall after leaf drop. X Disease is currently epidemic through­ Cherry (Prunus spp.) out the northern Great Plains. Many native and The most common and severe diseases of planted groves of chokecherries have been wiped cherry and chokecherry are black knot, plum out. X disease is caused by a phytoplasma which pockets and X disease. Short descriptions follow. becomes systemic in the plant. The disease is For more complete information on diseases of spread by leafhoppers and takes several years to cherries and chokecherries, see Circular PP-689, develop. First symptoms are early fall color. In Disease Control in Cherries, Plums, and Other subsequent seasons plants show bronzing or Stone Fruits, available from your county agent. reddening of leaves in mid summer. Shoot dieback Black Knot is easily recognized by occurs and eventually the plants die. There is the hard black swellings or "knots" on smaller no cure. Do not replant chokecherries near branches. These knots eventually girdle and kill affected plants. 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 Cottonwood weather. • See under Poplars. Control. Prune all developing knots at blos­ som time, cutting 4-5 inches below the knot. Annual pruning may be required for several years Crabapple to achieve control. Best control is achieved by • See under Apple. 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 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.

13 Elm (Ulmus spp.) (American or white elm, red elm, Verticillium Wilt causes a wilting very Siberian or so-called "Chinese" elm) similar to Dutch elm disease but leaves do not Despite many problems with disease and always yellow before wilting. Often a whole branch injury, the elms, especially American and Siberian or side of the tree may wilt suddenly and leaves elms, are by far the most commonly grown trees in may be shed from wilted branches. Laboratory North Dakota. diagnosis is the only sure way to distinguish DED Dutch Elm Disease. This lethal disease and verticillium wilt (see "verticillium wilt" under of American elm was first found in North Dakota Parasitic Diseases of Widespread Occurrence). in 1969. At the present time it has spread through­ Native Elm Wilt or Dothiorella Wilt, out North Dakota. Because of its importance, like the two other wilts above, is caused by a fun­ Dutch elm disease (DED) is the subject of a special gus which grows in the vascular, water-conducting NDSU Extension Circular, PP-324, Dutch Elm tissues of the tree. The native elm wilt fungus Disease. Several other diseases of American elm infects through wounds in leaves or branches. cause symptoms which may resemble DED. These It may cause leaves to wilt or turn yellow, cause are verticillium wilt, native elm wilt, botryodip­ shoots to die back or form cankers in branches. lodia canker, and wetwood. Major branches or entire trees are seldom killed Wilt. Several diseases may cause wilting of outright. foliage on elms. Wilted foliage usually dries up Control: Prune out and destroy infected quickly under dry North Dakota conditions. Wilt­ branches, cutting at least a foot below affected ing may be a symptom of Dutch elm disease. In areas. Fertilize trees of low vigor. Prevent insect this case the foliage almost always turns yellow injuries. before wilting and turns brown after wilting. Wetwood. Wetwood is a bacterial infection Symptoms of DED progress down the tree. Wilted in the heart of the tree. It is evidenced by discol­ brown leaves hang on the tree (See NDSU Exten­ oration of the wood and sap oozing from wounds sion Circular PP-324, Dutch Elm Disease). 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, Figure 10. Wetwood slime flux individual young elms on American elm. may be severely in­ The flux is oozing from jured by wetwood. an old branch stub. Leaves may wilt and yellow, and branches may die back. Usually these symptoms ap­ pear during or follow­ ing a drought. Affected trees may recover in following seasons with adequate rainfall.

14 The external symptom of wetwood, an oozing inserted into the wood has recently been ques­ of sap, is termed "slime flux." It is associated with tioned, as the drilling of drain holes may actually branch stubs, pruning wounds or other stem injur­ aid the spread of the bacteria inside the tree. ies. The slime flux on the outside of the tree results Leafspots. While several fungi cause leaf­ when internal pressure in the wood, caused by spots on elms, only one is of importance. This is growth of the bacteria, forces out the sap. This Stegophora (Gnomonia) ulmea, the cause of elm bacteria-laden sap supports the growth of yeasts black leafspot (Figure 11). Infected leaves show and molds which produce the foul odor and texture small grayish, irregular spots on the upper leaf on the flux area (Figure 10). surface. Often these are surrounded by a yellow Wetwood sap is toxic to living plant tissue. halo. Later the spots become black and thickened. Wounds with wetwood seldom heal completely If infection is heavy, leaves may turn yellow and and often become larger due to the toxic flux. fall prematurely. Elm black leafspot infects all Such openings may properly be termed wetwood species of elm. It is often more severe on leaves in cankers. shade and lower in the crown. Individual trees Wetwood is most often noticed on elm (both vary in susceptibility to elm black leafspot, so some American and Siberian). It is also very common on trees may be heavily infected while others nearby cottonwood and on willows. Wetwood occurs to a are not. lesser extent or with less obvious symptoms on Control: Rake up leaves in fall and burn, birches and maples. compost, or bury them since the fungus survives Severe wetwood may cause foliar symptoms the winter in fallen leaves. This will usually give and dieback of branches. On American elm and adequate control under North Dakota conditions. If cottonwood, foliage often shows characteristic elm black leafspot is severe or occurs repeatedly, toxicity symptoms of chlorosis and scorch; ends of fungicidal sprays may be used. If sprays are affected branches may die back. These symptoms needed, use a fixed copper fungicide or Fore fungi­ seem to be more pronounced during or following cide at 1.5 pounds or 1.2 quarts per 100 gallons of dry seasons and may disappear completely in water. Apply three sprays, the first as leaf buds succeeding wet years. open, the others at lO-day intervals. Fungicidal There is no cure for wetwood. The objection­ control depends on protecting leaves before able flux may be directed away from the trunk infection, hence timing is important. with a tube inserted into the bark so that the flux Scorch. Leaf scorch is common on elm in drips to the ground instead of running down the North Dakota. (See "scorch" under Non-Parasitic trunk. Dealing with fluxing by use of drain tubes Disorders). Scorch-like symptoms may indicate a root problem (see "root disturbance" under Figure 11. Non-Parasitic Disor­ Elm black leafspot. ders) or a wilt disease. The small spots are Herbicide covered by dark, raised structures Injury. Elms, espe­ (stromata) of the causal cially Siberian elm, are fungus. Spots are often sensitive to phenoxy (but not always) surrounded herbicide drift and by a yellow ring. fumes, as from 2,4-D. The response in elm is

15 curling and discoloration of newly developing frequently develop below girdling cankers and leaves (Figure 20, page 29). Trees need not be produce a witches' broom appearance. Eventually, sprayed directly to be affected. Individual elm Siberan elms may be killed. On American elm, trees vary in their sensitivity to 2,4-D. Symptoms Botryodiplodia is usually restricted to branch tips may be evident on some trees in a row and not and weak shaded branches. The leaves on can­ on others. Elms may appear to suffer no perma­ kered branches turn bright yellow in mid-summer nent injury from low levels of phenoxy herbicide but generally do not wilt. The bright yellow "flags" exposure. Later growth appears normal and caused by this canker are sometimes confused with trees survive for many years. (See "pollution" Dutch elm disease. Cankered branches usually die under Non-Parasitic Disorders). Research in back gradually from the tip. Botryodiplodia infects South Dakota has shown that phenoxy herbicide through wounds over a wide temperature range. increases the susceptibility of Siberian elm to Thbercularia ulmea can develop on trunks, canker fungi. branches, and twigs of affected trees, especially Elm Yellows. Elm yellows (formerly called Siberian elm. Dead branches with dead leaves still phloem necrosis), is a lethal disease of American attached can indicate the presence of the canker. elms. Once thought to be a virus disease, yellows The surface of infected bark is reddish-brown is caused by phytoplasma - a tiny relative of the and becomes brown to black as it dies and dries bacteria. The disease is "transmitted by leafhoppers out. T. ulmea readily colonizes dead or broken from tree to tree. Scattered reports of elm yellows branches, thus its presence does not necessarily have been noted in North Dakota and in adjacent mean it killed the branch or tree. states. Symptoms include decline and early fall Control: See under Parasitic Diseases of color. Inner bark on infected trees has a winter­ Widespread Occurrence - Cankers. green odor. There is no cure. Trees with elm yel­ lows will gradually die. They never recover. In­ fected trees should be removed because as they Hackberry (Celtis sp.) become weaker they will act as "brood trees" for Leafspots. See under Parasitic Diseases elm bark beetles. of Widespread Occurrence. Cankers. Several fungi cause cankers Scorch. See under Non-Parasitic Disorders. on elms in North Dakota. The most common are Powdery Mildew. See under Parasitic Botryodiplodia hypodermia and Thbercularia Diseases of Widespread Occurrence for mildew on ulmea. Tubercularia cankers occur mostly on leaves and also "witches' broom" below. Siberian elm, while Botryodiplodia attacks both Witches' Broom. Many people recognize Siberian and American elm. hackberry by the conspicuous witches' brooms on Botryodiplodia canker develops on the the branches (Figure 12). These bunched twiggy branches of American elm in natural woodlands growths occur as a result of a dual infection by a and in landscape plantings and on Siberian elm tiny gall mite and the powdery mildew fungus. The in all situations. Infected bark becomes reddish­ galls do not harm the tree but may be unsightly brown to brownish-black, watersoaked and very (or picturesque, depending on your taste). soft. Cambial and sapwood tissues immediately Control: There is no practical way to control beneath infected bark become dark reddish-brown, the brooming. If brooms are objectionable they can and a distinctive line of demarcation separates be pruned off. healthy and infected bark. Siberian elm foliage on stems girdled by cankers initially becomes chlo­ rohc, then wilts and dies. Adventitious sprouts

16 Figure 12. Witches' broom on common hackberry. Several brooms of different sizes are shown along with remnants of old ones that have died.

Nipple Gall. Not a disease. Galls formed on pruning wounds. Cankers may be more common underside of hackberry leaves are caused by the in periods of drought and on trees in stressful jumping louse. See Insect Pests of Trees and situations such as parking lots. Shrubs (NDSU Extension Circular E-296). Control: The best control for cankers is to prevent conditions which favor their development. Avoid wounds or injuries. Protect young trees from Honey-Locust winter injury. Plant only the hardiest cultivars, (Gleditsia triacanthos) such as Imperial' and Skyline.' Water and fertilize Winter Injury. The honeylocust and its young trees to promote vigorous growth. Prune off thornless clones ('Moraine,' 'Imperial,' 'Skyline,' cankered branches and destroy them by burying 'Sunburst') are of marginal hardiness in North or burning. Sterilize tools between cuts. Examine Dakota. Slow growing forms may be more hardy. nursery stock carefully before purchase. Avoid Avoid high fertility to minimize winter injury. (See buying trees with wounds or abrasions on bark. "winter injury" under Non-Parasitic Disorders). See under Parasitic Diseases of Widespread Leafspots. Several fungal leafspots may Occurrence for more information. occasionally appear on honey-locust but are of little consequence. (See "leafspots" under Parasitic Diseases of Widespread Occurrence). Hawthorn (Crataegus spp.) Powdery Mildew. See under Parasitic Although hawthorn is not generally consid­ Diseases of Widespread Occurrence. ered a fruit tree, the disease problems of hawthorn Cankers. May be caused by Tubercularia, are those of its closest relative, the apple, and are Nectria and Thyronectria. These cankers have covered in a companion NDSU Extension Circular caused serious problems in states where honey­ PP-454, Diseases of Apples and Other Pome locust was widely planted following the loss of elms Fruits." The most common problems of hawthorn to Dutch elm disease. Cankers occur on twigs, are rust, fireblight and leafspots. See under large branches or even the main stem. Both young Apple. 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

17 Hop Hornbeam or Powdery Mildew, Sooty Mold. See Parasitic Diseases of Widespread Occurrence. Ironwood (Ostrya sp.) Herbicide Injury. Linden is less sensitive The hop hornbeam is a North Dakota native to 2,4-D than boxelder or Siberian elm but may which deserves to be planted more widely. It has suffer damage if sprayed directly. Leaf curling or few disease problems. Occasionally leafspots, distortion is the usual symptom. mildew or scorch may be of minor importance. Linden is quite sensitive to injury from dicamba CBanvel), an herbicide often included in lawn fertilizer mixtures. Symptoms of dicamba Horse Chestnut irDury are leaf-cupping or leaf distortion, including • See Buckeye. leaves that are thicker and darker green than normal. Because this chemical is in the soil, the effects may persist for a long period. Ironwood Control: Do not use fertilizers containing • See Hop Hornbeam. dicamba around or under linden trees . Scorch. Lindens are quite susceptible to scorch on the leaf margins (Figure 19, page 28). Kentucky Coffee Tree See "scorch" under Non-Parasitic Disorders. Verticillium Wilt. Linden is susceptible (Gymnocladus sp.) to verticillium wilt although reports from North Few diseases have been reported on this tree. Dakota are rare. See under Parasitic Diseases of Several leafs pots (see Parasitic Diseases of Wide­ Widespread Occurrence. spread Occurrence) are known to occur, but none is Winter Injury. Lindens, especially young considered serious. This is one of the most disease­ trees, may be subject to both sunscald and freezing free trees suitable for planting in North Dakota. damage. See under Non-Parasitic Disorders. Such Winter injury (see Non-Parasitic Disorders) may be injuries may provide infection sites for canker­ a problem, although some large trees are found in causing fungi. eastern North Dakota. Trees grown from reliable Cankers. Canker fungi - especially seed sources should be winter hardy. Cytospora sp. - are sometimes associated with dying branches or with young trees which have died. Many of these canker-causing fungi do not Linden or Basswood attack healthy established trees but can attack (Tilia sp.). following injuries, sunscald, etc. - especially on newly transplanted or weakened trees. Leafspots. Several leafspots and a leaf Control: The best control for cankers is to blight or anthracnose have been reported on linden prevent conditions which favor their development. but none is serious in North Dakota. See Avoid wounds or injuries. Protect young trees from "leafs pots" under Parasitic Diseases. 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 be­ tween cuts. Examine nursery stock carefully before purchase. Avoid buying trees with wounds or abrasions on bark.

18 Maple (Acer spp.) (Silver, A. saccharinum; sugar, A. saccharum; Norway, A. platanoides; phenoxy-type herbicides such as 2,4-D. Low level red, A. rubrum; Amur, A. ginnala; exposure produces leaf distortion while higher mountain, A. spicatum; boxelder, A. levels kill foliage. Such trees have a scorched negundo) appearance as if burned by flame. These extreme The maples are a large diverse group of trees, symptoms are common in remaining hedgerows only a few of which are grown in North Dakota. alongside grain fields, but most of the boxelders Winter Injury. The sugar, silver, and Amur in shelter plantings have succumbed to this prob­ maples and the boxelder are winter hardy in North lem. See "pollution injury" under Non-Parasitic Dakota; even so, young trees may be damaged by Disorders. sunscald if not protected. Many cultivars of N or­ Leafspots. Severalleafspot fungi attack way maple are of questionable hardiness and may maples (Figure 13). See "leafspots" under Para­ suffer severely from winter injury, especially when sitic Diseases of Widespread Occurrence. The heavily fertilized and watered. See "winter injury" ocellate leaf gall insect causes a circular spot on under Non-Parasitic Disorders. maple leaves which is often confused with a Poor Growth, Chlorosis, Dieback of fungal leafs pot. red and sugar maples. Although their native range Leaf Galls. Several species of mite cause includes adjacent northern Minnesota and Canada, leaf galls on maple which may be confused with these species often do not succeed when planted in a disease. These galls may look like spines, or may North Dakota. Trees or saplings collected from the be woolly or felty patches on the leaf surface and northwest extremity of the native range may be of different colors, often bright red, (see NDSU better adaped to the Plains environment. This non­ Extension Circular E-296, Insects of Trees and specific disorder appears to be a root/soil problem. Shrubs, for description and controls). Neither species likes alkaline soils. There may also Powdery Mildew. See under Parasitic be problems with the ability of the essential symbi­ Diseases of Widespread Occurrence. otic root-fungus associations called mycorrhizae to Sooty Mold. Sooty mold is especially form in North Dakota soils. common on boxelder. See under Parasitic Diseases Herbicide Injury (Boxelder blight). of Widespread Occurrence. Boxelder is one of the most sensitive of all trees to Verticillium Wilt. Maples are especially susceptible to verticil­ lium wilt. See under Figure 13. Parasitic Diseases of A leafspot of boxelder Widespread Occur­ caused by Cristulariella rence. pyramidalis. The large white circular spots Chlorosis. The show concentric zones. symptom called chloro­ sis is a yellowish color­ ation of the foliage. The leaves may turn pale yellow-green to bright yellow, depend­ ing on the tree species and the severity of the

19 problem. The leaves may be uniformly yellowish or, Cankers. The fungus Eutypella parasitica more often, the veins are green while the area attacks maples and boxelder causing large peren­ between is yellow. Severe chlorosis may lead to nial cankers on trunks or main branches. On sugar scorch on leaf margins. and Norway maple, eutypella cankers are elongate Chlorosis is usually caused by lack of some with conspicuous ridges of callus tissue around the nutrient, especially iron or manganese, in the sides (Figure 14). On sugar maple these cankers leaves. This deficiency may be due to lack offertil­ seldom girdle the stem but the enlarged, distorted ity, but may also result because the element, while growth around the canker and subsequent expo­ present in the soil, is unavailable to the roots. This sure of wood to decay fungi may structurally condition is especially common in alkaline soils weaken the tree. On Norway maple, eutypella stem such as occur over most of North Dakota. cankers can completely encircle the trunk causing Birches and maples seem especially prone top dieback. Cankers on Norway maple are often to chlorosis, and should be avoided if high soil pH associated with frost crack or winter sunburn of problems are known to occur. Chlorosis is ex­ the lower trunk. tremely common on silver maples. Some trees may On boxelder, eutypella cankers tend to be show severe chlorosis year after year and yet make sunken and rounded, often nearly circular, while good growth otherwise. Individual trees vary in on Norway maple cankers are very elongate, appearance of chlorosis even when grown under especially when associated with cracks in the stem. identical conditions. No reason for this variability In contrast to sugar maple, where bark on the is known, other than genetic variation. canker face sloughs off, bark on faces of eutypella Control: Foliar applications of iron chelate or cakers on Boxelder and Norway maple remains other iron-containing compounds will sometimes attached. temporarily correct the chlorosis. Where practical, Control: Protect stem wounds and pruning adjustment of the soil pH and proper application stubs. Small cankers may be carefully excised. of micronutrient fertilizer may offer the best long­ When selecting those trees to leave on a new term solution. Micronutrient capsules implanted in building lot, reject any trees with large cankers. the trunk may provide a remedy lasting for several When purchasing Norway maple nursery stock, seasons but mixed results have been reported from examine the trunk carefully for canker infection. this therapy. Wood Decay and Heart Rots. The wood of maples, especially silver maple and boxelder, is very decay

Figure 14. Eutypella susceptible. Any canker on a sugar maple pruning wound or stem. This perennial injury which exposes canker causes the the wood is an avenue distortion shown here. Concentric rings on the for entry of decay canker face indicate fungi. Avoid major sur­ successive years of gery whenever possible growth. as large wound faces heal slowly and readily admit decay fungi. See "heart rots" under Parasitic Diseases of Widespread Occurrence.

20 The bright red coloration often seen in the ing on the source and time of infection. There are wood of boxelder is caused by a fungus, Fusarium three phases of this disease, twig blight, shoot lateritium, which infects the wood. This fungus blight and leaf blight. In the twig blight stage, does not affect the soundness of the wood, but its buds are killed before they can begin to grow. presence often indicates decay fungi are also Later, shoots may grow out from lower down on the present (Figure 5, page 7). branch. When this occurs repeatedly, "witches' Wetwood. See under Elm. brooms" or clusters of dead twigs may occur at ends of branches (Figure 15). In the shoot blight phase, new shoots are killed while they are ex­ Mayday Tree (Prunus sp.) panding. Often blighted shoots appear scorched as • See under cherry. 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 Mountain Ash (Sorb us sp.) of ash anthracnose, Figure 6, page 9). If infection occurs after leaves have expanded, The mountain ash is a member of the rose circular to irregular brown spots may develop on family and closely related to apple and pear. Many the leaf. These spots may enlarge or coalesce in of the same diseases such as fireblight and cankers wet weather, blighting large areas of the leaf. occur on both mountain ash and apple. See under Often all these phases of anthracnose are present apple. together on different branches of the same tree. Weather conditions in spring, when new growth is occurring, are the most important factors (Quercus sp.) Oak determining the severity of oak anthracnose. Mean Bur oak (Q. macrocarpa) is the common daily temperatures between 50 and 57 F during native oak in North Dakota. shoot growth are most favorable for oak anthra­ Anthracnose, caused by the fungus cnose infection. Cool, wet, late springs favor an­ Gnomonia veneta, is a serious disease of oaks, thracnose development. incl uding the bur oak. The twiggy habit of bur oak The anthracnose fungus is almost always is largely due to repeated infection by anthracnose. present, even though weather may be unfavorable The oak anthracnose fungus overwinters in for serious disease development. For this reason a fallen leaves, infected shoots, and in buds. The serious epidemic can occur even though symptoms disease shows several different symptoms depend- have been virtually absent for several years. Control: Re­ peated anthracnose Figure 15. attacks seldom perma­ ''Witches' brooms" on bur oak caused by nently damage estab­ oak anthracnose. lished bur oaks; in Repeated killing of fact, the anthracnose­ new shoots produces the twiggy branching caused, gnarled, twiggy shown here. growth is sometimes considered picturesque and characteristic of the trees.

21 Young or recently transplanted trees and Powdery Mildew. See under Parasitic trees in nurseries may need protection from an­ Diseases of Widespread Occurrence. thracnose. Spray protectant fungicides at properly Sooty Mold. Common on bur oak. See timed intervals to prevent anthracnose infection. A under Parasitic Diseases of Widespread Occur­ dormant spray of lime sulfur at 1 gallon (30 per­ rence. cent) per 10 gallons (1.5 cups per gallon) or copper Scorch. See under Non-Parasitic Disorders. Bordeaux at label rate may help prevent bud Oak Wilt. Oak wilt is an important and blight. Do not apply lime sulfur after bud break. serious disease of oaks in Wisconsin, Iowa and Rake up and destroy (burn, bury, or compost) southeastern Minnesota. The nearest known leaves in the fall. Broomed or cankered twigs may locations of oak wilt are in Minnesota in counties be pruned out. Fertilize and water to promote around St. Cloud. Oak wilt has never been vigorous growth. found in North Dakota or in the northwest­ Oak Leaf Blister. This disease may go ern part of Minnesota. unnoticed unless a large number of leaves are All oaks can be infected by the oak wilt fungus severely infected or leaves begin to fall prema­ but they vary in susceptibility. The red oaks are turely. The initial symptom is a slight yellowing of most susceptible and rapidly killed. White oaks are infected leaf tissue, followed by the formation of moderately resistant while bur oak is intermedi­ circular raised blisters on the leaves. Blisters form ate. when infected cells are stimulated to enlarge, A drought-induced decline of oak in the Red while surrounding noninfected cells remain rigid. River Valley and adjacent parts of northern Minne­ Blisters are usually less than an inch in diameter, sota sometimes appears similar to oak wilt symp­ and the lower surface will appear gray as the toms. A laboratory diagnosis is necessary to con­ fungus develops in the leaf tissue. Multiple infec­ firm the presence of oak wilt. Suspect trees should tions cause a single leaf to become distorted. have samples taken as for Dutch elm disease (see Premature defoliation may occur in early fall. The NDSU Extension Circular PP-324, Dutch Elm fungus survives on bud scales and in bark crevices. Disease) and submitted to the appropriate state Control: Particularly valuable ornamental laboratory for diagnosis. trees may warrant treatment with a fungicide, Decline. A slow decline or gradual deterio­ which must be applied as a dormant spray to be ration of bur oaks is a common problem in newly effective. Use lime-sulfur at 10 tablespoons per built-up areas where the oaks were native. At first, gallon in the fall or in spring before bud break. leaves may be scorched or fewer than before; later, Fungicides are not effective after leaves begin to twigs and then branches die and eventually the develop because infection has already occurred. tree becomes unsightly and is removed. Collecting and disposing of infected leaves has no Although disease or insects may playa role in effect on the disease. the final death of such trees, the real cause of the Leafspots. Several fungi cause spotting of decline is root disturbance. Oaks are particularly oak leaves. Few are serious in North Dakota. See sensitive to even small disturbances of their root "leafspots" under Parasitic Diseases of Widespread systems. Even slight changes in drainage caused Occurrence. Note: compare the leafspot phase of by installation of driveways near existing trees, anthracnose. installations of septic systems, etc., may send oaks into decline. Although care can prolong their demise, once large oaks begin to decline, the pro­ cess is irreversible. Extraordinary measures to preserve native oaks near new construction are impractical or incompatible with modern construc­ tion methods. See "root disturbances" under Non­ Parasitic Disorders.

22 Shoestring Root Rot. The oak root rot fungus Armillaria meUea often attacks roots of oak, especially trees which are stressed by drought or defoliation. This fungus fruits in the fall produc­ Poplar (Populus spp.) ing clusters of tan or yellowish mushrooms at the (aspen, cottonwood, hybrid poplars) base of the infected tree. The color of these mush­ Septoria Leaf Spot. This disease is rooms gives the fungus its other common name, caused by Septaria musiva and occurs on native "honey mushroom." Many people consider this and hybrid poplars commonly grown in windbreaks mushroom a delicacy and collect it in large quanti­ and used in landscape plantings. Symptoms vary ties to eat. according to time of infection, hosts, texture and Oak trees attacked by Armillaria often show a age of leaves. Four types of leaf spot symptoms decline similar to that described above. In addition occur: 1) Small flecks with

23 affected trees appear smaller than normal, turn covered by orange to brown waxy crusts which are yellow-bronze, and are shed prematurely. The the overwintering stage of the rust. fungus moves progressively upward in the crown. Spores produced in spring from these over­ If viewed from a distance the diseased leaves wintering stages on fallen leaves can infect nearby appear bronzed. On more established plantings larch trees, if present. Later, spores produced on and in native stands repeated outbreaks result in infected larch branches reinfect poplars. Since branch die back and predispose trees to secondary larches are absent over most of North Dakota, mid­ pests and low temperature injury. to-late summer infections probably arise by wind­ Control: Plant poplars resistant to or toler­ borne spores from adjacent areas. ant of marssonina leafspot. Remove dead and Control: Rust may become a problem in infected twigs from diseased trees. Rake up and North Dakota under certain moist environmental destroy fallen leaves during the growing season. conditions. Early defoliation by rust may weaken For ornamentals apply Ortho Multi-Purpose young or newly-planted trees and can be a serious Fungicide at three-fourths tablespoon per gallon of problem in nurseries. water, or Daconil 2787 at two-thirds tablespoon per Removal of fallen leaves will not control rust gallon (2 pints per 100 gallons) or Daconil Ultrex as the infectious spores are wind-borne, often from at 1.4 pounds per 100 gallons at 7 to 14-day inter­ great distances. vals starting July 1 of the growing season. Protectant fungicide sprays may be used Poplars may be affected by other leafspots, where protection of young trees or nursery stock is scorch, powdery mildew or wetwood, all needed. Use Funginex at 1 tablespoon per gallon at covered under "Parasitic Diseases of Widespread seven to lO-day intervals as needed. Occurrence" except scorch, which is covered under Cankers. The single type of disease caus­ "Non-Parasitic Disorders." ing more problems on poplars than any other in Shoot Blight. Young shoots and leaves North Dakota is cankering caused by parasitic shrivel and turn black. The fungus Venturia fungi. Fungi in the genera Cytospora, Phomopsis, populina and related fungi infect young succulent Septoria, and Dothichiza cause cankers on poplars. shoots and cause death. The disease is favored by These fungi kill areas of the bark on branches and moisture. It is more severe in wet weather and main stems. If the killed area (canker) is large in shaded locations or where dew is frequent. enough or if several cankers grow together, the Although temporarily disfiguring, shoot blight stem may be girdled and killed. Spores which seldom damages established trees unless it occurs spread the canker fungi are produced in tiny for several years in succession. pimple-like fruiting bodies in bark of cankers. In Control: Prune out affected shoots. Young wet weather they ooze from the bark and may be trees and trees in nurseries may require fungicidal splashed about by wind and rain. The canker fungi protection. Apply three sprays at lO-day intervals, infect healthy stems when the spores land on beginning at bud break. Spray a fixed copper wounds in the bark. Even minute cracks or scars fungicide according to label directions. may be sufficient to let in canker fungi. Rust. Poplar rust is caused by the fungus Bark on cankers is sunken and discolored. Melampsora medusae and related species. Rust Foul-smelling sap often oozes from cankers. The generally appears after midsummer. First symp­ pimple-like dark fruiting bodies form on the af­ toms are small yellow pustules on the lower leaf fected bark. Later, the bark may loosen and split surface. These contain the yellow to orange ure­ or fall off, exposing the wood. If healing is taking dospores by which the fungus spreads from poplar place, the canker will be surrounded by raised to poplar. Later in the season the leaf appears ridges of callus tissue which gradually grow to­ gether to heal the canker. Cankers may be any shape, but are often diamond-shaped or elongate (Figure 17).

24 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.

Control: Certain poplar clones, particularly Russian-Olive the Lombardy poplar (P. nigra 'Italica'), are ex­ (Elaeagnus angustifolia) tremely susceptible to cankers and should not Scorch, Herbicide Injury, Winter be planted. Drought stress greatly increases the Injury. See under Non-Parasitic Disorders. susceptibility of poplars to cankers. Keep trees well Leafspots, Verticillum Wilt. See watered and promote good growth with proper under Parasitic Diseases of Widespread Occur­ fertiliza tion. rence. Prune out cankered branches during dry Rust. A leaf rust may damage leaves weather and destroy by burying or burning. Avoid and cause some defoliation in certain seasons. No wounding trees. Treat pruning cuts with a fungi­ control is generally needed. cidal wound dressing. Protect young stems from Stem Cankers. Several canker fungi sunscald in winter. Septoria canker may be con­ attack Russian olive. These include Phomopsis trolled by utilizing septoria leaf spot recommenda­ elaeagni, Botryodiplodia theobromae, and tions. See "canker" under Parasitic Diseases of Tubercularia ulmea. Widespread Occurrence for additional information. 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.

25 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 Walnut (Juglans nigra) and Widespread Occurrence for leafspots and wetwood and under Non-Parasitic Disorders for scorch and Butternut (J. cinerea) winter injury. Leafspots. Several leafspot fungi attack Rust. Willow rust is caused by species of walnut. See under Parasitic Diseases of Wide­ Melampsora, similar to the poplar rust fungus. See spread Occurrence. "rust" under Poplar for description of symptoms Anthracnose, Leaf Blight. See and control. Funginex is not labeled for rust con­ "anthracnose" under Ash. trol in willows. Scorch. See under Non-Parasitic Disorders. Shoot Blight or Scab. Willow shoot Winter Injury. Trees from different blight, caused by the fungus Venturia saliciperda, seed sources vary in hardiness. Plant only hardy is closely related to the fungus causing shoot blight sources. Understock used on named cultivars may of poplars. See "shoot blight" of Poplar for descrip­ affect hardiness. Butternut is of questionable tion of symptoms and control. hardiness in North Dakota, although some large Cankers. Several fungi cause stem cankers specimens exist in the eastern part of the state. on willow. The most important willow canker fun­ See "winter injury" under Non-Parasitic Disorders. gus 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.

26 Non-Parasitic Disorders

rees deteriorate and die for many reasons. The most common pattern of leaf scorch is T As with people, trees may die from hunger, a drying out and browning of the leaf margins thirst, old age, toxic chemicals, or improper care. (Figure 18). The symptom is often more severe on Disorders of this sort are termed non-parasitic outer, more exposed leaves, especially on the sun­ or abiotic to indicate that the cause is not some ward and windward sides of the tree. living agent or pathogen. Abiotic disorders are Where hot drying winds occur for several seldom the cause of death of a tree but, all too days during the period of active tree growth, more often, they pave the way for some pathogen which severe scorch symptoms may appear. The outer could not attack a healthy tree directly. Non­ parts of leaves or whole leaves dry up. Following parasitic disorders of trees most commonly encoun­ such an episode new leaves will often form if mois­ tered in North Dakota are scorch (desiccation ture supplies to the roots are adequate. injury), winter damage, and root injury. Leaf scorch symptoms may also appear following damage or disease in the roots (see root disturbance below) or injury to the trunk. Scorch Leaf Scorch symptoms may also occur from salt damage, especially from deicing salt used on roads, side­ • The symptom known as leaf scorch results when the roots are unable to supply enough water to walks or driveways near the tree. Elm, maple, linden, and walnut appear to replace that used by the leaves. For this reason be the species most commonly affected by scorch. scorch is most commonly seen following dry spells, Large-leaved trees such as catalpa or buckeye may drought, or on windy sites. The combination of also be more susceptible. Some leaf scorch on environmental conditions which favor scorch susceptible trees by the end of summer must be occurs nearly every summer in North Dakota. By considered normal in North Dakota (Figure 19). the end of the season many trees show leaf scorch Control: In exposed areas, plant trees in symptoms. groups rather than singly. Plant scorch susceptible trees in protected locations. Water young Figure 18. Leaf scorch on silver maple. The dark trees as needed and areas on margins and water deeply. Fertilize tips of the leaves are trees of low vigor. killed by hot dry winds. Do not plant shallow­ rooted trees on droughty or excessively wet sites.

27 Figure 19. Leaf scorch on linden. Drying of leaf margins and between veins is common on many trees by late summer.

Winter Damage Symptoms first become evident in spring when plants fail to flower or to leaf out. When new • Winter damage may take several forms on growth finally begins, shoots arise from older parts deciduous trees: frost cracks, bud or shoot death of the branches. Winter-killed twigs may be quickly and sunscald. colonized by weakly parasitic fungi. This can give Frost cracks develop from internal a false impression that a parasitic disease is stresses in the tree trunk. Frequently these cracks the cause. Excessive fertilizer levels or failure to open up in coldest weather and close again when reduce irrigation frequency in late summer will the weather warms up. Ash and maple are espe­ delay hardening off, which makes shoots more cially prone to frost cracking. Frequently, cracks susceptible to injury. open in the same place year after year resulting in Control: Do not use high nitrogen fertilizer a callus ridge ("frost rib") down the side of the tree. (lawn fertilizer) under trees in late summer or As long as the tree is otherwise healthy, frost ribs early fall. Reduce or stop irrigation as fall ap­ do little harm, but if the tree also has heart rot or proaches. Plant trees known to be hardy in North is hollow they may constitute a serious structural Dakota. weakness. No control is known for frost cracking. Sunscald or sunburn occurs in late Bud or shoot death (freezing injury) winter when the sun is bright - especially if new occurs if temperatures get too low in winter or snow is on the ground. Sunscald usually occurs on from frosts in early fall or late spring. Prolonged the south or southwest side of a tree. Thin bark on warm spells during the winter will cause this the tree is warmed enough to become active. When condition, particularly in those trees in which the temperatures fall sharply at night, the inner living rest period can be broken by low temperatures. It bark is killed. Thin-barked trees such as maple are usually is aggravated if a warm period is followed most susceptible, especially when young. by an extremely cold one. Buds or entire shoots Control: Sunscald on newly planted trees may be killed. Buds may be injured sufficiently to may be prevented by wrapping the trunks with allow entry of disease-causing fungi. Flower buds burlap, kraft paper or special tree wraps. Trunks are more cold tender than leaf buds and may be may also be shaded on the south and west side killed while leaves are unaffected. This type of with boards or screens. Tree paints (white-wash) winter injury is most likely when non-adapted tree have also been utilized to reduce sunscald damage. species or marginally hardy cultivars are planted Plant trees of marginal hardiness only in protected in North Dakota. Occasionally, very unusual locations. Avoid frost pockets. Plant only hardy weather may cause injury even to otherwise hardy species or cultivars (see NDSU Extension Bulletin native trees. No. 13, Trees and Shrubs for North Dakota).

28 Air Pollution Injury • Although the levels of air pollutants such as sulfur dioxide (S02) or ozone (03) are generally low in North Dakota, locally high levels of S02 may occur downwind from power stations. Vegetation injury by S02 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 pollut­ ant in North Dakota is drift from herbjcides. 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. 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 concentra­ tions 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 drip line of trees. Do not spray trees with the same sprayer used to apply herbicides.

Figure 20. Herbicide injury to Siberian elm. Symptoms typical of 2,4·D injury. (top) Branches showing different severities of leaf distortion. Leaves which grew out before exposure are normal. (bottom)Extreme distortion resulting from a high level of herbicide.

29 Root Disturbance • One of the most common and yet least recog­ as oak and cottonwood, are extremely sensitive nized causes of tree problems is root disturbance, to any root disturbance and large trees of those often associated with some sort of construction species may die quickly from major damage. activity. This may be laying of pipe or cable (Figure Oaks vs. People. Bur oaks 21A), street widening, placing a new building in grow native along river valleys in North Dakota. an older site or turning residential areas into These wooded areas are often sought as prime parking lots. home building sites. Because young trees - and even moderately­ On such sites, the oaks frequently begin to decline sized ones - can be successfully transplanted, shortly after people sometimes think that root damage is of new construction. Even so minor a disturbance as no consequence. An established tree .has a wide, changing underbrush into lawn, or water drainage spreading root system which may extend as much changes from installation of a septic system may as two to three times beyond the width ofthe can­ send oaks into a slow, irreversible decline. Very opy. Loss of a substantial part of this root system careful planning and management are necessary if will result in decline of the tree, unless corrective oak woodland is to be preserved around a new measures are taken. Old or very large trees seldom home. Many prospective home buyers may find the have sufficient regenerative capacity to survive needed protection is too costly or forces unaccept­ major root damage, although it may take such able design requirements. a tree several years to die (Figure 21B). Home Control: Protect tree root systems, not builders or contractors frequently try to preserve just the trunk, during construction. Prevent heavy trees by fencing off an area around them. This equipment or traffic from travelling near trees. If protects the trunk from damage but is of little help this is unavoidable, aerate the soil immediately to the root system, which extends out well beyond following such activity. Do not allow utility trench­ the drip line of the tree crown. Excavation, even at ing near trees. a distance of 10 feet from the trunk of a large tree, Insist upon written guarantees by contractors may destroy 30 percent of the root system. It does working near valuable trees. If major root damage not take major soil disturbance to cause damage. to large trees is unavoidable, consider removing Cutting a narrow ditch for laying utility cable the tree and replanting. Often the savings by this effectively disconnects the entire root system on procedure will more than pay for specimen size that side of the tree. replacements. Do not pave over tree root systems. Changes in grade will also damage tree root Roots need air to live! systems by upsetting the balance of air and water If tree roots are damaged, careful pruning to the roots need to survive. Soil compaction from balance top and roots can sometimes help the tree heavy vehicles can kill roots. When tree roots are survive. Obtain the help of a professional arborist. damaged by any of these causes, the tree begins Your city forester may recommend someone or offer to decline. The first symptoms may be leaf scorch. direct assistance. Later, twigs and shoots die back, then whole branches may die. Death of the tree may follow. Photo Credits Trees vary in their ability to tolerate root damage. The American elm is one of the most tol­ Figure 6 - V. D. Pederson erant, and it may take several years to die follow­ Figure 10 - D. S. Welch ing severe root disturbance. Some other trees, such Figure 20A - Robert Heintz Figure 20B - V. D. Pederson All other photos by R. W. Stack 30 Figure 21. Root injury from construction damage. A. Major disturbance of adjacent trees. B. The tree on the left has lost more than half of its root system.

31 Helping You Put Knowledge To Work NDSU Extension Service, North Dakota State University of Agriculture and Applied Science, and U.S. Department of Agriculture cooperating. Sharon D. Anderson, Director, Fargo, North Dakota. Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. We offer our programs and facilities to all persons regardless of race, color, national origin, religion, sex, disability, age, Vietnam era veterans status, or sexual orientation; and are an equal opportunity employer. 3M-11-95 This publication will be made available in alternative format for people with disabilities upon request (701) 231-7881.