North Dakota Tree Handbook

Table 4-10. Diagnostic key of deciduous tree and shrub problems.

I.Injury to Foliage

  1. Leaves being eaten
  1. Webbing present, larvae forage from silken nests
    Chokecherry, Prunus, apples, wild rose: prairie tent caterpillar; R1-P170, R3-P3, R5-149, R8-P7, R18-P6.
    Chokecherry: ugly nest tent caterpillar; R1-P172, R3-P4, R5-P151.
    Many species: fall webworm; R1-P166, R3-P4, R5-P151, R8-P8, R18-P6.
  2. Webbing present, no organized nest, but larvae rest on a silken mat placed on the trunk
    Many species: forest tent caterpillar; R1-P168, R3-P3, R5-147, R8-P34.
  3. No webbing, larvae hang from a thread when the foliage is disturbed (cankerworms and associates)
    spring cankerworm; R1-P142, R3-P1, R5-P175, R8-P46, R18-P6.
    Green ash, boxelder, bur oak:
    fall cankerworm; R1-P142, R3-P1, R5-175, R8-P43, R18-P6.
    Many hardwood species: cankerworms; R1-P142. Linden looper; R1-P144, R5-177. Elm spanworm; R1-P144, R5-P177. The half wing geometer (Phigalia titea) may be found in any combination on a particular host; R1-P177, R5-P179.
  4. No webbing, free feeding caterpillars
    Many species of trees and shrubs: there are dozens of species of moth, butterfly and caterpillar-like sawfly larvae that may be present. Trees and pests evolved together and light defoliation poses no threat to the host.
  5. No webbing, free feeding, but not caterpillars
    blister beetles; R3-P7, R5-P115, R8-P20, R18-P6. Grasshoppers; R3-P7, R8-P24.
    Cottonwood, poplar, willow: cottonwood leaf beetle; R3-P7, R5-P111, R7-P73, R8-P20, R18-P5.
    Elms: elm leaf beetle; R1-P222, R3-P6, R8-P23.
  6. Leaves folded or tied, with a larvae inside
    Green ash, apples, many species:
    fruit tree leaf roller; R1-P172, R3-P3, R5-P165, R8-P3.
    Oak: oak leaf roller, R1-P214.
    Aspen: early aspen leaf roller; R5-P167, large aspen tortrix, R1-P216, R5-P163, R7-P65.
  1. Leaves not eaten, but missing or falling and have discolored spots or dead areas on them
    Poplars and Caragana: Septoria leaf spot disease; R2-P74, R4-P50, R7-P19, R12-P7, R14-P23.
    Green ash: ash anthracnose; R2-P70, R2-106, R4-P16, R12-P6, R13-P2. Mycosphaerella leaf spots; R4-P14, R14-P10.
  1. Leaves present but damaged
  1. Upper surface of leaf consumed
    Cotoneaster, Prunus: pear slugs; R1-130, R3-P6, R5-P199, R8-P29.
    Elms: elm leaf beetle; R1-P222, R3-P6, R8-P23.
  2. Curled leaves with a woolly mass of aphids inside American elm: wooly elm aphid; R1-P307, R3-P34, R5-P159, R8-P139, R18-P2. Woolly apple aphid; R1-P306, R5-P159, R8-P139.
  3. Aphids or aphid-like insects on underside of leaf, especially on new growth
    Aphids; R1-P13, R5-P107, R18-P2.
    American elm: European elm scale (June/July only); R1-P368, R3-P35.
  4. Yellowish stippled areas on upper surface, black deposits underneath, small black and yellow bug
    Green ash: ash plant bug; R1-P402, R3-P36, R5-P119, R8-P134, R18-P3.
  5. Leaves bronzed or faded with black deposits on underside, insects delicate in appearance
    Many species, especially elm, oak, hawthorn, juneberry and Cotoneaster: Lace bugs; R1-P426, R3-P37, R5-P117, R8-P134, R18-P3. Spider mites, R1-P472, R3-P37, P18-P4.
  6. Bladder-like or warty growths on either surface of the leaf, (insect or mite galls) R5-P155.
    American elm:
    elm leaf gall; R1-464, R8-P67.
    Hackberry: nipplegall; R1-452, R3-P28, R8-P58, R18-P5.
    Honeylocust: R18-P5. honeylocust pod gall midge; R1-P466, R3-P30,
    Many species: insect or mite galls; R5-P155, R18-P4.
  7. Oak leaves with dense woolly pile-like growths, colors vary from white to reddish or yellowish
    Bur oak: woolly oak gall wasp; R8-P77.
  8. Dense feltlike patches on the surface or depressions filled with feltlike hairs (Erineum), colors vary from green to brick red
    Boxelder, maples:
    Erineum galls; R1-483, R5-153.
  9. Distorted new growth, smaller than normal cupped leaves with wavy margins
    Boxelder, Siberian elm:
    phenoxy herbicides; R2-P458, R4-P33.
    Many species: herbicide damage; R2-P428, R4-P33, R6-P119.
  10. Leaves mined between surfaces
    leafmining sawfly; R7-P68.
    Elms: elm leafminer; R8-P68.
    birch leafminer; R1-P184, R5-199.
  11. Leaves with necrotic areas, sometimes joining to damage entire leaf
    Green ash: ash anthracnose; R2-P70 & 106, R4-P16, R12-P6, R13-P2, R14-P9.
    oak anthracnose; R2-P110, R4-P16, R12-P6, R13-P9, R14-P21.
  12. Leaf spots, may have lighter colored centers
    Septoria; R2-P74, R4-P8, R4-P50, R7-P19, P12-P7, R14-P23.
    Marssonia: R4-P6, R6-P73, R7-P25, R13-P12, R14-P23.
    Caragana: Septoria; R4-P8, R13-P2.
    Maples: Phyllosticta leaf spot; R4-P10, P13-P8, R14-P19.
    Prunus: leaf spots, may have holes in the leaves, fungal and bacterial leaf spots; R2-P66, R2-P160, R4-P22, R16-P9811. X-disease; R2-P394, R4-P24, R13-P4, R14-P13, R15-P6.
    Green ash: anthracnose; R2-P70, R2-106, R4-P16, R12-P6, R13-P2, R14-P9.
    Apples: frogeye leafspot (black rot), apple scab; R2-P96, R16-P9. Cedar-apple rust; R2-P240, R4-P136, R6-P83, R14-P9, R16-P7.
  13. Leaf spots, black spots resembling tar
    Maples and willows: tar spot fungus; R6-P91, R12-P6, R13-P8, R14-P19.
  14. Leaves with yellowish or whitish dusty deposits
    Lilac, honeysuckle, poplars and willows: powdery mildew; R2-P14, R4-P30, R6-P81, R12-P8, R13-P7. Melampsora leaf rusts; R2-P256, R4-P4, R5-P79, R7-P20, R13-P12, R14-P19.
  15. Leaf lesions, with hornlike structures, mainly on undersurface of leaf
    Apples, hawthorn and juneberry: cedar-apple rust; R2-P240, R4-P136, R6-P83, R12-P2, R13-P4, R14-P8, R16-P7.
  16. Leaves develop early fall color, sparse foliage
    Chokecherry: X-disease; R2-P24, R2-P322, R13-P4, R14-P13.
  17. Leaves yellowing between veins, veins remain green
    Maples and others: chlorosis, mineral deficiency; R2-P474, R4-P36, R14-P19.
  18. Leaf margins scorched
    All species,especially lindens and maples: Leaf scorch from hot winds; R2-P476. Salt damage; R2-P452, R14-P19.
  19. Leaf petioles with marble like galls on them
    Cottonwood, poplars: poplar petiole gall aphid; R1-P460, R3-P29, R5-P157, R7-P107, R8-P69, R18-P5.
  20. Frothy mass of spittle containing a small insect
    Many species: spittlebugs; R1-P240.

II. Injury to Twigs or New Shoots

  1. Shoots dying, deformed or stunted
  1. Shoots and leaves dying or deformed
    Many species: aphids; R1-P13, R5-P107, R8-P137, R18-P2.
    Herbicide damage; R2-P458, R4-P33.
  2. Witches' brooms
    Honeysuckle: honeysuckle aphid; R1-P315, R3-P34, R5-P107, R18-P2.
    Juneberry: black leaf witches' broom; R2-P20, R5-P87.
    Oak: oak anthracnose; R2-P110, R12-P6, R13-P9, R14-P21.
  3. Shoots ending with a convoluted "brain shaped" gall
    Cottonwood: vagabond aphid gall; R3-P30, R5-P209, R7-P95.
  4. Cauliflowerlike growth behind, or over buds
    Poplars: poplar bud mite gall; R3-P28, R5-P209, R18-P5.
  5. Black clusters hanging from twigs
    Green ash: ash flower/gall mite; R3-P28, R8-P60, R18-P5.
  6. Large globular galls on twigs
    Rose: rose twig gall; R8-P79.
    Oak: oak bullet gall; R3-P30, R5-P213, R8-P74, R18-P5.
  7. Twig terminating in a cone like structure
    Willows: pine cone willow gall; R5-P213, R8-P54.
  8. Swollen area on small twigs
    Boxelder: boxelder twig borer; R5-P218, R8-P64.
  9. Twigs dying, leaves blackened
    Apples, Cotoneaster: fireblight; R2-P162, R4-P76, R5-P99.
    Juneberry: black leaf witches' broom; R2-P20, R5-P87.

III. Injury to Trunks or Larger Branches

  1. Injury to branches
  1. Entire branch dying
    American elm: Dutch elm disease; R2-P366, R4-P92, R5-P105, R12-P10, R13-P5, R14-P14.
    Native elm wilt; R2-P172, R4-P94.
    Apples, mountain-ash, Cotoneaster: Fireblight; R2-P162, R4-P76, R5-P99, R12-P8, R13-P489, R14-P8. Black rot canker; R2-P96, R16-P9.
    Aspen: Hypoxylon canker; R2-P222, R4-P58, R6-P97, R7-P38, R13-P11, R14-P24.
    X-disease; R2-P392, R4-P24, R13-P4, R14-P13, R15-P6.
    Green ash, black ash: Western ash bark beetle; R3-P15, R5-P231.
    Poplars: Septoria; R2-P74, R4-P50, R7-P37, R12-P7, R14-P23. Cytospora; R2-P200, R4-P48, R7-P40, R13-P11, R14-P24. Dothichiza cankers; R2-P184, R4-P54, R7-P40, R14-P24.
    Siberian elm: Botryodiplodia
    canker; R2-P182, R4-P38, P13-P5, R14-P16. Tubercularia canker; R2-P208, R4-P40, R14-P16, R12-P5.
    Russian-olive: Botryodiplodia canker; R2-P180, R4-P42, P14-P25. Tubercularia canker; R2-P208, R4-P40, R13-P14, R14-P25. Phomopsis; R2-P140, R4-P44, R13-P14, R14-P25.
    canker; R2-P198, R12-P5, R13-P3, R15-P4.
  2. Branches or trunk with small immobile spot like structures that can easily be dislodged, may occur singly, in irregular patches or cover large areas, color varies with the species, (scale insects)
    scurfy scale (white); R1-P368, R5-P215, R7-P98, R18-P4.
    San Jose scale (grey); R1-386.
    American elm:
    European elm scale; R1-P368, R3-P35.
    Many hardwoods: lecanium scale (large dark-brown); R1-P364, R3-P364, R18-P385.
    Oystershell scale (small, elongate, grey-brown); R1-P370, R3-P36, R18-P3.
  3. Elongate thick blackish growths on branches
    Chokecherry, Prunus: black knot; R2-P152, R4-P70, R6-101, R12-P4, R13-P3, R14-P13, R15-P2.
  4. Holes in trunk of live or dying trees, meandering tunnels under the bark and sometimes into sapwood or heartwood, often associated with sawdust
    Green ash:
    lilac (ash) borer; R1-P261, R3-17, R5-P235, R18-P7.
    Carpenterworm; R1-256, R3-P16, R5-237.
    Poplars: poplar borer; R1-P284, R3-P19, R5-225, R7-P81, R18-P7.
    bronze poplar borer; R1-P272, R5-P229, R7-P85.
    Oak: two-lined chestnut borer; R1-P270.
    Birch: bronze birch borer; R1-P272, R5-P229, R18-P7.
    Many species: poplar/willow borer; R1-P268, R5-P221, R18-P7.
    Pigeon tremex; R5-P239, R1-P494.
  5. Evenly spaced, evenly sized, rows of holes in the bark of the thin barked species of trees
    Many species:
    sapsuckers (a woodpecker); R1-P500, R5-P125.
  6. Small regular tunnels under the bark, or shothole pattern in the bark (bark beetles)
    American elm:
    native elm bark beetle; R1-P249, R3-P15, R5-227. European elm bark beetle; R1-P249, R3-P15, R5-P227.
    Green ash: Eastern ash bark beetle; R3-P15, R5-P231, R8-P106.
    Chokecherry, Prunus: shothole borer and the peach bark beetle; R1-P250.
  7. Trunks or crotches oozing foul smelling sap, may kill vegetation where it drips
    Elms, poplars:
    bacterial wetwood (slime flux); R2-P382, R4-P64, R7-P52, R14-P14.
  8. Trunks with shelf-like or hoof-shaped growths, live but weakened or overmature trees
    Green ash: Perennipora (Fomes),
    heart rot fungus; R2-P344, R4-P80, R13-P2, R14-P9.
    Buffaloberry: Perennipora (Fomes), heart rot fungus; R4-P80, R14-P9.
    Aspen: Phellinus heart rot fungus; R2-P342, R4-P83, R6-P109, R7-P54.
  9. Burl-like or gall-like growths on trunks and larger branches
    Cottonwood and poplars:
    Diplodia gall and rough bark of poplars (a fungus disease); R5-P95.
    Many species: Crown Gall; R4-P66.
  10. Dead areas of bark extending vertically, usually on the south or west sides of main trunks
    Mountain-ash, maples, lindens, Malus and others:
    sunscald, mainly due to late winter low angle sun; R2-P478, R14-P28.
  11. Vertical trunk cracks commonly referred to as frost cracks or varied causes
    Many species:
    frost injury, drought damage, mechanical damage, improper pruning; R2-P340, R4-P89, R14-P28, R19-P2.
  12. Uniform dieback of top branches
    All species: usually associated with root injury or death. Some common causes of root dieback are: oxygen starvation due to soil compaction or poor drainage, excessively dry soil, chemical damage, or mechanical injury, girdling roots (see below). Insects or diseases are only rarely responsible for significant root injury. Decline from multiple or unknown factor; R2-P444. birch decline; R2-P448, R14-P30, R19-P2.
  13. Bark on branches and/or main trunk blackened
    American elm, and rarely other hardwoods: sooty mold fungus growing on "honeydew" produced by sapfeeding insects, most commonly woolly elm aphids and European elm scale on American elm.
  14. Witches'-Brooms or epicormic sprouts on the main trunk or basal flare.
    May be associated with tree decline: Ash yellows; R2-P394, R14-P10.

IV. Entire Tree Dying

  1. A. Dying With Leaves On
    American elm: Dutch elm disease; R2-P366, R4-92, R13-P5, R12-P10, R14-P14.
    Oaks: two-lined chestnut borer; R1-P270.
    Birch: bronze birch borer; R1-P272, R5-P229.
    Chokecherry: X-disease; R2-P392, R4-P24, R13-P4, R14-P13, R15-P6.
    Many species: girdling by: canker diseases, small rodents, beavers, or porcupines, mechanical damage from: lawn mowers, string trimmers, weed barriers. Trees may be girdled by their own roots, this may be indicated by a lack of root flare above the girdling portion of the roots of larger trees; R2-P488.
    Drought damage; R2-P340, R2-P476, R14-P27, R19-P2.


Borers and Bark Beetles

Ash Bark Beetles

Green ash and black ash
Eastern ash bark beetle; R3-P15, R5-P231, R8-P106.
Western ash bark beetle; R3-P15, R5-P231, R8-P106.

During the past several years, a number of resource managers have reported that relatively healthy green ash were being attacked and killed by the ash bark beetle in the genus Hylesinus (formerly Leperisinus). This type of damage and mortality was found, but the numbers of incidences were low. Few trees are killed in this manner, certainly only a fraction of a percent statewide. All members of genus Hylesinus are considered secondary, attacking trees severely weakened by drought, lawn mower damage, soil compaction or other stresses. Because of the drought conditions experienced in 1988 and 1989, increased tree losses were seen during this period. More favorable moisture conditions since 1991 reduced over all losses to pre-drought period levels.

Although more than one species of bark beetle is involved, the primary species is the eastern ash bark beetle, Hylesinus aculeatus.

One member of the genus, Hylesinus californicus, the western ash bark beetle, damages green ash across the state. Young, recently transplanted ash trees in parks, recreation areas and highway department plantings are especially hard hit. Stressed trees in any size class may be infested.

The western ash bark beetle, however, does not attack the main trunk of the tree en masse. This species prefers to attack live trees under stress, inflicting damage to individual limbs or the main stem in the crown. The formation of the brood gallery often girdles branches, killing them from the point of attack outward. This species tends to re-infest the same trees year after year and progressively kills young trees from the top downward. In larger trees, the crowns are progressively thinned until the tree loses its vigor, aesthetic value and ability to break the wind.

Pine Bark Beetle Complex

Ponderosa pine (Ips and Dendroctonus)
R1-P62, R5-P81, R3-P25, R5-P79.

In July 1988, scattered groups of recently killed trees were found in a native stand of ponderosa pine in Slope County. Closer inspection revealed groups of two to ten trees killed by bark beetles. These bark beetles were found to be in the genus Ips, commonly referred to as engraver beetles. This was the first time bark beetles had been reported killing ponderosa pine in the area.

The most prevalent species is the six-spined ips, Ips calligraphus. Two other bark beetle species; the pine engraver, Ips pini, and the red turpentine beetle, Dendroctonus valens, were also present. The latter two were not detected in every tree. None of the three is considered highly aggressive, but each is capable of killing trees weakened by other factors. Populations since 1991 have persisted at relatively low levels, this is no doubt due to the return of adequate moisture. No further significant outbreaks are likely, unless drought conditions return.

Ash Borer (Lilac Borer)

Green ash and black ash
R1-P261, R3-P17, R5-P235.

Present in young green ash statewide, the ash borer is most prevalent in the southwest and south central areas of the state. Moderate populations were found in street trees in several cities and shade trees in parks, recreation areas and highway department plantings, especially in locations south and west of the Missouri River. Light to moderate infestations were noted in shelterbelts surrounding the Bowman-Haley Reservoir.


Green ash, hybrid poplars, and Siberian elm
R1-P256, R3-P16, R5-P237.

Light to moderate populations appeared in green ash in southwest, south central and northwest North Dakota. Light to moderate populations also occurred in hybrid poplars in south-central and southwestern North Dakota. Moderate damage occurred in the shelterbelts surrounding Bowman-Haley Reservoir. Large green ash was the preferred host, with lesser populations in Siberian elm and hybrid poplars. Severe damage is common in both planted and native green ash south and west of the Missouri River.

Poplar Borer

Aspen, cottonwood, and hybrid poplars
R1-P284, R3-P19, R5-P229, R7-P81.

The poplar borer was most numerous in hybrid poplars in south central and southwestern North Dakota parks and urban areas. Oftentimes, both the poplar borer and carpenterworm were present in the same trees (see carpenterworm).

This pest was also present in native aspen, causing extensive damage to open grown trees and trees along edges of fields, roads or lakes. Because the poplar borer seems to show a preference for laying its eggs in sunny locations, it may be wise to delay or avoid pruning hybrid poplars if possible. Native cottonwood is seldom affected by poplar borer.

Bronze Birch Borer

R1-P272, R5-P229.

Bronze birch borers continue to attack birch statewide, especially cutleaf weeping birch weakened by drought or other factors. Drought conditions in 1988, 1989 and 1991 helped predispose many birch to attack, and there are likely to be continued losses over the next several years. Bronze birch borer infestations may be contributing to a continued decline of native birch in the state, but other factors are no doubt present.

Tip Borer

Scotch pine
R1-P48, R10-P84.

In 1989, larvae thought to be a species of Dioryctria were found tunneling in and killing the current year's growth, usually terminals, on Scotch pine. These insects were detected in McHenry, Cavalier and Pembina Counties. Highest populations were in Christmas tree plantations in Pembina County, but populations have not increased significantly. This insect could be of major concern to Christmas tree growers if it occurs in high numbers, because of the damage inflicted to terminarl shoots.

Tip Moth

Scotch and ponderosa pine
R1-P48, R3-P22, R8-P96.

The tip moth is most numerous on ponderosa pine in the south central and southwestern areas of North Dakota. It is occasionally found in the north central and northwestern areas of the state. The most severe damage was present on young natural reproduction in native stands in Slope County. Rarely has tip moth damage been observed north and east of a line extending from Crosby in the northwest to Ellendale in the southeast.

Two-lined Chestnut Borer

Bur oak

In August of 1990, large numbers of bur oak were reported dying at Sully's Hill National Game Preserve near Devils Lake. The trees were being killed by the two-lined chestnut borer. Subsequently damage was found across the state. Heaviest losses were in the Edinburgh to Walhalla areas. Southeastern North Dakota escaped significant damage. As the 1991 season progressed, it became evident that tree kill would be reduced compared to the 1990 season. Since 1991, more favorable moisture conditions have allowed the trees to resist attacks. Populations continue to decline to the point where it was difficult to find infested trees in 1992 and 1993.

Metallic Pine Pitch Nodule Maker

Ponderosa, lodgepole, and Scotch pine
R3-P23, R8-P98.

The metallic pine pitch nodule maker is found statewide on ponderosa pine. Historically the heaviest populations have been found in the south central and southwest regions. Populations collapsed in 1992 and 1993. Factors responsible for the reductions in population are not known, but may relate to cool wet summers.

Hardwood Defoliators


Elm, boxelder, green ash, linden, bur oak and hackberry
Spring cankerworm; R1-P142; R3-P1, R8-P46.
Fall cankerworm; R1-P142, R3-P1, R5-P175, R8-P43.

In 1992 and 1993, the perennial problem of defoliation of single row Siberian elm windbreaks by the spring cankerworm was noted in many single-row Siberian elm plantings across North Dakota. Fall cankerworm occurs across the state and complete defoliation of green ash, boxelder and bur oak is common in both native stands and planted windbreaks.

An additional problem has been high populations of the fall cankerworm occurring simultaneously with spring cankerworm in some areas. Where both species occur, spring cankerworms defoliate elms, and the fall cankerworm defoliates green ash, boxelder, lindens and hackberry. Where only spring cankerworm exist, green ash, boxelder and understory shrubs usually escape heavy defoliation. Fall cankerworm has been noted in single row green ash in several areas.

Common associates of both species, especially in native areas are three other geometrid loopers: the linden looper, R1-P144 and R5-P177; elm spanworm, R1-P144 and R5-P144; and the half wing geometer (Phigalia titea), R1-P177 and R5-P179.

Populations of all species seem to rise and fall independent of each other in native, planted or urban areas.

Prairie Tent Caterpillar

Chokecherry, Prunus, apples and wild rose
R1-P170, R3-P3, R5-P149, R8-P7.

The prairie tent caterpillar was common to locally abundant in the western two-thirds of the state. Chokecherry and wild rose are the preferred native species. Prairie tent caterpillar attacks occur in cities on Prunus and apple species. Usually, very little actual defoliation occurs, but control measures are often taken because people consider the nests unsightly.

Pear Slug

Cotoneaster and Prunus
R1-P130, R3-P6, R5-P199, R8-P29.

The pear slug has often caused complete defoliation of hedge cotoneaster statewide. The central and north central areas of the state historically are the hardest hit. Pear slugs have two generations per year and it is generally the second generation that builds up to the most damaging levels. Pear slugs skeletonize leaves by consuming the upper surfaces causing the damage to superficially resemble fireblight injury. Consequently, the two pests are often confused.

Conifer Defoliators

Introduced Pine Sawfly

Scotch and ponderosa pine
R1-P16 R5-P35, R10-P42.

Scattered light to moderate infestations of introduced pine sawfly occur on ponderosa and Scotch pine across the state, more commonly in the western half of the state.

Yellow Headed Spruce Sawfly

Colorado and Black Hills spruce
R1-P18, R3-P13, R5-P33, R8-P26.

Scattered light to moderate infestations of yellow headed spruce sawflies were found on Colorado and Black Hills spruce. Sometimes heavy defoliation occurs, mainly in the northern tier of counties. The geographical areas infested indicates a high likelihood that adults are being "blown in" from native spruce stands in Canada. Spruce cannot tolerate heavy defoliation, and a single heavy defoliation can kill them.

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