Insect and Disease
Management Guide
for
Woody Plants
in North Dakota -- continued
(Venturia inaequalis)
Host(s): Apple, crabapple, mountain-ash
Description/Biology: This fungus survives the winter
in leaf and fruit debris that falls from the tree.
Under appropriate environmental conditions in the spring,
the fungus produces primary spores which infect
young leaves and fruit. Secondary spores produced on
diseased tissue infect other leaves and fruit. The secondary
spores may have multiple generations per season if
appropriate environmental conditions persist.
Damage/Symptoms: The fungus causes olive green
to brownish velvety lesions on fruit and leaves of the
host. On the fruit, lesions are often corky, brown, and
may cause disfigurement. Lesion diameter averages 1/2
inch, but they may coalesce and appear larger.
Comments: Clean up and destroy fallen leaves
and fruit. Protectant fungicides can be used during
prolonged wet weather, including chlorothalonil, propiconazole, thiophanate-methyl, myclobutanil,
and mancozeb. There are resistant crabapple
cultivars including, Centurian, Donald Wyman, Prairifire,
and Spring Snow. Good air circulation between and
within trees reduces infection.
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Ash anthracnose
(Gnomoniella fraxini)
Host(s): Green ash, other ash species
Description/Biology: This fungus overwinters in
fallen leaves and twigs and in rachises (long, central part
of compound leaves), fruit, and twigs which may be retained in trees throughout winter. Under wet
conditions in the spring, spores are produced that
infect succulent new growth. Cooler weather favors
disease development. Dry weather reduces spore production
and disease development. Leaves, shoots, twigs,
branches, and fruit are infected.
Damage/Symptoms: The classic symptom is
brown, dead, distorted leaf tissue. These leaves may have
brown blotches associated with leaf veins and they often
fall prematurely. Heavy defoliation may cause
stunted growth and dieback. Small leaf spots with purple
halos develop instead of blotches during less
favorable weather. Orange or tan superficial lesions develop
on infected twigs.
Comments: Raking and destroying fallen leaves
may reduce infections in subsequent years. Fertilizing
to promote growth (high nitrogen) will allow for
optimum refoliation. In areas where trees are repeatedly
infected by anthracnose, protectant sprays may be
necessary. Three applications work best, and timing of fungicide
is critical for controlling disease development. The
first application should be made as buds are beginning
to swell, but before buds break. The second
application should be made when the buds show green tips, and
the third fungicide application should be made when
the leaves are half grown. The recommended spray
treatment should be lime sulfur first, followed by
two treatments with a chlorothalonil product. Spraying
after first infection can reduce late season infections.
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(Puccinia sparganioides)
Host(s): Ash
Description/Biology: This fungus requires two hosts
to complete its life cycle, ash and various species
of cordgrass (Spartina spp.). P.
sparganioides overwinters on grass species. Two rust spore types
(spermagonial and aecial) occur on the ash in the spring and
early summer. Aeciospores subsequently infect grass
species, where the fungus overwinters and produces
another spore type (basidiospores) in the spring that will
infect ash.
Damage/Symptoms: Infection occurs on
leaves, petioles, and green twigs of ash trees. The
initial symptoms are yellow to orange spots on the upper
leaf surface and chlorotic spots on the petioles and
stems. About two weeks after the appearance of those
spots, bright orange lesions containing aeciospores
appear, breaking through the plant surface on the lower side
of the leaf and on petioles and stems. These lesions may
be from 1/16 to 1/2 inch in diameter and the
pathogen causes swellings of 3/4 inch or longer in petioles
and stems. Diseased tissue may swell, causing distortion
of leaves, sharp bends in petioles, and roughly
egg-shaped galls on twigs. Severe defoliation occurs in
some locations under optimum conditions.
Comments: No control is needed on established
trees. On trees subject to severe infection in areas where
the cordgrass cannot be controlled (mowed,
sprayed), fungicides such as myclobutanil may be used.
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(Phytoplasma fraxini)
Host(s): Ash, lilac
Description/Biology: The disease caused by
this pathogen is called ash yellows in ash and lilac
witches'-broom in lilac. Phytoplasmas are bacteria that survive
in the phloem of plants and are vectored (carried)
by phloem-feeding insects such as leafhoppers.
Once infected, hosts remain infected. Stress from
insects, other diseases, drought, and other causes can hasten
the decline of trees infected with P.
fraxini.
Damage/Symptoms: Symptoms vary depending on
the age of the tree, stage of disease, and differing levels
of tolerance to the pathogen. Some of the symptoms
that may be present on infected ash include subnormal
leaf size, light green leaf color, slow-growing
lateral branches, witches'-brooms at the soil line or on
the trunk, branch dieback, or death of trees. Some
symptoms that may be present on lilac include clusters
of scorched leaves and witches'-brooms. Recent
research has shown that ash yellows phytoplasmas are present
in ash all over central North America and have been
found in lilac in southeastern North Dakota. Currently, we
do not know if ash yellows or lilac witches'-broom
cause significant damage in North Dakota.
Comments: Green ash is a popular tree species in
North Dakota and is often overplanted. Continued but
judicious use of ash and lilac is appropriate. If a plant
is showing symptoms of ash yellows infection, good
plant care principles such as proper irrigation,
fertilization, and pest control may prolong aesthetic qualities
of infected trees. The usefulness of removing infected
trees to reduce inoculum is questionable. Resistant
cultivars have been identified in the eastern and central
United States, including northcentral Iowa. White ash
is generally less tolerant than green ash. Ash
cultivars Bergeson, Dakota
Centennial®, Patmore, and Autumn Applause were least affected. Evaluations are
currently under way to determine if certain cultivars used in
North Dakota are more resistant than others.
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(Apiosporina morbosa)
Host(s): Chokecherry, plum, other
Prunus spp.
Description/Biology: This fungus overwinters
in `knots' (conglomerations of fungal fruiting
structures) on host twigs and branches. In the spring, spores
are produced in the knots and may be windblown or rainsplashed to wounds or, more commonly, new
green growth and cause new infections. A.
morbosa grows under the bark and in nutrient and water carrying
vessels for up to several months. Infected branches
eventually swell, and spores are produced in these swellings
during spring. Spores are produced at least one, but often
two, years after infection. In the fall, swellings may occur
on twigs infected that season. Occasionally, no
symptoms will be observed until the next growing season,
when the swellings begin to turn olive green with
development of the fungus in the host tissue.
Damage/Symptoms: Olive-green corky
swellings develop on twigs and branches the autumn or
spring following infection. During the second season,
these swellings become black and woody and can be
enlarged to three times or more the diameter of healthy twigs
and branches of the same age. The disease sometimes
causes twigs to bend at the swellings. Trees may have one
to many knots. Branches may be killed above knots,
and trees with many infected limbs may be severely stressed.
Comments: Removing the knots, and 3 to 4 inches
of healthy wood below the knot as they become
visible usually controls the disease but needs to be
repeated whenever new knots develop. Lime sulfur may be
used as a dormant spray after removal of knots, or thiophanate-methyl may be sprayed when the tree is
(1) dormant, (2) pink bud, (3) full bloom, and (4)
three weeks after full bloom. This disease may be easy
to manage if all sources of inoculum are controlled.
This can be difficult when there are many infected
wild plants or neighbors who do not actively manage for
the disease in their Prunus spp. ornamentals. Many
individual plants are resistant to black knot, so control
or removal of heavily infected plants will reduce
disease pressure. The disease is not a widespread
problem unless multiple years of weather favorable for
infection occur.
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(Botryosphaeria obtusa)
(this fungus also causes frogeye leaf spot)
Host(s): Apple, crabapple, mountain-ash
Description/Biology: This fungus overwinters in
bark scales, cankers, mummified fruit, and
fire-blighted twigs. Spores are produced in infected tree tissues in
the spring and are usually released after budbreak,
primarily by water. Spores can develop and be dispersed
during cool (~ 70oF) wet weather throughout the
growing season. Early infections occur in leaves and
petals, while later infections occur on fruit through
cuticular cracks. Black rot branch and stem cankers are caused
as the pathogen enters wounds through the bark.
These cankers often develop after winter injury to hosts.
Damage/Symptoms: Early infection of the floral
parts may appear as soon as bud scales loosen. Sepal
infections cause reddish specks that turn purple with a
red border. This type of infection may result in blossom
end rot in fruit later in the season. Leaf infections appear
as purplish flecks that enlarge to round lesions
approximately 3/16 inch in diameter. As the disease
progresses, the lesion margins remain purple while the
centers become tan to brown (giving a `frogeye'
appearance). Heavily infected leaves may become chlorotic and
fall. Leaf lesions are also susceptible to infection by
other organisms and may take on fuzzy, irregular shapes
or concentric ring patterns. Cankers (dark brown,
sunken lesions) may appear on limbs of the tree. A canker
may be only superficial on the bark or it may be in the
wood, weakening the limb and possibly resulting in
breakage during heavy fruit set or snow loads. Some
branches may be girdled and killed by cankers. Fruit
infection will appear first as reddish flecks that develop
into purple, raised bumps on the fruit. Infection on
more mature fruit will appear as black, irregularly
shaped lesions (blotches) with a red halo. As these
lesions enlarge, they can form concentric rings
alternating between black and brown. Infections can grow
at refrigerator temperatures, resulting in storage rot.
Comments: General sanitation measures that
include cleaning up leaf debris in the fall, removal of
mummified apples, and pruning out cankers and dead
wood while trees are dormant will help control this
disease. Fungicides are rarely necessary. See
"Maintaining Healthy Trees and Shrubs."
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(Botryodiplodia hypodermia)
Host(s): Elms
Description/Biology: This fungus overwinters
as cankers and primarily attacks stressed trees,
invading through wounds. Spores are produced in fungal
fruiting structures in the cankers all season long and spread
by wind and rainsplash, but damage is most severe
when infection occurs during the spring.
Damage/Symptoms: Cankers develop on
twigs, branches, and trunks and may girdle them in a
single season. In newly infected bark, tissue appears
water soaked, soft and reddish brown to brown. Foliage
may yellow, wilt and die beyond the cankered area.
On Siberian elm, cankered bark may slough off and
leave obvious scars. Leaves on American elms may turn
bright yellow in mid to late summer and shed prematurely.
This symptom can be confused with Dutch elm disease.
Comments: Maintain good general health of
trees, especially providing adequate water since
drought stressed trees are more susceptible. See
"Maintaining Healthy Trees and Shrubs." Avoid wounding
trees. Remove cankered and dead wood from trees.
No chemical treatments are available.
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(Monilinia fructicola)
Host(s): Plum, other stone fruits, and some pome fruits
Description/Biology: This fungus overwinters
in mummies (dried, infected fruit) on the tree and on
the ground and in twig cankers. In the spring, spores
are produced and carried by wind, rainsplash and insects
to blossoms, young leaves and shoots, where new
infections occur. M. fructicola is not active during
the summer but will infect ripening fruits in late
summer and fall. Disease development is rapid in warm, wet
and humid weather.
Damage/Symptoms: Infected leaves and shoots
become brown as they are covered with spore masses. The
most obvious symptom is the brown rot that occurs on
the fruit. On green fruits, this is characterized by
small, round, light brown spots. On ripening fruit, pale,
ash-gray to brown, felt-like fuzzy masses of spores
cover fruit. Fruit will rot in hours during wet weather.
Rotted fruit may cling to the tree or fall to the ground.
Brown rot may develop on fruit in storage.
Comments: Management for brown rot should begin
in late summer and fall. Remove all remaining
fruit, mummies, and cankered twigs. Remove or bury
mummies before blossom. Captan may be
used for controlling infection. The first application should
be made as soon as blossoms show color. When the weather is wet and night temperatures are above
55oF, apply the second spray at full bloom and the third
spray at petal fall. Another application should be made
when green fruits are fully developed but have no
color. Repeat the application 10-14 days later if the weather
is humid and temperatures are 60 to 80oF.
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Cedar-apple rust
(Gymnosporangium juniperi-virginianae
and related species)
Host(s): Rosaceae (apple, crabapple,
hawthorne, juneberry), juniper
Description/Biology: These fungal pathogens
require two different hosts to complete their life cycles.
They overwinter as galls or witches'-brooms on junipers.
In wet weather, orange, gelatinous spore-bearing
structures develop. These structures can develop several
times between May and August, producing spores each
time that can infect Rosaceous hosts.
Yellowish-orange lesions develop on leaves of infected Rosaceous
plants. Juniper-infecting spores develop in these lesions.
New infections on junipers will release Rosaceous
host-infecting spores either the following spring or the
year after. Some types produce spores perennially thereafter.
Damage/Symptoms: On Rosaceous hosts,
infection results in small yellow-orange lesions on the
upper surface of leaves and young fruit. Orange droplets
may form on these lesions. The infections expand to
the lower leaf surface, where 1/16 to 3/16 inch
diameter orange pustules are formed. Damage to Rosaceous
hosts may develop as reduced fruit quality and minor
to almost total defoliation of susceptible cultivars.
New infections on junipers result in small galls or
witches'-brooms that gradually enlarge until sporulation
occurs. Junipers are usually not damaged by even high
gall pressure, but their appearance is unsightly.
Witches'-brooms damage the function and appearance of
infected trees.
Comments: Remove one of the alternate hosts
(juniper or Rosaceae) if it is not a desired plant. If removing
an alternate host is not possible, picking the galls
or pruning the witches'-brooms off the junipers
should keep the disease to a manageable level.
Chlorothalonil, fenarimol, mancozeb, maneb,
myclobutanil, propiconazole, thiophanate-methyl, and triadimefon
are labeled for some ornamental Rosaceous hosts.
Check hosts listed on any products before purchasing.
They should be applied when the orange
spore-bearing structures develop on junipers. The crabapple
cultivars Donald Wyman, Indian Magic, Indian Summer,
and Prairifire are resistant to cedar-apple rust.
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(Leucostoma kunzei = [Cytospora
kunzei])
Host(s): Spruce
Description/Biology: This fungus overwinters in
the bark of infected branches. Spores may be released
any time temperatures are above freezing. Infections
occur in wounds or cracks in the bark. The fungus
expands and kills the bark. It continues to grow until the
branch is killed. Fungal fruiting bodies develop in the dead
bark to produce more tree-infecting spores.
Damage/Symptoms: Individual branches die
back, usually beginning in the lower crown. Often, sap
oozes onto the cankered area and dries on the
branches immediately below. The most characteristic symptom
is the presence of individual dead branches. As a branch
is dying, the needles will discolor to purple or
brown before they drop off. If infected limbs are left on
the tree, more branches will be infected in subsequent
years, and the disease may cause the tree to lose
aesthetic, wind control, and noise reduction benefits.
Severely affected trees may eventually die.
Comments: Avoid wounding and overplanting
(allow plenty of spacing between trees) and maintain
good plant health. Prune out infected branches as soon as
they are observed (best done during dry weather) or
in winter. Cytospora cankers are often less severe
on healthy trees. See "Maintaining Healthy Trees
and Shrubs." While Cytospora cankers occur on all
spruce species commonly planted around North
Dakota, Colorado and Norway spruce are more susceptible
than Black Hills spruce (and other white spruce
varieties). Removal of damaged trees may help any
remaining spruce trees to stay healthy.
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Dutch elm disease (DED)
(Ophiostoma ulmi)
Host(s): Elm (all species)
Description/Biology: Dutch elm disease overwinters
in infected elm trees or recently cut elm logs. The
disease may be spread by beetles carrying the fungus that
causes DED, by root grafts or through the aid of people.
DED is commonly spread short distances from infected
elms to healthy elms through root grafts, where the roots
of neighboring trees grow together. The disease is
spread over both short and long distances by the native
elm bark beetle and the European elm bark beetle
(See "Native elm bark beetle" and "European elm
bark beetle"). These beetles may fly up to 1/4 mile in
search of a feeding site or may be windblown for many
miles. Another way that DED is spread is by transport of
elm firewood. Adult beetles lay eggs in galleries between
the bark and wood of weakened elm trees or recently
cut elm firewood. If that wood was infected with
O. ulmi, the fungus produces sticky masses of spores in egg
and larval feeding galleries. When adult beetles emerge
in the spring, they carry fungal spores on their bodies
to healthy trees where they feed. The fungus grows in
the water-conducting tissues of elm trees and
spreads rapidly within the trees. As the tree responds to
infection, and the fungus continues to grow, water
conducting tissues become plugged and the tree wilts.
Damage/Symptoms: By plugging water
conducting tissues, DED results in wilting and dying of
leaves outward from the infected area in branches or
stems. Those leaves are often bright yellow early in the
wilting stage and result in "flagging." A small flag in the top
of a tree is where the initial infection occurred. As
the fungus moves down the xylem, more and more of
the tree is killed. Trees infected through root grafts
suddenly wilt either on one side or throughout the entire
tree. Trees may be killed in one season but usually die
after several years. Peeling back the bark on affected
limbs often reveals streaking in the sapwood. Confirmation
of the disease requires laboratory testing.
Extensive sanitation programs in larger communities have
limited the impact of DED in those cities. However,
smaller communities, conservation plantings, and native
woodlands continue to sustain heavy losses soon after
the disease moves into those areas. DED will kill
most American elms that are not in good, prolonged
sanitation programs.
Comments: In landscape plantings and urban
areas, sanitation is extremely important in managing the
spread of DED. There are no economically feasible ways
to reduce DED in natural stands. In an area where DED
is present, elm wood should never be taken to an area
with healthy elms unless all the bark is removed
before transporting. All elm firewood should be burned
or debarked prior to April 1 when beetles resume flight.
A few elm logs secreted away negates a community's attempts at DED management. Dying trees should
be promptly cut down and debarked, burned, buried,
or chipped and composted. After an infected elm is removed, trenching between the diseased tree
and nearby (within a distance equal to the height of
the infected tree) healthy elms may prevent
transmission through root grafts. Fungicide injections, by
trained professionals, have been effective in reducing
the probability that trees will become infected, but they
are expensive, must be repeated every three years and
do injure trees. Thiabendazole and propaconizole
are labeled for Dutch elm disease prevention.
Therapeutic fungicide injections have not proven effective in
North Dakota. Quick removal of infected elms to save
neighboring trees is the most prudent and beneficial
practice. DED usually kills only small to mid-sized branches
of Siberian elm. Such infections can jeopardize
sanitation programs because they are usually not detected.
There are several cultivars of American elm and Asian
elm species and hybrids that are resistant to DED and
appear to be hardy in North Dakota.
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Elm black leaf spot
(Stegophora ulmea)
Host(s): American elm
Description/Biology: This fungus overwinters in
fallen leaves. New infections begin in the spring during
moist periods when the temperature is approximately
45oF to 75oF. Secondary infections (from spores produced
in spots caused by the first infections) can
continue throughout the growing season if adequate moisture
and appropriate temperatures are maintained. Young,
rapidly growing leaves are more susceptible than older leaves.
Damage/Symptoms: Typically yellow, but
sometimes white, spots are irregularly distributed on the
upper surface of leaves in early spring. About two weeks
after the spots appear, shiny coal-black pustules
(acervuli) will be clumped around the center of the spots and
may appear to be surrounded by a halo. Lower leaves
are usually infected first, but the disease may spread
upward if temperature and moisture are favorable for
disease development. Even after heavy infections,
severely blighted trees appear to recover during dry
summers. This results from the growth of buds that would
typically remain dormant until the following spring.
Continual infection, year after year, can cause
eventual decline of trees.
Comments: Elm black leaf spot is rarely life
threatening to trees in established landscape plantings and
forest populations. Although pruning, raking, and
burning infected plant parts decreases the level of infection
in young isolated trees, it does little to decrease black
spot damage in areas where spores can be blown in
from nearby. Fungicide treatments, supplemented
with optimum fertility, will help control black spot.
Fungicides that may be used to manage elm black leaf
spot include mancozeb, maneb, and Bordeaux.
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(See "Winter injury of evergreens")
Host(s): Elm, linden, maple, poplar
Description/Biology: Some of the
environmental conditions that may cause leaf scorch include poor
soils (including highly saline soils), flood or drought,
soil compaction, nearby excavation, root rot, high
winds, severe temperatures, limited room for root growth,
and transplanting. Leaf scorch occurs when the roots of
the tree cannot translocate enough water to the leaves
to replace the water lost to transpiration. A lack of
available water may cause this, but a flood can also result
in the same symptoms when there is water available to
the roots but they are starved for oxygen and begin to
die. Soil compaction may also cause death of segments
of root mass, resulting in less water uptake. Excavation
and limited space result in a reduction in, or lack of,
root mass and less water uptake.
Damage/Symptoms: Injury symptoms include
yellowing along the veins or margins that progresses
to browning of leaves. Some leaves will become
generally brown or show brown lesions on portions of the
foliar tissue. The most diagnostic symptoms include
marginal yellowing and browning. Minor late season leaf
scorch may be unavoidable on susceptible tree species in
North Dakota. Defoliation may occur, and dieback from
the branch tips may occur in severe situations.
Comments: The most effective way to manage
this disorder is to provide adequate water to the plants.
Do not plant into poorly drained soils, and try to ensure
that there will be sufficient space for root mass expansion
as the plants grow. If excavation, compaction of soil,
or physical disturbance to the roots occurs, there may
be little that can be done to correct the situation. In
this case, supplemental water and fertilizer may be
warranted to help the plants recover.
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(Erwinia amylovora)
Host(s): Apple, cotoneaster, crabapple,
mountain-ash, hawthorne
Description/Biology: Fireblight bacteria overwinter
in cankered areas from previous years' infections. In
the spring, as the temperature warms and the host
and pathogen become more active, an ooze composed
of plant sap and high numbers of bacterial cells
exudes from the cankers. Additional infections occur when
this ooze is transported by rainsplash or insects to
open blossoms, succulent leaves, vigorously growing
shoots, and wounds.
Damage/Symptoms: The classic symptom is
dark brown or blackened leaves on a shoot with a "shepherd's crook" at the shoot tip. Holdover
cankers, where the bacteria overwinter, will be found in
branches and stems larger than a wood pencil in diameter and
will appear darker in color than surrounding tissue.
Sometimes these cankers will appear sunken. If the
environmental conditions are right (warm, humid),
infections may spread very rapidly and kill many shoots,
branches, and rarely, whole trees.
Comments: Plant resistant cultivars and prune
out cankers. It is essential to sterilize pruning tools
between cuts to avoid spreading the bacterial pathogen to
new cuts. Some sterilizing agents include 1/5
strength household bleach, Pine
Sol®, or denatured alcohol (not rubbing alcohol). Bleach and Pine
Sol® are corrosive to metal, so rinse and oil pruners well when done.
Avoid excess nitrogen fertilization and grass fertilizer to
limit succulent terminal growth (most susceptible) to no
more than 12 to 15 inches. Streptomycin (an antibiotic)
or copper-based fungicides may provide some early
season protection from fireblight infections. Spray
timing, labeled hosts, and other requirements vary for
different products; therefore, read product labels before
purchasing them. An application after hail injury may
offer some protection.
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(Botrytis cinerea)
Host(s): Arborvitae, dogwood, hawthorne, juniper,
lilac, pine, rose, viburnum
Description/Biology: This is a ubiquitous fungus
that overwinters in plant debris or as sclerotia (hard
masses of fungus) on plant debris. B.
cinerea enters host plants through wounds, dead plant parts, or possibly
through direct penetration of intact surfaces. Many strains of
the fungus produce dark resting bodies (sclerotia) in
moist tissue. In the spring, sclerotia germinate to
produce infective spores, which become air-borne for
dispersal. Moist, still air is most favorable for disease
development. Lower leaves, shaded or dense plantings,
and plantings that remain wet for prolonged periods are
at greatest risk.
Damage/Symptoms: The fungus infects and
kills leaves, buds, flowers, twigs, and new shoots
primarily. Sparse webs of grayish-brown fungal growth is
evidence of gray mold. B. cinerea may appear as tiny clusters
of spores that may be seen with a hand lens.
Symptoms usually develop after extended periods of high
humidity or leaf wetness and rarely threaten the health of
trees and shrubs in North Dakota.
Comments: It is important to facilitate good air
circulation when trying to manage this disease. Avoid
applying water to foliage and remove diseased or dying
plant material.
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Iron chlorosis
Host(s): Maple, occasionally other woody plant species
Description/Biology: Iron chlorosis is an
abiotic disorder that is the result of a lack of iron in the
plant. This deficiency may be caused by too little iron in
the soil. It may occur when there is iron in the soil, but
it exists in a form that is not available to the roots of
some plants for absorption. This is often the case with
alkaline soils in North Dakota.
Damage/Symptoms: Affected trees have leaves
that are pale yellow-green to bright yellow. Leaves may
be uniformly yellow, or more commonly the veins
will remain green while the areas between veins turn
yellow. Severe iron chlorosis may lead to scorched
leaves (dieback from leaf margins), defoliation, branch
dieback, and possible loss of the tree. Silver maple
is generally very susceptible to this nutrient disorder,
but some silver maples do not have iron chlorosis.
Comments: Foliar applications of an iron chelate
may provide temporary relief from the symptoms.
Soil application and injections with iron chelate will
help reduce severity of symptoms. If practical, adjustment
of the soil pH may offer the best long-term solution to
the problem. Micronutrient capsules implanted in the
trunk are available as a longer lasting remedy than
foliar applications, but this form of therapy injures the
tree, and repeated injections may stress or ultimately kill
the tree.
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Leaf and shoot blight
(Venturia sp.)
Host(s): Poplar, willow
Description/Biology: These fungi overwinter on
shoots killed in the previous season and on infected
leaves which fall to the ground. Spores infect young,
succulent tissue in the spring. As leaves and shoots become
more hardened later in the season, they become resistant
to infection.
Damage/Symptoms: Infection begins as small
brownish spots that expand to form brown to black leaf
and shoot spots. Young shoots and leaves shrivel and
turn black. The disease may cause disfigurement. It is
seldom a serious threat to established trees unless it
occurs during several successive years. Young trees or
nursery trees may be damaged to a greater extent.
Comments: Management of the disease in
established trees requires pruning affected shoots and
removing leaves in the fall. Fungicides containing fixed
copper may provide protection of young trees.
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(Marssonina spp.)
Host(s): Cottonwood, poplar
Description/Biology: These fungi overwinter on
the previous season's shoot growth and on fallen
leaves. Spores are produced in these tissues during wet
weather in the spring. These spores infect leaves and new
shoots. Spores are produced in the new infections
and rainsplashed onto adjacent leaves, causing more
infections and sometimes building to epidemic levels by
mid to late summer.
Damage/Symptoms: Spots on infected leaves appear
as small circular to angular brownish lesions, often with
a chlorotic halo. When spots enlarge and coalesce,
they may appear as angular rust-brown to black blotches
on leaves. Whitish fuzzy masses of spores may be
evident on some of the larger spots. Petiole lesions are
usually lens shaped with white centers and black
borders. Severe infections cause defoliation, but the disease
is not usually severe in North Dakota. Repeated
defoliation can predispose trees to other problems.
Comments: Plant resistant varieties when possible.
For existing plants, remove dead and infected twigs
from trees. Rake up and destroy fallen leaves during
the growing season. High value ornamental plantings
may require application of a fungicide such as chlorothalonil.
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(Melampsora medusae)
Host(s): Aspen, cottonwood, larch, poplar
Description/Biology: Two different host species
are required to complete this fungal pathogen's life
cycle, but a repeating stage may allow year-to-year
infections without completing the full life cycle. This
fungus overwinters on fallen aspen, cottonwood, or
poplar leaves, from which it produces spores in the spring
that can infect a conifer host and possibly another spore
type that infects a broadleaf host. On the conifer host,
the fungus produces spores which infect aspen,
cottonwood, or poplar leaves. Early-season and mid-season
spores produced on these hosts can re-infect the
broadleaf hosts. Some spores produced after midsummer
will overwinter in lesions on fallen leaves.
Damage/Symptoms: Yellow to orange pustules
appear in midsummer on broadleaf foliage. Later in the
season, these pustules will be covered by an orange to
brown waxy layer that contains the overwintering stage of
the fungus. Severe infections can cause early defoliation
of broadleaf hosts. Repeated defoliation can
predispose trees to other problems.
Comments: Larches are not prevalent in North
Dakota, so new infections are mostly caused by
windblown spores coming in from adjacent areas. If a
problem develops with rust, early defoliation may weaken
young or newly planted trees. Protectant fungicides, such
as triadefon, help prevent infection. Avoiding
monocultures of susceptible cultivars substantially
reduces damage. Some cultivars (e.g., Norway, Robusta,
and Imperial) are resistant.
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Plum pockets/leaf curl
(Taphrina spp.)
Host(s): Plum, other Prunus sp.
Description/Biology: These fungi apparently
overwinter as spores in buds and in distorted plant parts
and infect young shoots, leaves or flower parts as buds
break and growth begins. During the summer, the
fungus continues to grow in infected fruit, leaves, and
shoots. Fungal spores are produced later in the season, but
no further infections occur until the following spring.
Damage/Symptoms: Growth of the
Taphrina spp. in the plant causes distorted growth, overgrowth,
and pigment production in the host. This may result
in symptoms such as leaf curl and witches'-brooms
on some trees. Infected fruit is often hollow, distorted,
and enlarged, forming "pockets" up to 10 times normal
size. These pockets may range in color from greenish
yellow to bright red and lack developed seeds. Infected
shoots and leaves are thickened and deformed (curled). Later
in the season, infected tissues are dark gray or
black. Serious fruit loss and plant deformity can result
if infection is not managed.
Comments: It is not effective to manage this
disease during the summer and fall. The best
management practice is to prevent infection in the spring.
Infected plant parts on the ground and in the tree should
be removed from the area. Lime sulfur or Bordeaux mixture can be applied when the temperatures are
above freezing but before buds begin to swell.
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(multiple fungal species including
Cytospora spp., Hypoxylon spp.,
Phomopsis spp., and Septoria spp.)
Host(s): Aspen, cottonwood, poplar, other species
Description/Biology: These fungi generally
overwinter in cankered tissues on branches and stems. Some
also overwinter on fallen leaves. They generally infect
trees through wounds and cause greater damage to trees
under stress. Some of these fungi infect healthy bark
and reside there as latent (inactive) infections; such
infections allow the fungus to rapidly colonize branches
of stressed or dying trees. Branches and limbs die as
they are girdled by the expansion of the canker fungi.
Tiny fungal fruiting bodies develop on the cankered bark.
Damage/Symptoms: Expanding areas of
discolored bark radiate from infection sites. Peeling back the
bark will reveal discolored sapwood. Infection sites may be
at any wound that penetrates the bark and exposes
the sapwood, such as those caused by insects, hail,
or human activity around trees. As infected tissues die,
the discolored area becomes sunken. Individual branches
or entire trees may be killed. Leaves may remain
attached to killed branches as the branches are girdled by
the expanding canker.
Comments: Positive identification of canker
fungi requires laboratory examination. Except for
Hypoxylon spp., their presence does not establish their role
as pathogens. Some canker diseases are more severe
in nurseries where plantings are dense and excess
nitrogen is used. Old poplar stands should be thinned before
they begin to decline. Leaf diseases, such as Marssonina
leaf spot and Septoria leaf spot, can weaken trees,
causing them to be more susceptible to severe canker
infections. Avoid wounding branches and stems. All poplars
are susceptible to canker diseases; however,
columnar European aspen, Assiniboine poplar, Tower
poplar, white poplar and native cottonwood are believed to
have greater resistance than other species and
cultivars. Quaking aspen is very susceptible to cankers, but
the cultivar Pikes Bay is reputed to be more
canker-resistant than the type species. No fungicides have
reliably controlled these diseases. See "Maintaining
Healthy Trees and Shrubs."
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(several species of Erysiphe, Microsphaera,
Phyllactinia, Podosphaera, Sphaerotheca, and
Uncinula)
Host(s): Crabapple, currant, hawthorne,
honeysuckle, lilac, oak, plum, poplar, rose, willow
Description/Biology: These fungi overwinter
in infected leaves that fall to the ground. Spores from
these leaves infect current-season leaves. The fungi
grow inside the leaves as well as superficially on the
leaf surface producing asexual spores throughout the
season. These spores cause new infections through the
growing season. As the plant's dormant season approaches,
the fungi produce sexual fruiting structures on the
leaf surface that are at first round and colorless, then
becoming yellow, brown, and finally black. Spores
that continue the disease cycle are produced from
these structures in the spring .
Damage/Symptoms: Damage from powdery mildew
is most often an aesthetic concern. Lesions are not
formed in infected leaves as occurs with most foliage
diseases. Rather, a white to dirty white powdery mat develops
on the leaf and fruit surface. Premature defoliation
can occur.
Comments: Fungicides are seldom necessary, but
those registered include sulfur, thiophanate-methyl,
triforine, and chlorothalonil. Avoid planting in the shade,
watering the foliage, and excessive fertility. Promote good
air circulation by properly spacing and pruning plants.
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(Rhizosphaera kalkhoffii)
Host(s): Spruce (especially Colorado)
Description/Biology: This fungus overwinters
in infected needles on the tree and in fallen needles. In
the spring, spores are produced in infected needles and
are spread in splashing and dripping water. The
newly emerged needles are most susceptible. Fruiting
bodies develop in as little as several months or as much
as several years after infection.
Damage/Symptoms: Rhizosphaera needlecast
has historically been a greater problem in eastern
North Dakota than western areas of the state. The disease
will often begin in the lower part of the tree, killing
interior needles as it progresses up the tree. Needles will
first turn yellow (for a few days) and later
purplish-brown. Infected needles may turn brown within six to
eight months after infection or remain green for several
years. The infected needles may fall before browning
or remain attached for several months after browning.
The fruiting bodies of R. kalkhoffii will emerge through
the normally white stomatal openings of brown or
green needles, causing the stomatal openings to appear
black under 10X magnification. In severe cases, only
current-season needles remain green, and prolonged
defoliation results in dead branches in the lower crown.
Comments: Prevention of Rhizosphaera
needlecast begins with planting healthy stock and allowing
adequate space for mature trees to develop. This can
be difficult in windbreaks, where density is important
for wind management. When Rhizosphaera needlecast
does develop in established trees, chlorothalonil or
Bordeaux mixture applied once in the spring as needles are
50% elongated and a second time soon after needles are
fully elongated usually controls early infections. Two
consecutive years of fungicide treatments are necessary
for adequate control. This disease is a greater problem
on Colorado spruce than white spruce and its variety
Black Hills spruce.
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Rose black spot
(Diplocarpon rosae)
Host(s): Rose
Description/Biology: This fungus overwinters in
fallen leaves or infected canes. Spores produced in the
spring infect leaves only if the leaf surface is continually
wet for seven hours, the relative humidity is high, and
the temperature remains around 68 0F to
750F. Multiple cycles of infection can continue throughout the
growing season when temperature and moisture are correct.
Damage/Symptoms: Large black areas
(approximately 1/2 inch) appear on the upper leaf surface as
circular spots that may coalesce into irregular spots with
feathery margins. Small black dots (fungal fruiting bodies)
may be visible in the center of the spots. Leaf tissue
surrounding the spots may turn yellow and leaves often
fall off the plant. On susceptible cultivars, raised
and purplish blotches may appear on the first year
canes. These will eventually become black. Heavy
damage (defoliation, reduced growth, poor appearance)
can occur when susceptible plants grow together.
Comments: There are different races of this
pathogen. Roses which are not damaged by rose black spot in
one location may be damaged in others. Disease can
be reduced in susceptible cultivars by mixing the
susceptible plants with resistant roses. If this disease has been
a problem, ask for resistant cultivars when
purchasing new rose plants. Sanitation and cultural practices
should be sufficient to manage this disease in resistant
plants. Remove fallen leaves and prune canes that
appear infected. Avoid a dense planting to promote good
air circulation. Do not water excessively during periods
of humid weather. Fungicides such as
chlorothalonil, mancozeb, thiophanate-methyl, triforine, and
wettable sulfur are readily available and effective, but may
need to be used repeatedly if conditions favorable to
infection persist.
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(Phragmidium spp.)
Host(s): Roses
Description/Biology: There are as many as five
spore stages for these rusts. These fungi overwinter
on diseased leaves and stems. Disease develops as
spores produced in the spring on overwintering pustules
infect through natural openings on the leaves and shoots of
the plant. Standing water on the leaves is required
for infection to occur. Repeating spores are produced
in these infections that cause additional infections
throughout the growing season as long as moisture is
available. When the temperatures begin to decline and day
length shortens, the overwintering spore stage (raised,
black lesions) forms on the leaves and stems.
Damage/Symptoms: Initial infections appear as
yellow to orange pustules on the lower leaf surface and
on shoots. As the disease progresses, orange to
brown pustules develop on both the upper and lower
leaf surfaces and on the stems. Toward the end of the
season, the pustules develop a black, crusty appearance.
When conditions are right for disease development, leaves
may curl and drop from highly susceptible cultivars. Canes
of normally hardy roses can die above stem infections
by the next growing season.
Comments: Cultural and sanitation methods are
usually sufficient for managing rose rust. Periodically
scouting for and removing leaves with pustules on them
will remove the source of new infective spores.
Removing all old leaves in the fall or pruning out old material
will eliminate the immediate source of the pathogen.
For high value plantings where rust is a perennial
problem, available fungicides include mancozeb,
maneb, propaconizole, and triadimefon.
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(Schizophyllum commune)
Host(s): Ash, caragana, little leaf linden, maple,
mountain-ash, white poplar, willow, several fruit tree species
Description/Biology: This fungus overwinters in
dead and diseased stem and branch tissues. It is a
widespread organism that can aggressively colonize trees stressed
by heat, drought, or major wounds, causing a white
canker rot of the sapwood. After host infection and
colonization, S. commune produces small (up to 2 inches
wide), white, hairy shelf-like structures on the sides of
trees. Spores from these structures enter hosts through
openings such as sunscald lesions, freezing injury cracks,
fire scars, dead branches, or stubs. Since this fungus
colonizes stems which have been killed by other
organisms and parasitizes other fungi, the presence of the
shelf-like structures is not an absolute indicator that the fungus
is causing damage to the tree.
Damage/Symptoms: S. commune can be identified
by small (less than 2 inches wide), white, shelf-like
structures produced in lesions or cankers on trunks
or branches. Since this fungus may come in after
other problems have affected trees, the extent of
damage caused by S. commune can be difficult to determine.
It can cause cankers, ultimately girdling trees, and does
rot sapwood, which can structurally weaken trees.
Comments: The presence of this organism on a tree
is often an indicator that the tree is seriously stressed.
See "Maintaining Healthy Trees and Shrubs." Healthy
trees may wall off infections. Avoid making large
wounds. Remove trees if they become hazardous.
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(Septoria musiva) also see "Poplar cankers"
Host(s): Aspen, cottonwood, poplar
Description/Biology: This fungus overwinters in
fallen leaves from infected trees and in stem and
branch cankers. During periods of wet weather in the
spring, spores are released and wind-blown or
rain-splashed onto leaves, branches, and stems to cause initial
infections. Secondary infections occur throughout
the growing season as more fungal spores are produced
in infected tissues.
Damage/Symptoms: The photosynthetic area of
leaves decreases with severe infection, and tree growth
is reduced. Premature defoliation (as early as July)
is common on highly susceptible trees and may
predispose trees to cankers and other problems. Leaf spots
are commonly small and angular, sometimes coalescing
to form large spots, but also may be circular and up to
1/2 inch in diameter with brown or yellow margins,
large and irregular-shaped with dark margins and tan
centers, or very small and white or silvery.
Comments: Leaf spots may first develop in
early summer and widespread infection may occur by
early August. Cankers rarely develop until after a few years
of leaf infections. Cankers may be visible by mid to
late summer, but are easiest to see after leaf drop
(see "Poplar cankers"). Plant uninfected stock,
remove highly susceptible varieties if they are damaged
or threaten other varieties, and replant using
resistant clones. Maintain plant vigor. Leaf litter cleanup in
the fall is helpful. Fungicides labeled for use on this
disease include chlorothalonil and mancozeb.
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(many fungi)
Host(s): Many tree species, common on boxelder,
elm, linden, maple, and pine
Description/Biology: These fungi are not plant
parasites but opportunistic organisms that grow on the
sweet honeydew excreted by sap-sucking insects
(especially aphids). The fungi produce spores on the leaf
surface. Some of these spores colonize other leaf surfaces
during the same season and others overwinter.
Damage/Symptoms: Sooty mold fungi appear as
a black powdery or "sooty" growth on leaves, stems,
or branches of many trees. While unsightly, this
condition rarely has a negative impact on the tree. Sooty mold
can indicate an aphid (or other sap-sucking insect)
problem that may warrant control measures. It is most common
in shaded, high humidity areas.
Comments: If the trees are young and the sooty
mold appears to be covering much of the leaf surface,
hosing off the leaves and branches periodically throughout
the growing season often eliminates the honeydew that
the fungi grow on. Control the underlying insect problem
if necessary. Increased light or air circulation and
reduced moisture will reduce sooty mold development.
These fungi are rarely a concern for tree health but can
cause aesthetic damage.
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also called Diplodia tip blight (Sphaeropsis
sapinea)
Host(s): Pine
Description/Biology: This fungus overwinters
primarily in infected cones and shoots. The fungus usually
builds to high levels on cones before shoot blight
becomes serious on infected trees. Fungal spores are
dispersed during moist conditions. Current-year needles,
shoots, and seed cones are infected in late spring or
early summer. The fungus develops rapidly in needles
and shoots, usually killing the entire shoot before needles
or shoots are fully developed.
Damage/Symptoms: New shoots are killed, and
often have short, brown, dead needles. Resin-soaking
of shoots is common. Branch tips, major limbs, and
entire trees have been killed by this disease in other areas. It
is usually more severe on older cone-bearing trees but
may severely affect young trees if they are near older
infected trees. Continued infections deform and reduce
growth. Infected cones develop normally. Fungal fruiting
bodies are easily seen on the scales of mature cones.
Comments: Fungicides that may be used as new
growth begins include copper hydroxide + mancozeb, propaconizole, copper salts, and
thiophanate-methyl. This disease is not currently a major concern in
established trees in North Dakota.
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(many species of fungi)
Host(s): Nearly all woody plant species
Description/Biology: Most stem decay fungi
produce mushrooms or conks (shelf-like or rounded
woody growths) from decaying wood. They overwinter in or
on infected wood. As they grow and utilize nutrients in
the wood, they break down cell walls so that the structure
of the wood is weakened or destroyed. Once they have
an adequate food base they produce fruiting
structures. Mushrooms produce spores in one season and
then deteriorate, while conks may be annual or
perennial. Perennial conks produce a new spore-producing
layer each year. The spores are dispersed by wind. Spores
that are deposited on exposed wood may infect that wood
if substrate and environmental conditions are
favorable (usually warm and wet). The fungi may grow in
the wood for a few to many years before they cause
substantial decay and produce fruiting structures. Most of
the decay fungi are relatively host specific, but some
infect many different host species.
Damage/Symptoms: The most obvious symptom is
the fruiting structure; its presence indicates that the wood
is decayed. Some decay fungi also cause cankers. If
the tree is cut, the wood in the central part of the trunk
will be relatively soft. Depending on the fungus involved,
its color can range from white to yellow to dark
brown. Trees that have been infected by decay fungi for a
long time may be hollow. The most obvious damage
occurs when the wood is structurally weakened and the
tree breaks during a wind or snow storm. Some decay
fungi may weaken the host by preventing transport of
water and nutrients. Branch and top dieback are common
in trees with advanced decay, but it is difficult to
determine whether the decay caused the dieback of the decays
was advance because the trees were stressed.
Comments: Aspen, boxelder, buffaloberry,
caragana, cottonwood, green ash, plum, and willow are
particularly subject to stem decay in North Dakota. If a tree
is structurally weakened, it should be evaluated as
to whether it would be a substantial hazard if it fell; if so,
it should be removed. Otherwise, it is likely that there
are so many infection sources that removal of a given
tree would not markedly reduce the chances for infection
of nearby trees. Reasonably healthy trees can contain
(wall off) most decay fungi to the wood that existed
when they were first infected (an exception is when
canker-rot fungi are present). Then, any wood that
develops subsequently will be sound. However, if the
infected trees are severely stressed or wounded, the fungus
can escape from the contained area to invade all the
wood that is present at the time of the stress or
wound. Therefore, maintaining good tree vigor and
avoiding wounds is particularly important in dealing with
trees that contain decay fungi.
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(Rhytisma acerinum)
Host(s): Maple
Description/Biology: This fungus overwinters in
tar-like spots on fallen leaves. In the spring, spores
are produced that can infect new growth. New infections
do not occur during late summer and fall. The
infected leaves may be the last to fall, so the fruiting bodies
are often on top of the leaf litter.
Damage/Symptoms: Typical leaf symptoms
are irregular, circular, raised and textured black spots on
the upper leaf surface that resemble tar. Often there will be
a reddish or yellow halo around the spots. This disease
is unsightly, but it is rarely injurious to the tree.
Comments: Raking up infected foliage may reduce
the incidence of the disease in following years. This
disease is rare in North Dakota. Fungicide treatments are
not necessary.
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(Tubercularia ulmea)
Host(s): Honeylocust, Russian-olive, Siberian elm,
and many other species
Description/Biology: This fungus overwinters
on diseased and dead branches and stems. Spores
are dispersed in rainsplash, by adhering to birds or
insects, or on horticultural implements. The fungus infects
trees through wounds in the bark caused by factors such
as adverse weather conditions, insect feeding, and
frost injury. It may also enter through human-induced
damage caused by pruning, weed control, and harvest
implements.
Damage/Symptoms: As infected tissues die,
the infected area becomes discolored and sunken. If
the fungus encircles (girdles) the stem during the
growing season as it grows through the outer sapwood,
cambium and bark, the leaves on the dead branch turn brown
and remain attached, resulting in a flag. Light colored
fungal bodies develop on the infected tissues and usually
turn black within two weeks. Tubercularia canker can
deform or kill stressed trees and shrubs as it kills branches
and stems.
Comments: This disease is very common in
North Dakota and heavy losses have been seen in
North Dakota highway, urban, and conservation plantings.
See "Maintaining Healthy Trees and Shrubs."
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(Valsa spp. and Leucostoma spp.)
Host(s): Prunus spp.
Description/Biology: These fungi overwinter
in infected branches and stems. Existing cankers
may expand, produce spores, and cause new infections
when temperature and moisture is favorable, wounds
are present, and host resistance is impaired. The spores
are spread by windblown rain and dripping water.
Cankers may survive and produce spores for one to many years.
Damage/Symptoms: Sunken cankers form, and
fungal spores are produced in small black fruiting bodies in
the bark. In some cases the fruiting bodies appear to
be covered with a white to tan powdery-looking
substance. If moisture is adequate, these spores may be exuded
in tan to orange colored tendrils from mature
fruiting bodies. Limited cankers on large stems are oval, and
the canker margin generally enlarges gradually each year
to form a target canker that may be sunken or
swollen. Branch cankers may be very long. In the case of
Valsa canker, clear to opaque gums are exuded from the
host. The exudates may become black and hard or crusty
as they dry.
Comments: These fungi typically colonize
weak, wounded, or less vigorous hosts. See
"Maintaining Healthy Trees and Shrubs." Winter injury,
pruning wounds, mechanical damage, insect injury, and
leaf scars are all sites where infection may occur.
Management strategies include pruning in the early spring
to facilitate healing and prevention of lawn mower
caused damage. Remove and destroy cankered branches
to reduce local sources for further infection. Do not
plant new trees of susceptible species next to older,
diseased trees. Try not to plant on poorly drained soils.
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(Verticillium dahliae)
Host(s): Many tree species, but especially catalpa,
elm, maple, and stone fruits.
Description/Biology: This soilborne fungus
overwinters in hardened resting structures in the soil or in
host tissue. The fungus invades root tissue or enters
through wounds, such as those created by contaminated
pruning tools. It grows in xylem, the water conducting
tissue, and causes foliage to wilt. V.
dahliae may remain viable in trees for several years after they are dead.
Damage/Symptoms: There are acute and
chronic symptoms associated with this wilt disease.
Acute symptoms include wilting, abnormal red and
yellow color of leaves, dry and curling leaves that may cling
to the branch (flagging), and defoliation. Chronic
symtoms include slow growth, sparse foliage, stunted leaves
and twigs, leaf scorch, and heavy seed crops. Both
chronic and acute symptoms can appear at the same time.
Green (in maple), dark brown, or black streaks may be
observed in the sapwood of most species. When
severe, this disease can kill branches or entire trees.
Comments: There is no control for Verticillium wilt.
A tree may recover if good health is promoted by
appropriately watering and fertilizing. Prune out
affected branches. Do not plant another susceptible tree
where Verticillium wilt is suspected since the pathogen
most likely remains in the soil. Tree species reported to
be resistant to this wilt disease include birch,
flowering crabapple, hawthorn, honeylocust, mountain-ash,
pine, spruce, and willow .
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(Endocronartium harknessii)
Host(s): Hard pines (e.g., Ponderosa and Scotch)
Description/Biology: This fungus overwinters in
galls on pine stems and branches. Spores are produced
in galls for consecutive years. These spores infect
developing needles and shoots in May and June. Galls
will develop and usually begin releasing spores two
years after infection. Western gall rust does not require
an alternate host as do many rust fungi.
Damage/Symptoms: Roughly textured, elongate
or roundish galls appear on branches and stems. In
the spring of the second and subsequent years after
infection, bright yellow-orange powdery spores appear on
the surface of the galls. Branches may die above galls
or witches'-brooms may form around galls, both of
which deform trees. Trees may break at the point of
stem infection. This disease can be very damaging in
nurseries and can cause dieback and stunting in landscape
and Christmas trees.
Comments: Managing the disease requires removal
of galls from infected trees. Remove galls on infected
trees from within 300 feet of a nursery growing
susceptible stock, Christmas tree plantings, and other high
value sites. Mancozeb and maneb are labeled for this
disease. Fungicides should be applied before infections begin
in the spring or early summer and repeated after
heavy rains and at two week intervals as long as necessary.
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Wetwood,
also called slime flux (bacterial species are often,
but
not always, associated with this disorder)
Host(s): Cottonwood, elm
Description/Biology: The bacteria that are
normally associated with wetwood overwinter in the soil or
host tissue. They are common water and soil
inhabitants. These bacteria can infect trees through root wounds
and may also be transmitted from tree to tree by bark
beetles or on pruning tools. Once present, the bacteria reside
in older xylem tissue of tree indefinitely.
Damage/Symptoms: The most typical symptom is
light gray (when dry) or dark brown or black (when
wet) streaking on the trunk, caused by a bacterial
ooze (slimeflux) that leaks out of openings, often in
branch crotches or stubs. The exudate may be
foul-smelling. The infected wood is dark-stained and
somewhat weakened. The damage is mostly aesthetic as a result
of the bark staining and odor. If the wood is used
for lumber, its value is reduced.
Comments: There is no treatment for this
disorder. Sterilize tools after pruning an infected tree to
reduce the possibility of spreading the disease on
pruning implements. Avoid stressing or wounding trees to
reduce expansion of the wetwood area within trees.
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Winter injury of evergreens
(See "Environmental leaf scorch")
Host(s): Arborvitae, juniper, pine, spruce
Description/Biology: Reddish-brown dead foliage
first seen on evergreen trees in the spring is often the
result of injury sustained in the previous winter. This
injury may be the result of desiccation (drying of the
foliage) or an early fall freeze. Desiccation occurs when
the ground contains inadequate moisture due to
drought, frozen ground, or when a plant is unable to
access moisture in the soil. Trees are then unable to take
up enough moisture to replace water lost by the
foliage. Early fall or late spring freezes (especially rapid drops
in temperature to below freezing) can kill
evergreen foliage when it is not adequately hardened off.
Damage/Symptoms: Since wind can accelerate
water loss from foliage, desiccation is often directional
toward prevailing winds. Winter desiccation may be
greater near light colored surfaces (white siding, white
rock mulch, etc.) or when trees are improperly
planted, stressed by insects/diseases/other environmental
factors, fertilized at an improper time, or have poor
winter hardiness. Winter desiccation and early fall
freezes generally kill foliage but buds and branches are
mostly unaffected. Foliage killed by both early fall freezes
and winter desiccation often remains green as long
as temperatures are cold. Damaged needles then
turn brown when temperatures warm up. Snow
insulates needles, so damage may occur only above the snow
line. New foliage usually emerges the following spring
or early summer. Assessment of total injury should only
be made after new growth has occurred. Winter injury
is often an aesthetic problem in evergreens but
does occasionally kill trees.
Comments: Only plant trees from a hardy source.
The incidence and severity of winter desiccation may
be reduced by watering trees adequately during dry
periods, especially during late summer and fall.
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X-disease
(Phytoplasma - a type of bacterium)
Host(s): Chokecherry, many other stone fruits,
many herbaceous plants
Description/Biology: The phytoplasmas overwinter
in infected host plants and are moved from tree to tree
by grafting and by leafhoppers. They live in the
phloem (food conducting tissues of host plants) and cause
a decline of susceptible hosts. Wild chokecherries
and perennial weeds are natural host reservoirs, and
the disease can move from them into new plantings.
Damage/Symptoms: New growth on infected trees
may begin later in the spring than on healthy trees.
The characteristic symptom is the presence of bright
orange to red leaves before fall coloration. There may be
a second flush of growth in late summer. In
subsequent years, leaves and shoots may be progressively
smaller. Development of rosettes or tufts of leaves on the ends
of branches is common. Fruit on infected chokecherry
may be pointed and red in color. On sour cherries, fruits
may be smaller than normal, pale red to greenish white,
and pointed. It is common to see normal looking fruit on
the same branch with symptomatic fruit. In
susceptible plants, growth slows progressively over three to
four years, followed by progressively severe branch
dieback. This disease has damaged and killed many
chokecherry plants in North Dakota.
Comments: The only management tool
currently available is planting disease-free stock and
isolating plantings. Removal of all infected trees and
all chokecherries within a radius of 500 feet has
been shown to reduce disease spread in other
regions. Interplanting of chokecherry with other trees and
shrubs may reduce the spread of the disease and reduces
the aesthetic impact if plants are lost. X-disease
resistant plants are being developed for release in the future.
Wild plums (Prunus americana) may be symptomless hosts.
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Insect and Disease
Management Guide
for
Woody Plants
in North Dakota -- f1192, Revised 9/2005
|
