Pathological
Disorders –
Branch and Stem
Diseases
The fungus Cytospora kunzei causes a common and
severe spruce canker. As the fungus invades the limbs of
the tree, cankers are formed which cause the bark to
decay. The result is dead foliage
and limbs (71KB color jpg),
particularly near the base of the tree. A flow of bluish resin (38KB
color jpg) from the diseased branches is the
earliest symptom. When the outer bark surface is
carefully pared off, the circular embedded black fruiting bodies (59KB
color jpg) of the fungus can be seen in the inner
bark.
Cytospora is spread to healthy limbs or healthy trees
by rain or contaminated pruning tools. Measures used to
prevent sunscald also aid to prevent Cytospora because
the fungus enters through such wounds.
Control:
The most effective control measure for Cytospora canker
is removal of infected branches in winter. At this time
infected branches that are already dead will be brown,
while dying branches will show a grayish-green or dull
color.
Infected branches never recover. Do not leave them in
place hoping they will revive. Prune out all diseased
branches before the first spring rain to prevent spores
of Cytospora, which mature during the winter, from
splashing to new infection sites on healthy branches.
Dispose of all pruned-out branches away from spruce
plantings.
Spruce cancer is most easily controlled when the
disease is first noticed on one or
a few branches.
Disinfect all pruning tools such as knives and saws by
wiping them with rubbing alcohol. Treat pruning and other
wounds with a wound dressing such as asphaltum or sulfur
tree dressings.
The Cytospora canker fungus most readily attacks weak
or shaded branches and trees. Therefore, remove weak
branches and avoid crowding in plantings. When trees get
too large and are crowded together consider thinning out
some trees. Spruce trees are shallow rooted; do not
cultivate under established spruce trees as this destroys
fine feeder roots and weakens the tree.
Cytospora is most easily controlled by prevention.
Keep trees well watered and fertilized. Maintain good
drainage and air circulation. Avoid wounding trees. Prune
during dry periods in summer and during winter. Do not
bring or dispose of spruce Christmas trees or branches
near healthy spruce plantings. Black Hills spruce seems
less prone to Cytospora canker than Colorado spruce.
Although several stem rusts of pines are known, only
one occurs in North Dakota. This is the Western Gall Rust
or Pine-to-Pine Rust caused by Endocronartium
harknessii. This fungus is a perennial parasite of
pine stems. It causes the tree to form large, round galls (52KB
color jpg)at the site of infection.
The life cycle of western gall rust is simple. In late
spring, masses of orange spores burst from the surface of
the galls. These spores infect the new shoots or candles
of the same or nearby pines. Gall formation takes at
least one year to develop after infection. Galls may
remain alive for many years and continue to enlarge as
long as the branch is alive. If the branch or stem dies,
so does the gall.
Western gall rust is present throughout North Dakota.
It has been found in plantations, shelterbelts and on
ornamental trees. Infected planting stock appears to have
been the original source. The current high infection
levels in some Christmas tree plantations are also
attributable to use of infected stock purchased from
nurseries where the rust is present.
In North Dakota, infections occur in so-called `wave
years'. In such wave years infection rates may be high,
while in the intervening years few or no new infections
will arise. This gives rise to the appearance of a series
of even-aged galls on older trees. Wave years in North
Dakota may be 5 to 7 years or more apart.
The damage done by western gall rust is related to the
number of galls on a tree and their location. Heavy
galling may kill occasional
trees but usually does not. More often such trees may be
stunted and show distorted growth. Trunks of trees with
main stem galls may break off just above the gall. Main
stem galls may cause death of the shoot above the gall.
This results in a bushy, leaderless growth.
Ponderosa pine, jack pine, Scots pine, lodgepole pine
and mugho pine are susceptible to western gall rust, but
individual trees vary in susceptibility to infection.
Often a tree completely free of infection will occur
right next to a heavily-galled tree.
Control:
Inspect nursery stock for galls. Cull and destroy any
galled seedlings found. In new plantations examine trees
1 to 2 years after planting and rogue out any galled
trees which escaped earlier detection.
Certificates stating freedom from rust may be
requested from suppliers but since few states enforce
controls on movement of western gall rust infected stock,
such paper may be worthless. Most nurseries cull out
obviously galled seedlings but some infected seedlings
may slip past as not all infections occurring in the year
of lifting will have developed into galls.
In Christmas tree plantations, do not plant new stock
adjacent to older infected trees; cut and remove galled
trees as early as practical. Fungicidal sprays applied at
label rates during May and June to control foliar
diseases, can also protect trees in the nursery from new
gall rust infections. Three sprays at 10-day intervals
are needed.
Cedar-apple rust and related rusts are caused by
species of Gymnosporangium that attack evergreen
trees and shrubs in the juniper family. In North Dakota
these include eastern red cedar (Juniperus virginiana),
Rocky Mountain juniper (J. scopulorum), creeping
juniper (J. horizontalis), common juniper (J.
communis), and certain exotic junipers planted as
ornamentals.
Red-brown galls of cedar-apple rust form on eastern
red cedar over a period of nearly two years. The mature
galls (`cedar apples') produce orange
gelatinous tendrils (86KB color jpg) (`horns')
during moist spring weather. Heavily infected trees can
be quite striking. The spores formed on these tendrils
infect apple leaves and fruits. In G. clavipes (quince rust) (44KB
color jpg) the galls are perennial and may live
for several years, producing new crops of spores each
spring. Quince rust galls may be only slightly swollen.
Infection of Rocky Mountain juniper by juneberry rust (Gymnosporangium
nidusavis) results in `witches brooms' or `birds
nests', which are stunted bushy
groups (61KB color jpg) of
branches.
Like many other rust fungi, cedar-apple rust and other
Gymnosporangium rusts alternate parasitism between
two kinds of plants—one being the juniper (54KB
color jpg). The alternate hosts (plants on which
it completes its life cycle) are trees in the pome fruit
group of the rose family, including apple, pear, quince,
hawthorn, mountain ash, and juneberry.
Table 2 lists the Gymnosporangium
species occurring in North Dakota and the evergreen
and rosaceous (pome fruit) alternate hosts.
On pome fruit hosts, the symptoms and signs of these
rusts are very characteristic. Symptoms develop on both
leaves and fruits. Small, yellow-to orange spots develop
on the upper leaf surface shortly after bloom. Black dots
soon appear in these spots. The infected spots are often
thickened or blistered. In mid-summer tiny orange-colored
tubes form on the lower leaf surface opposite the spots
on the upper surface. These tubes
split open and curl back (46KB color
jpg). Heavy infection (57KB color jpg) can result in severe
defoliation. Spots on the fruits are similar except that
the tubes are not always formed on apples while they are
spectacular on juneberry and hawthorn.
Table 2. Hosts of Gymnosporangium
rusts.
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Pome Fruit
Juniper Host* Plant Part Attacked
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Cedar-apple rust- ERC, RMJ Leaves, fruits of apple.
G. juniperi-virginianae
Quince rust- ERC, RMJ, Fruits, especially
G. clavipes CJ, BJ hawthorn.
Hawthorn rust- ERC, RMJ Foliage, especially
G. globosum hawthorn; also on apples,
mountain ash and pear.
Juneberry rust- RMJ, ERC, CJ Fruit, stems, leaves of
G. nidus-avis juneberry, quince,
apple, mountain ash.
(No common name) RMJ Hawthorn foliage.
G. bethelii
(No common name) BJ Juneberry, Hawthorn,
G. clavariforme Cotoneaster foliage.
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*ERC = eastern red cedar (Juniperus virginiana)
RMJ = Rocky Mountain juniper (J. scopulorum)
CJ = creeping juniper (J. horizontalis)
BJ = common (Bush) juniper(J. communis)
Control:
Gymnosporangium rusts are seldom severe enough on
juniper to warrant control. In a home garden, removal of
galls by pruning out in late winter may give control.
Eastern red cedar (Juniperus virginiana,
`Canaert' and `Glauca') and Rocky Mountain juniper (J.
scopulorum, `Hughes', `Lakewood Globe', `Medora',
`Moffet', `Welch', `Pathfinder', `Cologreen', `Platinum'
and `Gray Gleam') are generally susceptible to cedar
apple rust. J. chinensis sargentii (Sargent
juniper) and many cultivars of J. sabina (`Arcadia',
`Skandia', `Buffalo' and `Broadmoor' junipers) are
resistant to cedar apple rust (G.
juniperi-virginianae) but may be susceptible to other
Gymosporangium rusts which occur in North Dakota.
Since the rusts must alternate between a juniper host
and a pome fruit host, do not plant these two together or
near each other. If you wish to plant crabapples in the
vicinity of junipers, use resistant varieties. Native
crabapples are susceptible to cedar-apple rust; Asiatic
crabapple varieties are generally resistant. `Dolgo',
`Centennial', and Manchurian crabapples are resistant to
rust.
In major lumber and pulpwood producing regions pines,
spruces and other conifers suffer extensive losses from
stem decay and rots. Timbersized, mature trees are those
most often affected by wood decay or heart rot.
Heart rot in living trees is caused by fungi which
have the ability to decay wood. These fungi gain entrance
to the heartwood of the tree through wounds, branch
stubs, etc., which expose the bare wood. The fruiting
bodies, or "conks", are common on trunks of
decaying trees. Production of fruiting bodies is a sign
of extensive decay in the stem. Heart rot fungi do not
invade living wood of healthy trees. As long as the tree
is growing vigorously the rot will be confined to a small
central core of the trunk and the structural integrity of
the tree will be maintained. If the tree is weakened for
any other reason or fresh wood exposed by severe pruning
or storm damage, then the decay fungi can advance to more
and more wood. When this happens the tree may become
unsafe and the risk of wind or storm breakage greatly
increases.
Control:
Avoid pruning wounds which expose large areas of wood.
Train trees when young so major branch removal will not
be necessary later. Remove broken branch stubs following
storm damage. Keep trees growing vigorously. Heart rot is
primarily a disease of mature or over-mature trees.
Except in native stands in southwestern North Dakota, few
evergreen trees in North Dakota have yet reached the age
where heart rot is likely to be a problem.
While root injury and disturbance (see earlier
section) are most often the cause of root-related
problems in evergreens in North Dakota, certain diseases
do cause problems in some cases. Root diseases are
difficult to diagnose because the affected parts are
hidden below ground. Symptoms of root rot appear in
above-ground tree parts as decline, dieback of shoots or
branches, stunted growth and abnormal or discolored
needles. These symptoms are not specific to root rot.
B A C K | C O N T E N T S
Environmental
Injury
Pathological Disorders - Foliage
Diseases
Pathological Disorders - Shoot
Blights
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