SHADE TREE DECLINE AND RELATED PROBLEMS
by J. R. Hartman, M. L. Witt
Maple and Oak Decline
Shade tree decline is a complex disease
situation since the cause frequently cannot be traced to one single factor.
Typically, one or more primary stresses cause the health of a tree to deteriorate,
and then secondary pathogens and/or insects contribute to further decline.
Determining causes of decline requires careful examination of the tree
and growing site as well as knowledge of the tree's history. Even then,
diagnosis may be difficult because the original stress may be obscure or
no longer present. Trees most commonly affected in Kentucky are Norway
and sugar maples, ash and oaks (particularly pin oaks).
There are several symptoms that may
indicate decline. Early symptoms can include: late spring flush, a decrease
in twig and stem growth, and premature fall coloration and leaf drop. As
stress continues, foliage may appear small and pale in color. Water sprouts
may grow from the branches and trunk.
Thinning in the upper crown foliage
becomes evident as twigs die back. Larger branches die as the decline progresses.
Heavy seed formation may be another indication of decline.
Affected trees may survive indefinitely
while continuing to gradually decline, or death may occur in a year or
two. Trees that have been declining gradually may appear to suddenly die.
This can occur when already weakened trees are subjected to a particularly
severe environmental stress (e.g., drought) resulting in an accelerated
decline and then "sudden death."
A wide variety of factors can be involved
in shade tree decline:
Poor Soil Conditions
Compacted or poorly drained soils,
poor fertility, moisture stress due to competition with turfgrasses, excessive
drainage, change in soil level around tree roots.
Unfavorable Weather Conditions
Late spring frost; drought; severe
winters; dramatic temperature drop after a warm, wet fall; plant variety
not suited to local climate.
Construction, de-icing salts, trunk
injury (e.g., mower damage), paving over roots, improper pruning (e.g.,
topping), improper planting (girdling roots).
Insects and Diseases
Canker fungi, defoliation by leaf spotting
fungi, root and butt rot disease, vascular wilt diseases (e.g., Verticillium
wilt of maple, ash yellows bacterial scorch of oak), insect borers, defoliation
In addition, lime-induced iron deficiency
is often associated with oak declines and girdling roots can contribute
to maple decline. These problems are explained below.
1.Attempt to determine stresses associated
with the growing site and alleviate them if possible. Early recognition
2.Water thoroughly during dry periods.
Water over several hours using a slow running stream of water from a hose.
Move the hose periodically to distribute the water over the entire root
area. Mulch soil over the roots of the tree.
3.Avoid site disturbances, like soil
compaction, as well as injuries to the trunk, roots and branches.
4.Apply a fertilizer if needed based
on soil test.
5.Prune out dead wood. Do not top tree.
6.See below for methods of correcting
iron chlorosis and girdling root problems.
Iron deficiency or chlorosis is a common
problem on pin oaks in some locations in Kentucky. The condition, also
referred to as lime-induced chlorosis, occurs where soil pH is neutral
or alkaline (7.0 or above). Other woody ornamentals particularly sensitive
to iron deficiency problems include: American holly, azalea, magnolia,
various black and white oaks, and rhododendron.
A pale green or yellowish coloration
between leaf veins is the most distinctive symptom of iron deficiency.
As the problem becomes more severe over the years, leaves appear smaller
than normal with brown dead areas along the margins, resembling leaf scorch.
Small, brown angular spots may develop between veins. Shoot growth is stunted
and twigs die back. Over a period of years, unless treatment is given,
affected trees can die.
1.Soil in pots or in small beds can
be acidified by adding sulfur, aluminum sulfate or ammonium sulfate. However,
it is difficult to change soil pH under a tree in the landscape.
2.Iron can be added to soil in the
form of water-soluble iron chelates commercially available as Sequestrene
138 or 330 Fe. This material can be injected into the soil as a liquid
or applied to holes in dry form. A maximum rate of 10 pounds of iron chelate
per 1000 square feet of soil can be used. Lower doses should be sufficient
in neutral or slightly alkaline soils. Injection sites or holes should
be 2 to feet apart and 1 to 6 inches deep. Apply equal portions of liquid
or dry material to these sites in the feeder-root zone beneath the canopy
drip line. Best results are obtained from treatments made in April, May
or June. Treatments usually remain effective for several years.
3.Implantation or injection of iron
directly into the trunk of pin oaks is being done with some success. Commercial
products such as MEDICAPS and INJECT-A-MIN make use of soluble iron such
as ferric (iron) citrate, ferric ammonium citrate or chelated iron. This
method, often used by commercial arborists, may be effective for a few
months to 1 year. For more detailed information on iron chlorosis, see
UK publication ID-84, "Iron Deficiency of Landscape Plants".
Tree roots normally grow outward in
a radius from the trunk. When a lateral root intertwines with another main
lateral root or encircles the trunk, a girdling root problem can occur.
The girdling root, in effect, causes "self-strangulation" by restricting
the flow of water and nutrients in the tree. This problem is more common
on maples (particularly Norway, sugar and silver maples) and pines than
on other species.
Affected trees are generally well-established
(20 or more years old), often with a past history of having done well in
their growing site. Gradually, one or more of the aboveground symptoms
of shade tree decline become evident. These symptoms may occur over the
entire tree or on one side only. If the problem is not corrected, the tree
will eventually die.
To confirm a girdling root problem,
carefully examine the base of the trunk. Normally, by the time a tree is
20 years old, lateral roots flare out at the soil surface. A girdling root
restricts normal buttress flare so the tree appears to ascend straight
up from the ground like a telephone pole. Often the girdling root will
be at least partially exposed at the soil surface, forming a noose around
the tree. In other cases, careful digging near the trunk to a depth of
6 to 12 inches may be necessary to locate the problem root.
A girdling root can be caused by factors
originating as far back as transplanting. When roots are cut as the trees
are moved from the nursery, secondary roots become dominant and grow across
the path of future trunk expansion. If container-grown plant material is
root bound at transplanting, the roots may continue to spiral around within
the planting hole. Roots of barerooted transplants can become twisted when
placed in an improperly dug planting hole. Girdling roots may also be associated
with obstacles that prevent normal outward growth of roots. For example,
pavement, buildings, boulders, ledges or compacted soil can all impede
root growth. As a result, lateral roots may grow back across the main system.
In all of these cases, decline symptoms do not develop until the twisted
or turned root actually restricts further trunk expansion.
To minimize the likelihood of girdling
1.Avoid planting in locations where
space for proper root development is limited.
2.Make several vertical slits at least
one inch deep in the root ball of root-bound container-grown plants before
3.Be sure the planting hole is large
enough to accommodate the roots of the transplant. It is best to dig the
hole larger than the size of the root ball. Contact your county Extension
office for information on proper transplanting techniques.
4.Inspect for potential girdling roots
several years after transplanting, but before problems develop. Roots near
the trunks of girdling-root-prone trees such as maples can be carefully
excavated and examined. Lateral roots posing a potential threat can be
Treatment for an already existing girdling
root is effective only if the problem is corrected early. If a tree is
allowed to decline for a number of years, its chances of recovery are slim.
To remove a girdling root:
1.Sever both ends of the girdling root
and allow it to decay in the soil. Remove a couple of inches from the severed
ends to prevent the cut ends from rejoining.
2.Fertilize the tree and prune out
dead wood the top growth as described under Nos. 4 and 5 of oak and maple