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Chlorosis,
a yellowing of the plant leaf due to a lack of chlorophyll, may
be caused by a variety of factors. Among the more common causes
are compacted soils, poor drainage, root damage, alkaline soils,
and nutrient deficiencies. Probably the most common cause is iron
chlorosis, where iron is unavailable to the plant. Iron (Fe) is
an essential element for plant growth. It is required for the formation
of chlorophyll, the green pigments that capture light to produce
food for the plant. Iron is also necessary for the proper functioning
of many plant enzyme systems that influence respiration and plant
metabolism.
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Chlorosis
may develop because of unfavorable conditions for the utilization
of iron in the plant or in the soil. Under neutral or alkaline
conditions at a soil reaction (pH) above 6.5 to 6.7, iron
changes into insoluble forms and becomes unavailable for uptake
and utilization by the plants.
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Figure 1. Iron Chlorosis of Mountain Laurel
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Causes
of Chlorosis
Iron chlorosis can develop under conditions that reduce the availability
of iron to the plant. Iron is usable by plants only as the Fe++
ion, and is available in this form only when the soil pH is between
5.0 and 6.5. The presence of relatively large amounts of zinc, manganese,
or copper in the soil also contributes to iron chlorosis. Excessive
amounts of these elements decrease or prevent iron absorption by
the plant. Large amounts of limestone or ash, insufficient potassium,
or excessive application of fertilizers that are high in phosphorus
also contribute to iron chlorosis.
A deficiency
of available iron is aggravated by low temperatures and plants under
stress from poor root aeration or soil drainage, restricted root
growth, soil salinity, or a lack of iron in the soil. In addition,
many plants that are sensitive to iron chlorosis are planted without
regard to their adaptability to existing soil conditions.
Iron
chlorosis occurs in a wide variety of crop and ornamental plants
that grow in neutral and alkaline soils (pH above 6.5). Chlorosis
is most common in Illinois near brick, stucco, or cement foundations
and concrete sidewalks and drives, where high lime subsoils from
basement excavations were used as fill, or where excessive amounts
of limestone were applied.
Over
250 species of plants are susceptible. The crop plants that are
commonly affected include apple, blackberry, blueberry, cherry,
citrus, corn, flax, grape, turf and pasture grasses, mint, peach,
pear, plum, quince, sorghum, soybean, and strawberry.
Woody
ornamentals that are susceptible include arborvitae, azalea, bald
cypress, birch, black locust, boxelder, boxwood, camellia, crabapple,
cotoneaster, flowering dogwood, Douglas fir, elm, gardenia, ginkgo,
honeylocust, hydrangea, juniper or red cedar, lilac, silver and
sugar maples, certain oaks (bur, pin, red, shingle, white, willow),
pine, poplar, privet, pyracantha, rhododendron, rose, sour gum,
spirea, spruce, sweet gum, sycamore, tulip tree, walnut, and willow.
The most commonly affected woody ornamentals in Illinois are the
pin oak and the sweet gum.
Symptoms
A lack of available iron produces a variety of symptoms on trees
and shrubs, from a partial or complete yellowing (chlorosis) of
young leaves to poor growth or the death of the plant. Symptoms
may include the death (necrosis) of leaf tissues, premature leaf
drop, a stunting and dieback of twigs and branches, and the failure
of ornamentals to produce flowers or fruit. Occasionally, only part
of a tree or shrub will show symptoms.
Chlorosis,
the most common symptom, is characterized by a pale coloration of
interveinal leaf tissues, yellowish green to golden yellow. The
network of veins remains green (Figure 1). If severe, the leaves
may be dwarfed, turn ivory colored, and appear scorched along the
margins; or angular brown spots may develop between the veins. Such
leaves eventually curl, wither, and drop prematurely. Trees under
the stress of a high iron deficiency may have severely stunted or
dead terminal twigs and branches, while the lower and older branches
appear normal. In conifers, an overall yellowing of the needles
occurs. If severe, the needles progressively turn brown and drop;
shoots may die back.
Iron
chlorosis can be confused with zinc and manganese deficiencies.
Iron chlorosis can often be differentiated because the chlorosis
appears first on the younger or terminal leaves, spreading later
to the lower parts of the plant. Zinc and manganese deficiencies
appear first on the older, basal leaves.
Control
Iron chlorosis can be controlled when plants are supplied with available
iron. The iron may be sprayed onto the chlorotic foliage, introduced
into the trunk, or added to the soil. The most lasting results are
obtained through treating the soil. Generally, the treatment is
most effective when done by a licensed and experienced arborist
with the proper tools and equipment.
- Foliar Treatment. A thorough spraying of the foliage
with iron (ferrous) sulfate, a soluble organic iron complex, or
an iron chelate usually corrects chlorosis in leaves that are
treated, but will not benefit leaves that are produced later in
the season. Several sprays at 2- to 4-week intervals may be needed
to keep the developing foliage green. Iron chelates are more effective
in inducing recovery than spray applications of iron sulfate.
For treating shrubs, dissolve 2 1/3 ounces of iron sulfate in
3 gallons of water and thoroughly spray the foliage. Add a tablespoon
of a commercial spreader-sticker to the spray mix to help the
spray adhere to the leaves.
For treating trees, dissolve 1 pound of iron sulfate in 20 gallons
of water and add 2 ounces of spreader-sticker. Several ounces
of liquid household detergent can be substituted for the spreader-sticker.
The foliage should be wetted uniformly until the spray begins
to drip off the leaves.
If applying a spray of soluble organic complex or an iron chelate,
follow the manufacturer's directions on the package label carefully.
Spray the foliage in the late spring or early summer when the
leaves are expanding in size. Sprays are best applied in the early
morning or in the evening when the air is calm and both the humidity
and soil moisture are high. Foliar applications of iron materials
give a quick but short recovery from chlorosis, but the effects
will not last more than one season.
- Trunk Implantation. Introducing an iron salt into the
trunk of an affected tree should control chlorosis for 2 to 4
years. Treatment is quick, neat, and effective. Recovery is often
noted within 30 days. Three trunk implantation methods are available.
- Implanting ferric or ferrous citrate powder. Bore
holes 3/8 to 1/2 inch in diameter and 1 to 2 inches deep into
the white sapwood. Bore holes at an oblique angle, slanted downward,
and 2 to 4 inches apart in a spiral around the tree beginning
1 to 3 feet above the soil line. Make a hole for each inch of
trunk diameter. Trees with trunks up to 4 inches in diameter
at breast height should receive 0.4 grams of ferric citrate
per hole, with the holes 2 inches apart. Trees with trunk diameters
of 4 to 12 inches should receive 1.4 grams per hole, with the
holes spaced 3 inches apart. Trees with trunks larger than 12
inches in diameter should receive 2.8 grams per hole, with the
holes 4 inches apart. Seal the holes with grafting wax, putty,
glazing compound, or asphalt. Wood dowels or corks are not recommended
for this purpose.
- Placing plastic capsules containing ferric ammonium citrate.
Place capsules in holes bored into the sapwood as described
in A above. The plastic capsules have slotted sides that allow
a timed release of the iron salt into the sap and beveled heads
that keep the sap from bleeding, thus promoting rapid wound
closure (usually within a year). Several sizes of capsules are
available. The one to use depends on the size of the tree being
treated. Follow the manufacturer's recommendations concerning
the size and quantity of plastic capsules needed. Encapsulated
iron salts should be applied in the late spring or early summer.
Research in Illinois and Ohio indicates that the use of encapsulated
iron salts is a very effective method of controlling iron chlorosis,
particularly in pin oak and bald cypress.
- Injections of chelated iron into the trunk. Solutions
containing iron chelate can be injected under pressure into
the tree trunk. A commercial product comes with the chelate
solution in small plastic canisters. The iron chelate is injected
into the tree in a pattern similar to that described for methods
A and B. The holes made in the tree using this method are small
and usually heal quickly. No foreign object or material is left
in the tree trunk. Such tree injection treatments should be
made by an experienced commercial arborist.
- Soil Treatments. Iron chelates are commonly used and
are effective. For best results, follow the directions on the
package label carefully. The iron compound is placed in a series
of holes, 1 1/2 to 2 inches in diameter and 12 to 15 inches deep,
that are made with a punch bar or a soil auger. (A soil auger
attached to an electric drill provides a quick and easy method
of making holes). For uniform distribution of the chemical, the
holes should be spaced at 2- to 3-foot intervals in a series of
parallel lines 2 feet apart under the complete spread of the branches
and extending just beyond the drip line. No holes should be made
within 2 1/2 to 4 feet of the tree trunk. Approximately 200 to
250 holes are required in a soil area of 1,000 square feet.
Another method of treating the soil is to dissolve the iron chelate
in water and inject the solution into the soil, using a feeder
needle or lance connected to a power sprayer that delivers 150
to 200 pounds of pressure per square inch. Up to 10 pounds of
iron chelate in 200 gallons of water can be applied per 1,000
square feet. This is the maximum rate of iron chelate for highly
alkaline soils (pH 8.0 to 8.5). Lesser amounts are needed for
neutral soils (pH 6.5 to 7.2) and slightly alkaline soils (pH
7.2 to 7.5). Approximately 140 to 160 injection sites are required
in 1,000 square feet of soil.
Soil treatment is best done when the soil is moist in April, May,
or early June. If the soil is dry, irrigate immediately, using
1/2 to 1 inch of water or 300 to 600 gallons per 1,000 square
feet. The trees and shrubs should green up within 30 days after
application. A single soil treatment, properly done, should be
effective for up to 4 years.
If chlorosis is due to inadequate soil drainage or excessive soil
moisture (poor root aeration), the problem should be dealt with
accordingly. Avoid overwatering. Install plastic drain tile, or
make other arrangements to get rid of the excess water.
For
further information about woody plants, contact Nancy R. Pataky,
Extension Specialist in Turf and Ornamentals and Director of the
Plant Clinic, Department of Crop Sciences, University of Illinois,
Urbana-Champaign.
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