Reports on Plant Diseases |
RPD No. 603 - Iron Chlorosis of Woody Plants
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March 1996
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[ Causes ] [ Symptoms ] [ Control
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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 cap-ture light to produce food for the plant. Iron is also
necessary for the proper func-tioning of many plant enzyme systems that
influence respiration and plant metabolism.
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.
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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.
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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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