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Introduction
The
micronutrients iron, manganese, and zinc are required by all plants for
proper growth and function. In alkaline to only slightly acid soils (pH
above 6.5) these micronutrients may not be readily available to plants.
Some plants when lacking one or a combination of these essential
micronutrients show symptoms of yellowing or chlorosis of leaves (Figure 1
in the Diagnostic Lab Fact Sheet). Pin oak, red oak, Norway maple,
rhododendron, azalea, mountain laurel, other maples and oaks and other
ericaceous plants often show symptoms of foliar chlorosis when growing on
soils with a pH of 6.5 or above.
In acid soils chlorosis of
sensitive plants can be induced by the careless disposal of waste
materials. Lime, plaster, building stone, ashes, wood waste, caustic
chemicals, detergents, and limestone driveways create conditions that can
lead to chlorosis if they are buried or located near sensitive plants.
When symptoms may also be expressed by plants growing on poorly drained
sites or in heavy clay soils.
Symptoms
Chlorosis caused
by iron, zinc, or manganese deficiency first appears as a yellowing or
lighter green color of the foliage. These symptoms may be distributed on a
few branches, on one side of the plant or throughout the entire plant.
Typically, the veins in the leaves remain green while the area between the
leaf veins turns yellow. This yellowing becomes more pronounced on the new
foliage later in the growing season. Symptoms may gradually worsen over a
period of several years or they may become more severe in a single growing
season. In severe cases the leaves of pin oak become pale yellow, curl,
turn brown along the margins and display angular brown spots between the
veins.
Figure 1:
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Figure
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Management Strategies
Whether iron or zinc or manganese is the
deficient micronutrient is difficult to determine from the symptoms on the
leaves. Furthermore, application of the micronutrient that is not
deficient may aggravate the plant's condition. For this reason, foliar and
soil analysis is recommended PRIOR to treatment.
Micronutrient
chlorosis may be treated by incorporating chelated formulations of either
iron or manganese or zinc into the soil. They may be applied as a dry
powder in holes bored or punched around the plant or as a solution forced
into the soil under pressure at a controlled rate. Follow directions on
the manufacturer's label concerning the rate of application. For trees,
holes 1 to 1 1/2 inches in diameter are bored 18 to 24 inches deep into
the soil and spaced 24 to 30 inches apart in two concentric circles. The
outer circle should be just beyond the limit of branch spread. The inner
circle should begin 2 to 5 feet from the base of the tree depending on its
size. After treatment these holes should be filled with good loam soil,
sand, or coarse gravel and watered in thoroughly. Similar spacings may be
used if a solution of one of these compounds is injected into the soil.
These soil treatments can be effective for 5 years or longer. Plants
treated with soil applications of a micronutrient early in the growing
season should begin to show improvement later that same season.
For trees, chlorosis may be treated by implanting into the trunk
capsules containing solutions of the needed micronutrient. Several kinds
of prepackaged micronutrient solutions are available. Follow the
manufacturer's directions when treating trees with these. For best results
treat trees in early summer as symptoms show up. Improvement of foliage
color should be noticed in 2 to 4 weeks. The tree eventually will revert
to the chlorotic state once the supply of micronutrient is exhausted. This
may take up to 2 or 3 years after implantation. Pin oaks suffering from
sever chlorosis may not respond to any treatment and eventually die.
Treatment of soil directly with elemental sulfur to lower the pH
to 5.5 or below will allow sensitive plants to take up needed amounts of
iron, manganese and zinc. When preparing a bed for ericaceous plants have
the soil tested and incorporate into the soil the recommended rate of
elemental sulfur to lower the pH. For established trees and plants
exhibiting chlorosis on soils with pH above 6.5, elemental sulfur in a
dust-free granular formulation may be broadcast directly over the soil
surface. The required amount of granular sulfur (usually 10 to 20 lbs/100
sq. feet) may injure grass or groundcovers. If this is a concern, then
apply one half of the required amount and the rest 2 to 4 months later.
Foliage color improvement may not occur for several months or until the
following year. However, the effects of lowering soil pH with granular
sulfur can last for 5 to 10 years or longer.
In the future the
best practice is to avoid planting on soils having a pH of 6.5 or above
plants prone to micronutrient chlorosis.
Created, KLS,
8/99
This publication contains pesticide recommendations. Changes in
pesticide regulations occur constantly, some materials mentioned may no
longer be available, and some uses may no longer be legal. All pesticides
distributed, sold, and/or applied in New York State must be registered
with the New York State Department of Environmental Conservation (DEC).
Questions concerning the legality and/or registration status for pesticide
use in New York State should be directed to the appropriate Cornell
Cooperative Extension Specialist or your regional DEC office.
READ THE LABEL BEFORE APPLYING ANY PESTICIDE.
__________________________________________________________________________________
The Plant Disease Diagnostic Clinic at Cornell
University is located at 334 Plant Science Building, Ithaca, NY,
14853. Phone: 607-255-7850, Fax: 607-255-4471, Email: kls13@cornell.edu |