Iron chlorosis and yellow trees
When I was just a kid, they used to talk about "iron blight" in field crops, especially fall row crops. I thought it was a disease. Actually it was a joke about someone getting too close with the row crop cultivator to the actual row and wiping out part of the crop or clipping the roots. This was in the days before we had real effective herbicides, so we drove between the rows carefully and tilled out the weeds with shanks or small v-blades. They also called it "cultivator blight." It was in other words--mechanical injury.
That brings me to the topic of the day--iron chlorosis or what you really could call "iron blight." Iron chlorosis occurs on trees, lawns and field crops. In our area the trees and soybeans get it the worst. It seems to be more obvious when you have good soil moisture and we have had that the last month. My focus today will be on the trees and we will address other chlorotic situations in the coming weeks.
When iron in the soil is either deficient or unavailable to trees, iron chlorosis can occur. In most Kansas soils, iron is plentiful, but unavailable to trees. Iron is readily available to trees growing in soils with a pH of 5.0 to 6.5. In soils with a pH of 7.0 (neutral) and above, iron changes to an insoluble form that is not as available to trees. Many locations in our area have soils with a pH of 7.5 or above and thus are said to be alkaline or sometimes called calcareous or basic.
Several species of ornamental and native trees in Kansas suffer from iron chlorosis. The most susceptible species are pin oak, silver maple, bald cypress, and sweet gum. White pine, river birch, walnut, sugar maple, red maple, eastern red cedar, sycamore, ornamental pear, and some crab-apple species may develop iron chlorosis.
Trees that exhibit yellow leaves with prominent green veins are often suffering from a micro-nutrient deficiency called iron chlorosis. Symptoms may range from a mild yellowing of the leaf tissue between the veins to severe yellowing. This can lead to the death of leaves, dieback of branches, lack of vigor, and possibly death of the entire plant.
Initially, the problem may affect only a few branches, though it is common for the whole tree to take on a yellow-green hue. Trees may not show symptoms every year if only mildly affected. Trees with distinct symptoms will continue to deteriorate in following seasons.
Since iron chlorosis is often a problem of pin oaks and silver maples, I would not plant these species, even though they do grow rapidly. Often you acquire such trees when you purchase property from the previous owner. Other tree and shrub species can be affected. Chlorosis is typically associated with one of three site conditions--a soil with a high pH, a heavy soil containing a high percentage of clay, or a site where there has been extensive soil fill or removal, compaction, or construction injury that has damaged the root system. Older, established trees are especially subject to the latter type of injury.
Treating iron chlorosis can be expensive. There are three ways to treat iron chlorosis problem with iron-providing fertilizer: foliar sprays, soil incorporation, or trunk injection. The first two can be done by anyone. Trunk injection is best done by a trained arborist, because of the specialized equipment and procedures involved.
The foliar spray treatment uses an iron chelate and water mix sprayed directly onto the leaves of the affected tree or shrub. The results are usually noticed within a few days, but the problem is only temporarily corrected. A fertilizer that supplies iron, such as Greenol, Mir-Acid, or Acid-Gro should be used. Follow label instructions carefully.
Some type of spray equipment will be necessary for applying the fertilizer. Large trees may need to be treated by a tree company with powerful sprayers that can reach the upper portions of the tree. A hose-end sprayer may be adequate for smaller trees or shrubs.
A K-State Extension publication, MF-718 titled "Iron Chlorosis in Trees" is available on-line or at your county extension office.