Chlorosis — Yellow Leaves
Marianne C. Ophardt
Washington State University Cooperative Extension
Area Extension Agent
Leaves turning yellow is called chlorosis. Chlorosis is an abiotic (not caused by a living organism such as a fungus or virus) disease. It's characterized by greenish-yellow to yellow leaves. What causes chlorosis? The answer isn't easy because the cause is probably not the same in every case... but there are some very likely possibilities.
Much of the chlorosis seen in this region on trees during the summer is actually iron chlorosis or chlorosis caused by a lack of iron in the plant tissues. Iron is needed for the formation of chlorophyll, the green pigment in leaves. Without the green pigment which allows a plant to utilize sunlight for production of food and energy, the plant will eventually die.
Plants with iron chlorosis first turn yellow-green to yellow between the veins, with the veins remaining a darker green. With more severe chlorosis the leaves become pale yellow and develop brown spots between the main veins. Leaf margins may also turn brown with the leaves later drying up and falling off. Tree growth slows to a stop and dieback of branches can occur when iron chlorosis is extreme.
Iron chlorosis is quite common in our area because we tend to have alkaline soil. Alkaline soils are characterized by a pH above 8.0. While our soils actually contain adequate amounts of mineral iron, its in a chemical form unavailable to the plants due to the high pH of the soil. The yellowing or chlorosis can involve the entire tree, or may be restricted to one side or even just one branch. Within the same yard, there may also be perfectly healthy green trees growing right next to ones with iron chlorosis.
Certain types of trees and shrubs are more prone to iron chlorosis than others because they're more sensitive to high pH soils. Those trees most likely to show symptoms of iron chlorosis include pin oak, flowering dogwood, sweet gum, silver maple, tulip tree, magnolia, catalpa, white oak, holly, and white pine. Acid-loving shrubs, like azalea, blueberry and rhododendron, are also prone to iron chlorosis. These types of trees and shrubs should be avoided when planting in soils where the pH is extremely high.
While it's common to encounter highly alkaline, calcareous soils in this region, a high pH is not the only cause of iron chlorosis. First of all, it may not even be iron chlorosis. Chlorosis can be confused with similar symptoms expressed by mineral deficiencies such as magnesium, manganese or boron deficiencies.
Cultural factors can also lead to symptoms of chlorosis. Over watering is probably the most common cause of chlorosis, in fact iron chlorosis can be induced if soils are kept excessively wet as a result of over watering, compacted soils, or poor drainage. Tree and shrubs in this region often develop "lime-induced chlorosis" as a result of over watering.
Chlorosis can also be the result of root damage, girdling roots, or trunk damage from mowers and weed eaters. This is because root restriction, root injury and trunk injury all impair a tree's ability to take up and transport soil nutrients.
Chlorosis can also develop in extremely dry soil situations because mineral nutrients must be in solution for a tree to be able to absorb them from the soil. In dry soils they can't absorb the nutrients. Chlorosis is often seen on large silver maples in situations where half of the root zone or more is located in an area that isn't irrigated regularly, such as a dry lot, a gravel driveway, or a ditch bank area.
What can you do about chlorosis? If the cause is excessively wet soil, adjust the watering so the it doesn't remain saturated for any length of time. You will still need to water the tree adequately so it doesn't undergo drought stress. Watering should be done slowly enough to moisten the soil to a depth of at least 18 to 24 inches without saturating the soil. If the soil is compacted, aeration may help the water penetrate the soil more quickly. If the soil is too dry, the remedy is simple... water regularly to maintain moist soil to a depth of 18 to 24 inches in the entire root zone of the tree.
If watering or compacted soils doesn't seem to be the problem, check for trunk or root problems. Look for trunk injury from physical wounds to the base of the tree from mowers or weed trimmers. See if the bark on the southwest side of the tree has been damaged from winter injury. Check for girdling roots, constricted roots, or damage to the root system. When possible, these problems should be corrected. In some cases the tissue damage can not be corrected and the tree may eventually succumb to its injuries.
Finally, correcting iron chlorosis in alkaline soils isn't an easy task. Before you do anything, it's a good idea to have a soil test to find out the alkalinity of your soil. Once you've determined that your soil is alkaline, there are several approaches you can take in an attempt to correct iron chlorosis.
One of the most simple approaches is to acidify the soil. This is most easily achieved by adding sulfur to the soil prior to planting, but acidification is a slow process and the pH change will be slow. The easiest sulfur to use is prilled sulfur. This should be applied at the rate of 25 pounds per 1000 square feet of landscape bed and mixed thoroughly into the soil to a depth of 18 to 24 inches. If trees and shrubs are already established you will need to apply acidifying fertilizers, such as ammonium sulfate, instead.
Another method of correcting iron chlorosis is the application of iron chelates to the soil. Chelated iron is less affected by soil pH and more readily available to trees. Iron chelates must be placed in the root zone by drilling holes in the soil or working it into the soil.
Iron chelates in soluble form or iron sulfate can also be applied to plants through a spray to the leaves. These foliar sprays often result in a quick "greening" of the leaves, but the effects are temporary. New growth that develops after application will still be chlorotic.
There are also methods available for injecting iron right into trunk tissues with implants or injections, but these cause wounds to the tree trunk and many arborists advise against using them.
So if your trees are turning yellow, determine the cause and take action.