Iron chlorosis, or just chlorosis, has begun to show on pin oaks in the state. Chlorosis is another word for yellowing. It usually refers to leaves or needles that are light green or yellow rather than a healthy, dark green. Often the leaf veins remain dark green, while the rest of the blade is lighter in color. This condition is very common in Illinois on pin oaks, silver maple, red maple, sweetgum, and birch.

In Illinois landscape areas, the soils typically have a high (alkaline) pH level. Where irrigation is done, the pH of the water is also an issue. City water often has a very high pH level and can influence the soil pH where supplemental watering is frequent. These higher-pH soils may cause problems to trees. Minor nutrients are often tied up or bound within the soil chemistry, making them unavailable to the tree. In fact, these nutrients might be present in the soil, but they cannot be absorbed by roots. Iron or manganese seem to be the most limiting nutrients in a high-soil-pH system in Illinois. The symptoms caused by iron or manganese deficiency are very similar. Manganese deficiency is most likely if symptoms are worse on older leaves. Iron deficiency is often more a problem on new leaves first. An Illinois Extension report discussing iron and manganese chlorosis (Report on Plant Disease, no. 603, “Iron Chlorosis of Woody Plants: Cause and Control”) can be accessed from this site, http://www.ag.uiuc.edu/%7Evista/horticul.htm.

We have seen a few cases of chlorosis at the Plant Clinic this season and expect more. Whenever roots are injured, stressed, or growing poorly, absorption is limited. It follows that symptoms intensify in wet periods. Soils with high clay content or poor drainage also aggravate the problem. The question is how do we treat trees that are deficient because of the pH of the soil? Start by determining the soil pH. Consult your local soil-testing lab for this service. Determine the soil pH at which your tree thrives. Michael Dirr, in Manual of Woody Landscape Plants, often lists the desired pH for trees and shrubs discussed. Soils that have a pH below 6.7 seem to be ideal pH levels for red maples. Birches thrive at a pH of 6.5 or lower. Illinois landscape soils usually have a pH of about 7.4. Changing the soil pH around an established tree is not a quick process.

If a tree is severely affected, you might consider spraying the foliage with a chelated iron or manganese product available at garden centers. By spraying only a small area with one of these products, and watching for plant response, you may determine which element is limiting. During the growing season, the leaves should become darker green within a week or two of foliar applications. Such treatment works only on the leaves sprayed, so effects are very temporary and benefit only the leaves currently expanded. New leaves emerging after the spray do not turn darker green unless a more permanent solution is used.

Adjusting the soil pH has long-lasting efficacy but is the slowest treatment in terms of plant response. One such method is to add sulfur to holes dug 12 to 15 inches deep and 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 of the tree. Details are discussed in Report on Plant Disease, no. 603. This process may take several years to change the soil pH.

A short-term alternative that lasts for a couple of years is to inject chelated iron or manganese into the soil in the same holes that you dug for the sulfur. Actually, combining the two processes may give you the longest control. The chelates can be injected directly into the trunk but many horticulture specialists would rather avoid wounding the tree in this manner. Soil treatment is best done when the soil is moist in April, May, or early June.