Reports throughout Illinois over the past several years indicate that many homeowners are having problems with their pin oaks. At least a part of this problem is due to chlorosis and soil pH, but I suspect that root injury has intensified the situation in many cases.

Yellowing of plant foliage, usually with green veins, is called chlorosis. If chlorosis is caused by a lack of iron, it is called iron chlorosis. Usually, the newest leaves show symptoms most intensely. A closely related problem is manganese chlorosis. Symptoms are similar except that older leaves show symptoms first.

Iron chlorosis is a common problem in Illinois on several tree species, including pin oak, sweetgum, maple, and birch. In most cases, the soil has plenty of iron for tree growth, but our high-pH soils bind up the iron, making it unavailable to the roots. No pathogen is involved in this noninfectious problem. Soil conditions cause the chlorosis.

As chlorosis intensifies, we see brown speckling of the leaves, then totally necrotic leaves, branch tip die-back, and eventual death of branches and even mature trees. The process is a slow one, taking several years before dieback occurs and branches die. However, if the condition goes untreated, the tree will decline and could eventually die.

We have seen more than the usual amount of chlorosis on pin oaks in the last several years. Some of the chlorosis may be due to wet soil conditions that cause root injury, inhibiting the uptake of nutrients. Many of the cases we have received at the Plant Clinic are on oaks and appear to be iron chlorosis. Why is the situation worse? And why does a tree that has been unaffected for 15 or 20 years start to show chlorotic symptoms?

Iron chlorosis seems to occur when roots grow into an area of high-pH soil, such as the foundation of a building, the area under a sidewalk, a gravel parking lot or driveway, or many other alkaline sites. This can explain why an older tree would start to show symptoms. Logically, any factor that affects root health could aggravate a nutrient uptake problem. In the past several years, we have had an abundance of rain early in the season, often in flooding quantities. Such conditions rob soil of oxygen, causing root injury and inefficient nutrient absorption. The cause of poor growth on pin oaks is not just chlorosis but high soil pH, location of planting, abundant moisture, poor drainage, and many other site and environmental factors that affect root health.

What can be done to remedy the situation? There may be nothing we can do to help older trees. Prune out dead wood to avoid secondary wood rots, and try to improve drainage on the site away from the tree. Consider treating the tree for chlorosis and possibly using an acid fertilizer. It might be wise to start with a soil pH test to determine the extent of that problem. There are several types of treatment that can be used for chlorosis. These are discussed in Report on Plant Disease No. 603, “Iron Chlorosis of Woody Plants: Cause and Control,” or horticulture fact sheet NC-380. The method you choose will depend on the intensity of the problem, the age of the tree, the pH of the soil, and site restrictions.