Chlorosis is the condition of yellowing of plant foliage, usually with green veins. If the chlorosis is due to a lack of iron, we call it iron chlorosis. Usually, newest leaves show symptoms most intensely.
In Illinois, iron chlorosis is common on many tree species, most commonly 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 and make it unavailable to the roots. Iron is available to plants only as the Fe++ ion and is available in that form only when soil pH is between 5.0 and 6.5. Soils with high levels of zinc, manganese, or copper also aggravate the iron chlorosis problem. This is also the case with large amounts of limestone or ash, a deficiency of potassium, or excessive applications of fertilizers high in phosphorus.
As chlorosis intensifies, we see brown speckling of the leaves, then totally necrotic leaves, branch tip dieback, and eventually death of branches and even mature trees. The process is a slow one, taking several years before dieback occurs and branches die.
We have seen more than the usual amount of chlorosis on trees this summer. Some of the chlorosis may be wet soil conditions that cause root injury and thus inhibit uptake of nutrients. Many of the cases we have seen at the Plant Clinic are on oaks and appear to be iron chlorosis.
Why is the situation worse this year, and why does a tree that has gone 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: 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 cause a nutrient uptake problem.
This year we've had an abundance of rain, often in flooding quantities. Such conditions rob soil of oxygen, causing root injury and inefficient nutrient absorption.
What can be done to remedy the situation? Any of several treatments can be used, depending on the intensity of the problem, the age of the tree, the pH of the soil, and site restrictions. Options are discussed in Report on Plant Diseases No. 603, Iron Chlorosis of Woody Plants: Cause and Control, or "Horticulture Facts" NC-3-80, Iron Chlorosis of Woody Plants: Symptoms and Control.