Chlorosis is the name we give to yellowing of plant foliage due to a lack of chlorophyll development. If the chlorosis is due to a lack of iron, we call it iron chlorosis. Often, the term iron chlorosis is used improperly to refer to general chlorosis. To make the correct management decision to treat the chlorosis, it is important to accurately identify the cause of the chlorosis. Conditions that can induce chlorosis include soil compaction, poor drainage, nutrient deficiencies (especially micronutrients), high pH soils, root injury, flooding, and drought.
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 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. I’ve been watching the slow demise of many pin oaks with chlorosis in central Illinois for the last 12 years.
We have seen more than the usual amount of chlorosis on trees this summer. Some of the chlorosis may be the result of wet soil conditions, which cause root injury and thus inhibit uptake of nutrients. Many of the cases we have seen at the Plant Clinic are on oaks, birch, and sweetgum.
Why is the situation worse this year, and why does a tree that has grown apparently 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 such as around 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. Obviously drought stress can also injure roots.
What can be done to remedy the situation? Any of several treatments can be used for iron chlorosis, depending on the intensity of the problem, the age of the tree, the pH of the soil, and site restrictions. Suggestions for attacking the chlorosis problem 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. For many trees, especially shallowly rooted trees, it would help considerably to water in periods of drought lasting 2 weeks, remove dead wood in the tree to promote vitality and avoid wood rots, and fertilize in the fall with a balanced tree fertilizer. For details, see the references listed. The Report on Plant Diseases is available on the Web at http://www.ag.uiuc.edu/~vista/horticul.htm.