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Why Is "Iron" Chlorosis So Severe Now?

July 20, 2005
Chlorosis of trees has been particularly intense in Illinois in recent weeks. Before we can address possible causes, letís review the chlorosis facts. Chlorosis is another word for yellowing. With chlorosis of trees we generally see yellowing of the foliage, but veins remain green. A chlorosis due to lack of iron is called iron chlorosis, but in Illinois we also see chlorosis involving manganese deficiency. As a rule, iron chlorosis usually causes symptoms most intensely on the newest leaves, whereas manganese chlorosis symptoms appear on older leaves first. If left untreated, chlorosis progresses to brown speckling of the foliage, branch tip dieback, and eventually branch death.

Chlorosis is a common problem in Illinois on several tree species, including pinoak, sweetgum, maple (especially red and silver), and birch. In most cases the soil has plenty of macro- and micronutrients for tree growth, but high pH soils bind up the iron or manganese, making it unavailable to the roots. No pathogen is involved in this noninfectious problem, although secondary leaf-spotting fungi often invade the weakened tissues. Soil conditions are the cause of the chlorosis.

So why are we seeing more chlorosis than usual? There is a good chance iron and manganese are in the soil, but they are bound to the soil. Some iron and some manganese are absorbed by roots, but less than needed by the tree. If roots are compromised, even less absorption takes place. Whenever roots are injured, stressed, or growing poorly, absorption is limited. Most of Illinois is currently in moderate to severe drought status, adding to root injury and limiting absorption of nutrients. Soils with high clay content or poor drainage aggravate the problem. Some growers have been watering trees, especially shallow-rooted trees such as birch. This is helpful, but 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. Any and all of these factors may explain the increased appearance of this noninfectious problem in 2005.

You may have observed that often a tree is not affected with chlorosis until it is about 15 to 20 years old. 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 one of many naturally alkaline sites. This may explain why many older trees seem to acquire this problem with age.

What can be done to remedy the situation? For older trees there may be nothing to be done to help. Prune out dead wood to avoid secondary wood rots. Try to improve drainage from the site to areas away from the tree. Consider treating the tree for chlorosis and possibly using an acid fertilizer in spring or fall. It might be wise to start with a pH test of the soil to determine the extent of that problem. Look for a local soil-testing lab in the advertising pages of the phone book. An Extension Web site lists some soil testing labs (http://www.urbanext.uiuc.edu/soiltest/index.html). The several types of treatment that can be used for chlorosis are discussed in RPD No. 603, Iron Chlorosis of Woody Plants: Cause and Control, which addresses both manganese and iron deficiency. The treatment method you choose will depend on the intensity of the problem, the age of the tree, the pH of the soil, and site restrictions. The RPD can be found in Illinois Extension offices and online (http://www.ag.uiuc.edu/%7Evista/horticul.htm).