Many Illinois gardeners have been noticing yellowing leaves on their trees this spring states University of Illinois Extension Horticulture Educator, Kelly Allsup. This leaf condition is most likely caused by the frequent rains and consistently wet soils. Iron chlorosis is the yellowing of the leaves with the veins remaining green. It is most common to pin oak, red maple, white oak, river birch, tulip tree, sweet gum, bald cypress, magnolia and white pine. Some food crops like apple, blackberry, blueberry, cherry, grape, pear, plum and strawberry can show symptoms of iron chlorosis.
Iron is integral in the making of chlorophyll, and when it is limited plants show poor growth and health. The reduction of chlorophyll can hurt flower and fruit production. Most likely the issue is not the lack of iron in the soil but the pH of the soil. If the soil pH is too high (basic) it limits the iron's availability to the tree or food crop.
Some other causes of iron chlorosis could be excessive applications of fertilizers, poor soil drainage and restricted root growth. Chlorosis may be more common near brick or cement buildings or sidewalks. Plastic sheet mulching, compaction and water saturation can exacerbate the symptoms. In excessively wet or poorly drained soils, iron becomes unavailable.
If iron chlorosis is suspected,
1. Take a professional soil test and send to a lab to analyze for pH.
2. Correcting the pH to test below a 7 will make iron more available to the plants. Professional soil labs will give you exact instructions on how to amend the soil to alter the pH. If pH is too high, they will suggest a product made from sulfur that you can spread on the soil surface.
3. Improve soil conditions. If you did not amend your soil before you planted, add organic compost as a side dressing. Spread 2 to 4 inches of organic compost on top of soil around the base of plant and allow it to dissolve. Then add mulch.
4. A more immediate response can be utilized by spraying iron chelate on the soil or foliage, but this method is short lived and will not correct the issue in the long term.
If you are having problems with your newly planted or existing trees this rainy season, visit the Master Gardener help desk at your local extension office.