Chlorosis is caused by a deficiency of iron, manganese or zinc in the soil and afflicts oaks, especially pin oaks, and some maple, elm and other trees. It results in a yellowing of leaves, sometimes on a single branch, other times on one side of a tree or scattered throughout the tree.
Chlorosis causes the area between the veins of oak leaves to turn yellow; the veins remain green. The yellowing is more pronounced on foliage appearing later in the growing season. The yellowing of leaves can progress over several years or become severe in one growing season. When chlorosis is severe in pin oaks, the leaves turn pale yellow with angular brown spots between the veins. The leaves curl and turn brown at the edges.
It difficult to determine which deficient micronutrient is causing the chlorosis. Applying iron, zinc or manganese when it is not deficient in your soil can make the condition worse. For that reason, the soil should be analyzed before treatment.
Treatment involves incorporating formulations of the appropriate micronutrient into the soil. These micronutrients should be chelated, that is in a form the plant will absorb.
You can bore or punch holes around the tree and apply the minerals as a dry powder or you can force a solution of micronutrients into the soil under pressure. The Plant Disease Diagnostic Clinic at Cornell University recommends boring holes 1 to 1 1/2 inches wide 18 to 24 inches deep in two concentric circles. The inner circle should be 2 to 5 feet from the base of the tree. The outer circle should just beyond the spread of the branches. The holes in these two circles should be spaced 24 to 30 inches apart. Use similar spacing if you inject the micronutrients into the soil. After adding the micronutrients, fill the holes with loam, coarse gravel or sand and water thoroughly.
The amount of chelated iron, zinc or manganese to add varies; follow the manufacturer's directions for the correct rate of application. Plants treated early in the growing season should show improvement later that year.
If your oak exhibits signs of chlorosis and the soil has a pH above 6.5, apply a dust-free formulation of sulfur around the tree.The plant disease clinic at Cornell recommends applying 10 to 20 lbs. of granular sulfur per 100 square feet. This may injure ground covers or grass; if this concerns you, apply half of that amount followed by the rest two to four months later. The color of the foliage may not improve for several months or longerr. The benefits of using sulfur to lower soil pH can last five to 10 years or longer.
You can buy kits to test your soil online and sometimes from garden supply centers. Some of these kits will test for pH only, not for amounts of iron, zinc and manganese. You can also send your soil into companies to have it tested. These kits and services are often expensive; you can usually get your soil tested for free by contacting your local agricultural extension service.
Pin oaks suffering from severe chlorosis may not respond to any form of treatment and will eventually die.