Source: James Schuster (708) 352-0109 Contact: Bob Sampson (217) 244-0226 Extension Communications Specialist e-mail: rsampson@uiuc.edu
Yellowing leaf tissue is result of chlorosis, said a University of Illinois Extension horticulture educator.
"It is caused by a lack of chlorophyll," said James Schuster. "Possible causes of chlorosis include poor drainage, damaged roots, compacted roots, high alkalinity, and nutrient deficiencies in the plant.
"Nutrient deficiencies may occur because there is an insufficient amount in the soil or because the nutrients are unavailable due to a high pH--alkaline soil. Or, the nutrients may not be absorbed due to injured roots or poor root growth."
One of the more common nutrient problems associated with chlorosis is lack of iron. Manganese or zinc deficiencies in the play will also cause chlorosis.
"The way to separate an iron deficiency from zinc or manganese deficiency is to check what foliage turned chlorotic first," said Schuster. "Iron chlorosis starts on the younger or terminal leaves and later works inward to the older leaves.
"However, manganese and zinc deficiencies develop on the inner or the older leaves first and then progress outward."
Plants need iron for the formation of chlorophyll. Chlorophyll gives leaves their green color and is necessary for the plant to produce the food it needs for its own growth. Iron is also necessary for many enzyme functions that manage plant metabolism and respiration.
Iron becomes more insoluble as the soil pH climbs above 6.5 to 6.7 (7.0 is neutral--below 7.0, the pH is acidic; above 7.0, the pH is alkaline). With most plants, iron can only be absorbed as a free ion (Fe++) when the pH is between 5.0 and 6.5.
"Other elements such as calcium, zinc, manganese, phosphorus, or copper in high amounts in the soil can tie up iron so that it is unavailable to the plant," he noted. "However, a shortage of potassium in the plant will reduce the availability of iron to the plant. Insufficient iron in the soil is also a problem.
"In many parts of Illinois, most soils were formed from limestone bedrock. Thus, the chlorosis problem is often due to high soil pH."
Herbaceous plants as well as woody plants are susceptible to chlorosis. On all plants symptoms can vary depending on several factors. How alkaline is the soil? The higher the pH, the more chlorotic the plant.
How long has the plant been chlorotic? In general, the longer the plant has been chlorotic, the more severe the chlorosis. Generally, mid chlorosis starts as a paling (lighter green to lime-green color) of interveinal (between veins) tissue, whereas a yellow color indicates a more serious condition.
"In some cases, only part of the plant is chlorotic," he said. "Affected areas or the entire plant may be stunted or fail to produce flowers and fruit. In addition, chlorotic leaves are more prone to scorching and leaf diseases. With severe chlorosis, the leaf veins will turn yellow, followed by the death of the leaf. The affected branch may die back, and the death of the entire plant can occur."
Treatment for chlorosis varies with the cause.
"If the chlorosis is due to soil compaction, poor drainage, poor root growth or root injury, then core aerification, tiling, mulching, or some other cultural practice may be needed," said Schuster. "Nutrient deficiencies can be treated in one of several ways.
"Foliar applications of nutrients in a water-soluble or chelate form can correct the problem for a while, but only affects the leaves that are present during application. Leaves that develop and grow after the treatment are not affected by the treatment. Therefore, several treatments per growing season may be necessary to keep the foliage green."
Another method is trunk application. Trunk application is quick and may last several years.
"However, you should allow up to 30 days for the tree to respond to trunk applications," Schuster said. "There are a couple of ways to apply nutrients via the trunk. Both methods involve drilling holes in the trunk--the number of holes is based on trunk diameter.
"With the first type of application, containers with tubes are then attached to the holes. The tree's movement of moisture will help draw the nutrients into the trunk. After the containers are empty, they are removed and the holes are plugged. The other method requires plastic capsules to be hammered into the drilled holes. These capsules are designed to be left in the tree. In both cases, consider hiring a professional to do trunk applications."
The final method for treating chlorosis is via soil treatment.
"Soil tests should be taken to determine soil pH as well as the availability of nutrients that can cause chlorosis," he said. "Based on a soil test, the pH is corrected or the nutrients are applied by drilling holes in the ground at 45-degree angles to a depth of 12 inches starting three to five feet from the trunk. These go out as far as the tree is tall or the property lines, foundation, streets, driveways, etc., allow."
