Extension Educator, Horticulture
Yellow leaves are sometimes a normal characteristic of a plant cultivar or species. 'Margarita' sweet potato or 'August Moon' hosta are naturally yellow. However, not all plants are yellow by design.
Chlorosis is a fancy name for yellowing of plant foliage (of a normally green plant) due to a lack of chlorophyll development. Often the leaf veins remain green. At first the plant may appear lighter green than usual. Usually, the youngest leaves show the most yellowing. In some cases only part of the plant may be affected. I have seen one side of a pin oak that looks fine with the other side yellow.
As chlorosis progresses, the leaves may show brown speckling, become stunted and some of the newer leaves may die. As the disease progresses, branch tips and eventually entire branches may die. Even mature trees may die. However it may take several years before dieback occurs and branches die.
Many different situations can cause chlorosis. These include soil compaction, poor drainage, nutrient deficiencies (especially micronutrients such as iron), high pH soils, root injury, flooding, and drought. After our early rainy period I saw many warm loving annual flowers with yellow leaves. Figuring out why a plant is turning yellow is crucial in trying to remedy the problem.
Iron chlorosis is often seen in our area on pin oak, sweetgum, maple, birch, azalea, rhododendron and blueberry. These plants have a high requirement for iron.
In most cases, the soil has plenty of iron for plant growth, but as Nancy Pataky in the University of Illinois Extension Home, Yard and Garden Pest newsletter explains soils with pH above 6.5 bind up the iron and make it unavailable to the roots. Soils with high levels of zinc, manganese, phosphorus or copper also aggravate the iron chlorosis problem. This may occur with additions of large amounts of limestone or wood ash, or excessive applications of fertilizers high in phosphorus.
Treatment for iron chlorosis will vary with the cause. If the cause is related to the site as in soil compaction or poor drainage, than core aerification or tiling may help. Regular maintenance of mulching with wood chips and watering during drought periods is always a good idea. Remove any dead wood in the tree and fertilize in the fall with a balanced tree fertilizer if soil tests reveal deficiencies. Ideally before planting a tree or shrub get a soil test and do some homework to find out which plants will do well in that site.
Several options for supplying nutrients are available. Which option is used depends on the severity of the problem, tree age, soil pH and site restrictions.
Foliar application of nutrients in a water-soluble form or chelate form can help quickly, but the treatment is temporary since it only affects the leaves sprayed. Several treatments would be necessary. Foliar treatments are best in combination with longer-term solutions.
Another method for trees is trunk application of iron. It may take up to thirty days for the tree to respond, but the treatment may last several years.
Soil treatment may be the best option in less severe cases. Take a soil test to determine pH levels. Correct pH with sulfur and supply iron in a chelated form. Acidifying fertilizers may also help.
In severe cases check with a certified arborist to determine the best course of action for your tree.
Several fact sheets are available through your local extension office: Report on Plant Diseases No. 603, Iron Chlorosis of Woody Plants: Cause and Control; "Horticulture Facts" NC-3-80, Iron Chlorosis of Woody Plants: Symptoms and Control. The Report on Plant Diseases is also available at http://www.ag.uiuc.edu/~vista/horticul.htm.
The Champaign County Garden Walk is June 17 from 10-5. Tickets available at area garden centers and the Idea Garden on South Lincoln in Urbana.