Diggin In - Advice on Iron Deficiency and Fungal Disease in Trees

By Scott Aker
Special to The Washington Post
Thursday, January 1, 2009

QI have a crape myrtle and a franklinia tree on one side of my house, both apparently suffering from a nutrient deficiency. In the past two growing seasons, the leaves have turned yellow and only the veins remain green. They are planted in poor soil: fill dirt from the time the house was built. What can I do?

AThe trees are showing signs of a condition called iron chlorosis. Iron is used to make chlorophyll, and without adequate iron, there is less chlorophyll. What little there is tends to be near the veins, since that is the point where iron and other minerals are delivered to the leaf tissue.

Iron is rarely deficient in soils, but its availability to plants is controlled by the soil pH. Highly alkaline soils trap the iron. Possibly, the soil may have buried rubble that is pushing up the pH. Have the soil tested to determine the pH and to unearth any other problems with nutrient deficiencies.

If the pH is higher than 7.0, add organic matter to reduce it. Try using pine bark or pine needles as a mulch around the trees. The lab that does the soil test may recommend adding iron sulfate to the soil and will tell you the quantity required. The effort will be rewarded with greener leaves next spring.

I have a cherry tree that grows out of a bed of pachysandra, which I fear is harming the tree through root competition. By July, nearly all of the cherry leaves displayed a yellow spot with a dark spot in the center.

Pachysandra has a rather shallow root system, and it probably benefits the tree by shading and cooling the soil. Your problem is not related to the pachysandra. It is most likely a fungal disease brought on by the wet weather we had last spring and early summer.

It may be a problem again, depending on the weather conditions in the spring when the new leaves are growing.

In most instances, you can allow fallen leaves to sift into the pachysandra so they decay gradually, recycling nutrients and building soil organic matter. The exception to this is when the leaves have potential to carry disease spores through the winter. A fungus named Blumeriella jaapii is the most likely cause of the leaf spots, and it can be carried over to the following spring on fallen leaves. Complete removal of the infected leaves can provide good control of the disease, but the pachysandra may impede this step.

One way to collect infected leaves in the fall is to place a net over the pachysandra so the leaves may be blown off after they fall.

Scott Aker is a horticulturist at the U.S. National Arboretum.

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