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Ash Problems Abound

June 5, 2002

The Plant Clinic has recently received many samples and calls about ailing ash trees. We see this every year but not always so early. Major possibilities include ash yellows, ash decline, and Verticillium wilt.

Management is going to be similar, so why distinguish among the three? Ultimately, these problems may kill your tree. Then, you may want a replacement. Knowing what killed the tree may help you avoid problems by properly selecting a replacement. If you have no such intentions, remove the dead wood, water in periods of drought lasting 2 weeks, and apply a balanced fertilizer in the fall.

One cause of ash decline is ash yellows, a disease mainly infecting white and green ash in the north-central and northeastern United States. It is a problem in Illinois but is hard to quantify because its presence is hard to confirm. Ash yellows is caused by a phyto-plasma (formerly, mycoplasma-like organism). These pathogens are somewhat like virus particles, cannot be cultured in a lab, are spread by phloem-feeding insects, and are limited to the phloem tissue of a tree.

A tree shows a loss of vigor for 2 to 10 years before it dies. Symptoms include short internodes and tufting of foliage at branch ends. Leaves become pale green to chlorotic (yellowed) and might develop fall colors prematurely. It might defoliate, with a sparse canopy. Cankers form on branches and trunk, causing twigs and branches to die back. Witches’-broom sprouts might appear on branches but more commonly on the trunk near the ground. Cracks may also appear.

It is rare for an ash tree to recover from ash yellows. Many of the ash trees in our landscapes are green ash. It is likely that this yellows disease is more common than we realize because with green ash only the cankers and stem dieback appear.

Ash decline is a term used loosely to refer to vari-ous conditions--sometimes ash yellows or even Verticillium wilt; but it often indicates any decline for which no cause is identified. It usually includes branch tip death, defoliation of leaves to give the tree a sparse look, and a slow decline over years. Trees with ash decline may appear to be recovering in spring and then decline in July and August. Often a viral disease is also involved, so leaves may be mot-tled, thickened, and slightly deformed.

To complicate matters, Verticillium wilt on ash also results in cankers and dieback, not the typical vascular discoloration of most Verticillium. Refer to Report on Plant Disease (RPD) no. 1010 (available in Extension offices and on Extension's Vista Web site). The Verticillium fungus can be isolated in labs such as the U of I Plant Clinic. Hundreds of plants may host the Verticillium fungus. Also, it lives years in the soil without a plant host. Logically, it follows that replacement plants must be resistant to infection, or the problem will recur. The list of options is small.

Ash yellows is caused by a phytoplasma, a phloem-inhabiting pathogen. It cannot be cultured in the lab on artificial media. Some services that offer specific PCR (polymerase chain reaction) tests can detect phytoplasmas in plant tissues. This service is not of-fered at the U of I Plant Clinic. I spoke with AGDIA, Inc., a company in Indiana that offers this service; see .http://www.agdia.com/ (There are likely other labs that can help.) The cost varies with the number of samples. The procedure is time-consuming and uses expensive equipment; unit costs are lower when mul-tiple samples are run, ranging from $134 to $315. Turnaround time also affects cost. For this test, AGDIA needs live, thick bark from the tree base. It must include phloem tissue and be deep enough to prevent the phloem from drying out. Call them before sending a sample.

Ash decline cannot be confirmed with laboratory isolations because many factors are involved: sometimes Verticillium, ash yellows, or a virus, and always some sort of site or environmental stress.

There are no cures for these maladies. Suggested management to slow disease progress includes remov-ing trees with severe dieback, watering trees during extended drought of 2 weeks, and fertilizing in the fall or early spring with a balanced tree fertilizer. Removal of dead limbs may help.

Author: Nancy Pataky


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