Questions about ash trees have been common the last few weeks. If your ash tree looks healthy, don’t be too concerned, but be aware of these symptoms. Many homeowners have seen decline and dieback of even 20- to 25-year-old trees. A number of disease problems might be involved in decline of ash trees.
One possible cause of decline is ash yellows. This disease primarily infects white and green ash in the north-central and northeastern parts of the United States. It is a problem in Illinois, but one that is difficult to quantify because its presence is difficult to confirm. Ash yellows is caused by a phytoplasma (formerly called mycoplasma-like organism). These pathogens are somewhat like virus particles, cannot be cultured in a lab, and are spread by phloem-feeding insects. They are definitely limited to the phloem tissue of the tree. This disease is characterized by a loss of vigor over 2 to 10 years before the trees die. Symptoms include short internodes and tufting of foliage at branch ends. Leaves become pale green to chlorotic (yellowed) and might develop fall colors prematurely. The tree might defoliate, and the canopy generally appears sparse. Cankers form on branches and the trunk; and twigs and branches die back. Witches’-broom sprouts of growth might appear on some branches but are more common on the trunk near the ground. Cracks in the trunk may appear in this area as well. It is rare for an ash tree to recover from ash yellows. Many ash trees in our landscapes are green ash, which do not show ash yellows symptoms as clearly as white ash. It is very likely that this yellows disease is more common than we realize because the typical witches’ brooms and yellowing are not always seen with green ash, even when the disease is present. Instead, we see only the cankers and stem dieback.
Ash decline is a term that is often used loosely to refer to more than one condition. Ash decline might involve the ash yellows disease or even Verticillium wilt, but it is often used to indicate any decline of ash for which a single pathogenic cause has not been identified. Ash decline usually includes branch tip death, defoliation of enough leaves to give the tree a sparse look, and a slow decline of the tree over a number of years. Trees with ash decline may appear to be recovering each year in the spring and then decline in July and August.
To complicate matters, Verticillium wilt on ash also results in cankers and dieback and does not cause the typical vascular discoloration of most Verticillium infections. Refer to RPD no. 1010 for more information on Verticillium wilt. It is difficult and time-consuming to distinguish between ash yellows, Verticillium wilt, and ash decline in Illinois. Diagnosis depends almost entirely on symptoms that could be caused by a variety of problems.
Ash yellows is caused by a phytoplasma, a phloem-inhabiting pathogen. It cannot be cultured in the laboratory on artificial media. Some testing services offering specific PCR (polymerase-chain-reaction) tests can detect phytoplasmas in plant tissues. This service is not offered at the U of I Plant Clinic. I spoke with AGDIA, Inc, a company in Indiana that has such a service. You can read about AGDIA on the Web at http://www.agdia.com/. There are likely other labs that can help. Let me know, and I will include these in future newsletters. The cost for phytoplasma testing varies with the number of samples being tested. The procedure is very time-consuming and involves expensive equipment, so unit costs are lower when multiple samples are run. The cost ranges from $134 to $315. Turnaround time also affects the cost; so if you need results quickly, it costs more. For this test, AGDIA needs live, thick bark from the base of the tree. The sample must include phloem tissues and be deep enough to prevent that layer from drying out. It is advised to call the testing service before sending a sample. It is obvious why this disease has not been confirmed frequently in Illinois.
Verticillium wilt can be detected by traditional laboratory isolations of live leaf petioles at the Plant Clinic. The fungus grows to the point of positive identification in about 7 days. Samples should include live, symptomatic branch terminals with leaves attached.
Ash decline cannot be confirmed with laboratory isolations because many factors are involved. Sometimes Verticillium is involved, sometimes ash yellows, and always some sort of site or environmental stress.
There are no cures for any of these maladies of ash. Suggested management to slow disease progression includes removing trees with severe dieback, watering the trees during extended drought, and fertilizing in the fall with a general tree fertilizer. Removal of dead limbs may help as well. I have heard some very good testimonials involving the value of fertilization and watering to ash tree recovery.