Ash problems have begun to appear at the Plant Clinic again this year. Earlier in the season, we saw considerable ash anthracnose. That fungal disease causes brown to black spots on the leaves, followed by considerable leaf drop. Anthracnose is worse in cool, wet weather as leaves emerge. Usually, a new flush of leaves emerges later, in warmer, drier conditions; and ash anthracnose does not appear again. This year, we had cool, wet weather early, followed by hot, wet weather and then cool, wet weather again. It would not be surprising to see a second wave of anthracnose on ashes. If anthracnose is the problem, leaf spots are present. Refer to Report on Plant Disease, no. 621, “Anthracnose Diseases of Shade Trees,” available in Extension offices or on the Internet at http://www.ag.uiuc.edu/%7Evista/horticul.htm.
Another problem of late on ash trees is curling of new leaves and petioles. This symptom could be due to herbicide drift. Look for a pattern on your tree. Drift from application of a plant-growth-regulator herbicide (PGR) such as 2,4-D or dicamba can cause leaf curling on trees. Ash and redbud seem to be particularly sensitive. Look for a pattern in the landscape. Drift causes injury that is more severe on one side, usually the side exposed to wind. Also look for the same curling on other nearby broadleaf plants. Lawn herbicides can cause this injury, as can PGR herbicides applied to rights-of-way, fence rows, and agricultural fields. Some insects cause curling, so look closely for insects and insect injury.
One possible cause of decline in ash 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 disease is caused by a phytoplasma (formerly called a 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 limited to the phloem tissue of the tree. This disease is characterized by a loss of vigor over a period of 2 to 10 years before the tree 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. The tree might defoliate, and the canopy generally appears sparse. Cankers form on branches and the trunk, causing twigs and branches to 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.
Ash decline is a term that is often used loosely by many diagnosticians to refer to more than one condition. I think this problem is very common on Illinois ash trees. 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 no single pathogenic cause has 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 recover each spring and then decline once again 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 Report on Plant Disease, 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 of these ash problems is dependent almost entirely on symptoms that could be caused by a variety of problems.
Ash yellows disease is caused by a phytoplasma, which is a phloem-inhabiting pathogen. It cannot be cultured in the laboratory on artificial media. Some testing services that offer specific PCR (polymerase chain reaction) tests can detect phytoplasmas in plant tissues. This is not a service offered at the University of Illinois Plant Clinic. AGDIA, Inc., a company in Indiana has such a service. You can read about AGDIA at http://www.agdia.com/. There are likely other labs that can help. 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. Turn-around time affects the cost; so if you need results quickly, it costs more. For this test, AGDIA would need live, thick bark from the base of the tree. It must include phloem tissues and must be deep enough to prevent phloem tissue from drying out. It is advised that you call the testing service of choice 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. Ash decline cannot be confirmed with laboratory isolations because there are many factors involved, many of which are nonpathogenic. Sometimes Verticillium is involved, sometimes ash yellows, and always some sort of site or environmental stress.
There are no cures for any of these ash maladies. Suggested management to slow disease progression includes removing trees with severe dieback, watering trees in periods of extended drought of 2 weeks, and using a balanced tree fertilizer in the fall. Removing dead limbs may help. I have heard some very good testimonials involving the value of fertilization and watering to ash tree recovery.