When ash species were introduced into the landscape they were considered to be problem free. For various reasons, several disease and insect problems have tarnished their reputation. In reality, no tree is problem free (even plastic trees have to be dusted), and it is helpful to know what problems might occur on a given species. There have been many ash tree problems in Illinois over the last decade. Some of these are insect problems and have been dealt with by entomologists through this newsletter (see issue no. 11, 2005). This article addresses the disease problems of ash that we have seen in Illinois.
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 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. It is rare for an ash tree to recover from ash yellows. A great percentage of the ash trees in our landscapes are green ash. They 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. Some images of ash yellows can be found in an ash yellows brochure at this USDA Forest Service site: http://www.na.fs.fed.us/spfo/pubs/howtos/ht_ash/ht_ash.htm.
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 (RPD), no. 1010, for more information on Verticillium wilt. It is difficult and time-consuming to distinguish among ash yellows, Verticillium wilt, and ash decline in Illinois. Diagnosis of these ash problems depends almost entirely on symptoms that could be caused by a variety of problems. Background information on symptom development and site stress is extremely helpful in reaching an accurate diagnosis.
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 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 recover each spring and then decline in July and August.
How do we diagnose these ash 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. I spoke with AGDIA, Inc., a company in Indiana that has such a service. (The University of Illinois Plant Clinic does not.) You can read about AGDIA on the Web 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, but 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 would need live, thick bark from the tree base. The sample must include phloem tissues and 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 maladies of ash. Suggested management to retard disease progression includes removing trees with severe dieback, watering the trees in drought lasting 2 weeks, and fertilizing in the fall with a balanced tree fertilizer. Removal of dead limbs may help as well. I have heard some good testimonials involving the value of fertilization and watering to ash tree recovery.