There have been many ash tree problems in Illinois over the last decade. This summary has been written to help explain some possibilities, as well as limitations, in proving the problem.
One possible cause of decline is ash yellows. This disease mainly infects white and green ash in north-central and northeastern United States. It is a problem in Illinois that is difficult to quantify because it is difficult to confirm. Ash yellows disease is caused by a phytoplasma (formerly called a mycoplasma-like organism). These pathogens are somewhat like virus, cannot be cultured in a lab, and are spread by phloem feeding insects. They are limited to the phloem tissue.
This disease is characterized by a loss of vigor over 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 may develop fall colors prematurely. The tree may defoliate, and the canopy appears sparse. Cankers form on branches and the trunk, causing twigs and branches to die back. Witches’-broom sprouts of growth may appear on 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 percent of ash trees in our landscapes are green ash, which do not show symptoms as clearly as white ash. It is likely that this disease is more common than we realize because the typical witches’-brooms and yellowing are not always seen with green ash. Instead, we see only the cankers and stem dieback.
Ash decline is often used loosely to refer to more than one condition. I think this problem is very common on Illinois ash trees. Ash decline may involve the ash yellows disease or even Verticillium wilt but is often used for 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 over years. Affected trees may appear to recover each spring, then decline in July and August.
To complicate matters, Verticillium wilt on ash also results in cankers and dieback, without the vascular discoloration typical of most Verticillium infections. (See Report on Plant Disease, no. 1010.) 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.
Verticillium wilt can be detected by traditional laboratory isolations of live leaf petioles. Ash decline cannot, as many factors are involved, many of which are nonpathogenic. Sometimes Verticillium is involved, sometimes ash yellows, and always 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 in periods of drought lasting 2 weeks, and fertilizing in the fall with a balanced tree fertilizer. Removing dead limbs may help. I have heard some very good testimonials involving the value of fertilization and watering to ash tree recovery.