Ash yellows 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 organisms). These pathogens are somewhat like virus particles, cannot be cultured in a lab, and are thought to be spread by phloem-feeding insects.

This disease is characterized by a loss of vigor over a period of two to ten years before the trees die. Symptoms include short internodes and tufting of foliage at branch ends. Leaves become pale green to chlorotic and might develop fall colors prematurely. The tree might defoliate, and the canopy 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. Ash trees seldom recover from ash yellows.

Ash decline is a term that is often used loosely to refer to more than one condition. This term most commonly refers to the ash yellows disease, but it is often used to indicate any decline of ash for which a cause has not been identified. 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 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. 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. It is difficult and time consuming to distinguish between ash yellows, Verticillium wilt, and “ash decline” on our green ash trees in Illinois. Diagnosis of these ash problems depends almost entirely on symptoms that could be caused by a variety of other problems.

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 extended drought, and fertilizing in the fall with a general tree fertilizer. Removal of dead limbs may help as well.