The white ash tree grows to a height of approximately 120 feet in its native habitat in the eastern United States. The dark brown bark of the tree has a unique diamond shaped fissures that become more pronounced as the tree ages. White ash trees become susceptible to disease when under stress or suffering drought conditions, according to the National Forest Service.
White ash trees are susceptible to mycoplasma-like organisms (MLO). The disease occurs predominately in the northeastern United States. Fusicoccum spp. and Cytophorna pruinosa cause canker girdling to occur on the white ash tree. Nectria galligena cankers affect the branches and branch bark of the tree. Perenniporia fraxinophilus, Tyromyces spraguei, Laetiporus sulphureus, Pleurotus ostreatus and Phellinus igniarius enter the tree through cracks in the tree's bark. Once inside the tree's wood they can cause widespread heartwood damage and rot in an older, weak tree.
MLO cause rapid decline in the tree's health. The white ash tree's foliage begins to yellow and the tree suffers severe die back. Most of the fungal infections cause conks or fungi growths on the bark that are similar in appearance to mushrooms.
In the 1920's and 1930's large stands of ash trees began to show symptoms of MLO and they rapidly died. More ash trees began to die in the 1930's and 1940's along roadways. In the 1950's the disease spread to valuable forest timberland ash trees. Unusually low rainfall and air pollution is also believed to have contributed to the spread according to the North Carolina State University.
Young ash trees usually die within two years of the first symptoms of MLO, while older trees often hang on for up to 10 years before succumbing according to the North Carolina State University. A few trees recover from the disease. The other common bark diseases that occur on white ash trees may take years to kill an older tree, while younger trees often recover from them. Fusicoccum spp. and Cytophorna pruinosa usually only invade the tree for one season and then the tree recovers fully.
A white ash tree suffering bark damage from from extreme die back rarely recovers. However, if the tree still has only moderate die back it, a regular watering and fertilizing schedule can help it recover. Removal of dead branches can also help.