Ash Dieback |
| USDA Forest Service. 1979. A guide to common insects and diseases of forest trees in the northeastern United States. Northeast. Area State Priv. For., For. Insect and Disease Management., Broomall, PA. p. 123, illus. |
The cause of ash dieback, which affects white ash and to a lesser degree green ash, is unknown. Some scientists suspect the cause is a virus or mycoplasma, but this has not been confirmed. Drought apparently plays an important role. The disease became epidemic throughout the East in the 1950s and early 1960s. Surveys showed that 20 to 30 percent of the white ash trees throughout the East were affected. In recent years, reports of ash dieback have subsided.
The disease, which appears to begin in the tips of small twigs, gradually works into larger branches. Early symptoms are easily overlooked. The leaves are pale yellow-green and may be slightly dwarfed. Small twigs and branches begin to die throughout the crown. On any one branch, the line between dead and living wood is evident. Callus ridges form, particularly where small limbs join larger ones. The trees may produce epicormic branches or "water sprouts" along the trunk or on the branches. Yellowish- to reddish-brown sunken cankers form on dying twigs and branches. The cankers are caused by the fungus Cytophoma pruinosa, a weak pathogen that hastens the death of twigs and branches.
Eventually the crown becomes thin and tufted, and the leaves and leaflets dwarfed. The foliage in the stag-headed crown may be reduced to a small area near the top of the tree. Many trees die 2 or 3 years after symptoms appear; others linger for 10 years or longer. Infected trees apparently do not recover.
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