Beech Bark Disease
Beech bark disease represents a unique relationship between the beech scale insect, Cryptococcus fagisuga Lindinger, and the fungal pathogen Nectria coccinea var. faginata for which beech bark disease is named. This disease appeared on beech trees following introduction of the beech scale from Europe to Nova Scotia at the turn of the century. Since then the scale insect has proceeded to spread throughout New England and into Pennsylvania by 1958, and is now present in West Virginia.
Beech bark disease is a canker disease caused by the Nectria fungus. Entry of the pathogen is facilitated by the beech scale insect in which the scale insect inserts a stylet (needle-like mouthpart) into the bark and into the underlying live tissues where sugars and other nutrients are sucked into the insect. These wound sites are available for colonization by the Nectria fungus which produces spores that are transported passively by insects or wind. Under the right conditions spores germinate and enter wounds created by the scale insect.
The scale insects produce white, waxy filaments which form a small but noticeable waxy crust on tree trunks. The scale insects that reside beneath this waxy material are a pale yellow color. The insect lays eggs in the summer which soon hatch and the young crawlers move into bark fissures or may be carried to other trees by wind or wildlife. An area recently infested with beech scale may not become cankered for several years, eventually the Nectria fungus does become established and form cankers. The scale insect and fungal pathogen work in combination to kill patches of inner bark. Cankers can expand and join together to girdle the tree, in this instance the tree may die. Many beech trees die but others do survive in spite of severe canker development.
Fortunately, there do appear to be beech trees that are resistant to the scale insect. A resistant tree is less likely to have feeding injury and consequently attack by Nectria is reduced. Leaving resistant trees intact will allow these trees to produce seed that survives to produce more scale resistant trees and fewer diseased trees in the future.
