Beech bark disease causes defects and mortality in American beech trees. In North America, this disease is tied to a combined effect of fungi and insect attackers. The introduced beech scale insect, Cryptococcus fagisuga, begins the cycle by feeding on the bark of a beech tree. Feeding activities of the scale insects create tiny wounds such that the tree becomes susceptible to two different species of Neonectria fungus: N. faginata and N. ditissima. Once the bark is wounded by the scale insect, Neonectria ditissima, which is already present in other nearby hardwoods, can readily infect the beech trees. N. ditissima is more common in recently affected stands, while N. faginata is more common in beech stands that have been affected for a long time. Both Neonectria species make red, superficial perithecia in clusters, as seen in the photos in this post. 

Once it has invaded the beech bark, the Neonectria fungus kills the phloem and cambium layers below the bark, resulting in cankers on the beech trunks, which are typically diffuse and spreading. Neonectria faginata is more aggressive than N. ditissima, and can cause larger cankers and produce more perithecia (the tiny red spore-producing growths that appear on the surface of the bark).

Although the initial pathogen (N. ditissima) is native and was already well distributed in northern hardwood forests, the spread of the disease has been tied to the dispersal of the introduced associated scale insect. Since the scale insect cannot fly, its low mobility has allowed this disease to spread much more slowly than the chestnut blight did. However, although its spread is slow, it shows no signs of stopping. The main disease front is now in Pennsylvania, Virginia, West Virginia, and Ohio, but it still has much of the beech range to invade.