Beech bark disease (BBD), a complex affecting American beech (Fagus grandifolia), includes both insect and fungal components. The classic concept of BBD, first articulated by Alex Shigo in 1972, remains the standard for today's forest pathologists. Alex described three arbitrary, temporal disease phases: the initial scale front phase, the second killing front phase, and the final aftermath forest phase. During the initial phase, the exotic scale insect disperses through the forest, causing scale-induced alterations to patches of bark.
The killing front phase begins 1 to 19 years after the arrival of the scale. Throughout this phase, the scale-modified bark is killed and colonized by species of Neonectria, rendering the dead tissues vulnerable to additional decaying fungi. The resulting beech snap and mortality levels may reach 50 percent in 5 years. The final aftermath forest phase results in an ecological accommodation to the disease, resulting in either a change in species composition or the death of re-emergent beech. The genetically identical stump sprouts and root suckers, which appear following the initial BBD deaths and/or salvage, die in a second wave of BBD. When there are few other stressors acting on the beech, the trees can live for many years with sub-lethal Neonectria infections and under conditions of multiple stressors such as drought, out of season frosts, and insect attacks the disease acts like a decline complex.