Beech bark disease in North America: Over a century of research revisited

Abstract

American beech (Fagus grandifolia) in the eastern United States and Canada has experienced more than a century of mortality under a steady expansion of disease agents collectively causing beech bark disease (BBD). In North America, BBD consists of insect (e.g., Cryptococcus fagisuga [beech scale] and Xylococculus betulae) and phytopathogenic fungal (Neonectria faginata and N. ditissima) components that can variously interact with host and environment to cause extensive aboveground mortality of beech (i.e., killing front stage). This heavy mortality causes cascading impacts on forest ecosystems, resulting in altered biotic and abiotic conditions in which BBD-inducted mortality persists at reduced levels (i.e., aftermath stage). New evidence suggests that in aftermath conditions, disease agents interact with each other and their environment in ways neither observed nor anticipated from earlier studies of initial disease onset. Previously unrecognized factors such as atmospheric pollution and nutrient imbalances may exacerbate tree susceptibility to this disease. A clearer understanding of BBD-related organisms and their behavior, disease progression and impacts, and interactions between biotic and abiotic factors is needed to support the development of management options aimed at ameliorating economic and ecological consequences of disease-induced forest change. We comprehensively review over a century of literature in order to clarify these aspects in forests newly- and chronically-affected by BBD. Furthermore, we discuss new concerns of a reemergence of heavy mortality in some aftermath forests and present critical knowledge gaps and key considerations to help inform future BBD research and management programs.