Spatial distribution of Neonectria species associated with beech bark disease in northern Maine.

Beech bark disease (BBD) requires prior infestation of bark by an exotic scale insect, Cryptococcus fagisuga, to permit infection by one or more fungi, primarily Neonectria ditissima and Neonectria faginata. Previous studies in North America report a progression in which N. faginata replaces N. ditissima as the dominant pathogen in the BBD complex. To examine the status of the Neonectria populations in forests that have developed for decades with and without BBD a survey was conducted 2005-2006 in northern Maine. Ascospore measurements from 201 beech bark disks containing mature perithecia support reports that, once established, N. faginata dominates the BBD complex. However stands did contain more N. ditissima when other highly susceptible hardwood tree species were present (R2 = 0.775), regardless of disease severity. Abundance of N. ditissima in areas long affected by BBD suggests that N. ditissima, by continually supplying inocula from nonbeech hosts, continues to affect BBD.