Major New Tree Disease Epidemics: Beech Bark Disease

Beech bark disease (BBD) of European beech, Fagus sylvatica L., and Amer­ ican beech, F. grandifalia Ehrh., begins when the beech scale insect, Crypto­ coccusfagisuga Lind. (== Cryptococcusfagi Baer.), feeds on living outer bark and culminates when these bark tissues are invaded and killed by several species of Ascomycete fungi of the genus Nectria Fries (7,25,26). This causal complex has created an epiphytotic in North America with patterns of occur­ rence and development that are determined primarily by the spread and devel­ opment of the beech scale. The BBD epiphytotic can be considered a composite of three epidemics: the first, of the introduced insect initiator; the second, of the probably intro­ duced pathogen, N. caccinea var. faginata Lohm. and Watson (31); and the third, of the native pathogen, N. galligena Bres., on its new host, scale-in­ fested beech. Although the incidence and abundance of both pathogens are determined by the insect epizootic, those of N. galligena also are influenced strongly by the abundance of inoculum from non-beech hosts and by the timing of the arrival in the stand of N. coccinea var,faginata. The relationship-beech trees + the beech scale + Nectria spp. � BBD-is the basic model of the disease in both North America and Europe (20, 38). In Europe, BBD is a long-standing endemic, but in North America BBD it shows aspects typical