Strip-cankering of beech (Fagus sylvatica): Pathology and distribution of symptomatic trees

The pathology and distribution of European beech trees bearing elongated ba rk lesions (strip-cankers) were investigated. Two types of canker were reco gnized: those on small trees (< 40 cm diameter at breast height (dbh): 1.4 m above ground level) which bore fruit bodies of the xylariaceous ascomycet e Biscogniauxia nummularia (Bull.) O. Kuntze, and those on larger specimens (>40 cm dbh) which were consistently associated with the diatrypaceous asc omycete Eutypa spinosa (Pers.) Tul. & C. Tul. All cankers were strictly ann ual, having formed during single growing seasons following periods of low w ater availability. The regional and local distribution of trees bearing les ions also appeared to be correlated with environmental conditions, being mo st severe where low rainfall or high temperatures had occurred. Population studies of the associated fungi, generally considered as saprotrophs, indic ated the presence of unique genotypes within individual cankered trees and provided no evidence for the existence of pathotypes within either species. Within the decay columns which underlay canker surfaces, both B. nummulari a and E. spinosa formed longitudinally extensive: genets, implying non-myce lial spread in colonization. Suppression of both inter- and intraspecific i ncompatibility between fungi occurred in regions of canker decay columns wi th elevated water contents. The possible significance of coexistence betwee n the ascomycetes B. nummularia and 'Hypoxylon purpureum' (sensu Sharland & Rayner, 1989b) and of the formation of heterokaryons by E. spinosa is disc ussed.