Fungal growth within reaction zones of beech (Fagus sylvatica) challenged by three basidiomycetes, Inonotus
hispidus, Ganoderma adspersum, Fomitopsis pinicola, and one ascomycete, Ustulina deusta, was studied in naturally
colonized and artificially inoculated wood. All the fungi, except F. pinicola, breached reaction zones, but the
mechanisms involved were all somewhat different. Both I. hispidus and U. deusta bypassed blocked cell lumina by
tunnelling through cell walls (soft-rot mode), but the latter caused far more decomposition of cell walls.
Degradation of polyphenols was slight with I. hispidus and absent with U. deusta. By contrast, G. adspersum
preferentially degraded the polyphenolic occlusions in the cell lumina. The failure of F. pinicola to invade reaction
zones was typical of a brown rot fungus having limited enzymatic potential and a uniform growth pattern.
Mechanisms of lesion expansion, illustrated and summarized in schematic diagrams, are consistent with earlier
observations that reaction zones in beech sapwood are static boundaries, which may be successfully breached by
white and soft-rot fungi.