Mechanisms of reaction zone penetration by decay fungi in wood of beech ( Fagus sylvatica )

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.