Pattern of delignification in Ailanthus excelsa Roxb. wood by Inonotus hispidus (Bull.: Fr.) Karst

The pattern of delignification in Ailanthus excelsa Roxb. wood, naturally infected by Inonotus hispidus (Bull.: Fr.) Karst., was studied by light microscopy. Inonotus hispidus produced a typical pattern of soft rot decay even though it is grouped with white rot basidiomycetes. Fungal hyphae colonised all cell types of the secondary xylem but more damage was observed in xylem fibres. In the early stage, infection commenced on the cell wall corners and middle lamellae of the fibre wall without any pronounced effect on the primary and secondary wall layers. Delignification of fibre wall became apparent when all cell types became completely invaded by fungal hyphae. It started from within the lumina towards the middle lamellae, occurring initially in the immediate vicinity of hyphae growing on the luminal surface by forming an erosion trough. At an advanced stage of decay, localised degradation of lignin, cellulose and hemicellulose resulted in the formation of small cavities within the secondary walls (S 2 ) of fibres. These cavities were never observed to contain any fungal hyphae. Though the vessels were resistant to infection, xylem rays and fibres were relatively less resistant to attack by I. hispidus. In severe infection, vessel lumens were found to be filled with sclerotic tissue which blocked them, resulting in complete collapse. The formation of cavities and the extent of cell wall damage are described in detail.