813 Myca/. Res. 99 (7), 813-820 (1995) Printed in Great Britain Soft rot and multiple T-branching by the basidiomycete Inonotus hispidus in ash and London plane F. W. M. R. SCHWARZE!, D. LONSDALE 2 AND S. FINK! 1 Albert-Ludwigs-UniversitiH Freiburg, Institut fur Forstbotanik. Bertoldstr. 17, D-79098 Freiburg i.Br., Germany 2 Forestry Authority Research Station, Alice Holt Lodge, Farnham, Wrecclesham, Surrey GUlO 4LH, UK Wood degradation by the white-rot basidiomycete Inonotus hispidus was studied in ash (Fraxinus excelsior) and London plane (Platanus x hispanica). After 6 or 12 wk incubation of inoculated wood blocks the loss of dry weight was greater in plane than in ash, corresponding with a more intense visible degradation of the secondary walls in the lignified axial cells. Atypically for a white-rot. degraded cell walls contained internal cavities. The cells thus affected were the late-wood fibres in ash and the fibre tracheids in plane. The srructure of the cavities and the formation of multiple T-branches by the associated hyphae were typical of a soft-rot normally associated with certain Deuteromycotina and Ascomycotina. Naturally infected wood from a liVing plane tree contained cavities identical to those found in the wood blocks, and also showed typical white-rot degradation where the decay was more advanced, with dissolution of the middle lamellae and the indudion of progressive wall thinning by hyphae in the lumina of axial and ray cells. In the inoculated blocks, only ash showed degradation of ray cells, even after 12 wk incubation. Another form of degradation, observed in the early-wood fibres of the ash blocks, was notch-erosion at the cell wall/lumen interface, typical of both white-rot and some soft-rot fungi. The taxonomy of wood-degrading fungi bears some relation to the kinds of degradation that they cause. Brown-rots, in which cellulose is broken down with little or no breakdown of lignin, are caused exclusively by baSidiomycetes. This cellulose breakdown occurs mainly in the SI and S2 layers of the secondary cell wall and is effected by hyphal secretions which diffuse from the cell lumen through the S3 layer. The S3 layer itself and the compound middle lamellae between cells remain relatively intact. except at the numerous bore-holes by which these fungi pass between adjacent cells. White-rots, in which lignin is degraded either simultaneously with cellulose or preferentially, are caused both by basidiomycetes and certain ascomycetes. White-rot fungi initiate wood degradation from within either the cell lumina or the middle lamellae between cells (Blanchette, 1980). Soft-rots can be considered as chemically more similar to brown-rots than white-rots since they strongly decompose cellulose while merely modifying lignin (Liese, 1963). They differ from both brown- and white-rots in their pattern of development. which involves a process of hyphal tunnelling inside the lignified cell walls. This distinctive mode of attack was described as early as 1863 (Schacht. 1863; Cartwright & Findlay, 1958) and was elucidated by Savory (1954) who proposed the term 'soft-rot', The hyphae that are involved in this process grow in alignment with the cellulose microfibrils (Savory, 1954; Courtois, 1963; Liese, 1964). Hyphae of soft- rot fungi can also lie on the cell wall/lumen surface, where they cause local degradation in the same way as do some white-rot fungi. In contrast to both brown- and white-rots, soft-rots are attributed to deuteromycete and ascomycete fungi and are generally not thought to be caused by basidiomycetes. Although internal cavity formation in cell walls is generally regarded as characteristic of soft-rots, there is some evidence that it can also occur in association with other types of decay. Thus, diamond-shaped or rhomboid cavities have been found in the cell walls of wood decayed by basidiomycetes causing white- or brown-rots (Liese, 1963, 1964; Courtois, 1965; Liese & Schmid, 1966; Nilsson & Daniel, 1988). As in true soft-rots, such cavities may follow the helical course of cellulose microfibrils, as observed by Duncan (1960) in coniferous wood degraded by the brown-rot basidiomycete Poria nigrescens Bres. From later studies on P. nigrescens by Courtois (1965) it is apparent that it can also produce T- branching in artificially inoculated pine wood. This type of growth, which is more typical of soft-rot fungi (Corbett, 1965), occurs when a fine hypha enters the cell wall transversely and then forms a branch which grows axially within the wall. A definite soft-rot pattern has also been observed in wood blocks of birch, Betula pendula L. and pine, Pinus sylvestris L. inoculated with a white-rot basidiomycete, Oudemansiella mucida (Schrad.: Fr.) Hahn. (Daniel, Vole & Nilsson, 1992). However, soft-rot patterns have not yet been found in wood degraded by basidiomycete decay fungi within living trees. Another difference between soft-rot fungi and wood-