Belian (Eusideroxylon zwageri) is highly regarded among the naturally durable Malaysian hardwoods. Visual inspections at the groundline of belian transmission poles from Sarawak, Malaysia, showed only surface decay of wood after 20 years in service. The cause of decay in belian was investigated by light and transmission electron microscopy. Light microscopy of transverse sections of surface tissues showed circular to oval cavities in wood cell walls, reminiscent of cavities produced by soft rot fungi. However, unlike soft rot decay, where cavities in the secondary cell walls are oriented along the direction of cellulose microfibrils, no such correlation was observed in the belian wood tissues examined in this study. This was confirmed by transmission electron microscopy (TEM) which revealed that such "cavities", detected by light microscopy, appeared somewhat like tunnels varying widely in their size and orientation. The fact that all areas of the secondary cell walls, including the highly lignified middle lamellae, were degraded by this way suggests that the decay pattern observed in belian was not typical of soft rot, but of a type which has not been previously identified and which will be ascribed to as hyphal tunnelling.

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