Wood rotting fungi, the fungal species causing biodeterioration for wood building, are generally classified into white-rot, brown-rot and soft-rot fungi based on their decay modes. Since white-rot and brown-rot fungi are known to reduce wood strength significantly, it is important to clarify the mechanisms of their wood degradation. White-rot fungi reduce wood strength as the decay progress and decompose all main component of wood cell wall such as cellulose, hemicellulose and lignin. In contrast, brown-rot fungi are known to cause rapid strength loss of wood and decompose cellulose and hemicellulose with the chemical modification of lignin. However, the knowledge of the decay mechanisms at the early stage is limited. In this study, cedar sapwood blocks were decayed by two brown-rot fungi, Gloeophyllum trabeum and Coniophora puteana, and one white-rot fungus, Trametes versicolor, to clarify the morphological changes at the early stages of decay. Especially, the hyphal migrations from ray parenchyma to adjacent cells were examined by scanning electron microscopy. All wood rotting fungi preferentially degraded terminal cell walls in ray parenchyma and extend their mycelia from parenchyma to tracheid through cross-field pit. In brown rotted wood, small particles were observed not only around hyphae but also wood cell wall, and they were partially degraded in areas where particulates were accumulated. Degradation by G. trabeum were observed that the hyphae elongated from the ray parenchyma to the interface between S1 and S2 layers of the tracheid cell walls. Degradation by C. Puteana, particulates accumulated between the interface between S1 and S2 layers of the tracheid, and the cell walls were delaminated.

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