Fractography of white charcoal reveals past fungal infection and embolism in the secondary xylem via SEM.

The internal structures of fractured white charcoal were investigated using a scanning electron microscope. The charcoal was fractured using a razor blade and hammer, gold-coated, and observed under the electron microscope. Both embolized and conductive vessels coexisted across the transverse surfaces of the fractured charcoal. Vessels were predominantly ellipsoidal, with an average diameter of approximately 250 μm. Embolized vessels exhibited membranous tyloses within their lumens. The presence of ring-porous wood, axial parenchyma cells, and xylem fibers within the secondary xylem implies that the white charcoal was produced from a Quercus species. Fungal hyphae were observed to proliferate, branch, and sporulate on the secondary cell walls of vessels. The conidiogenous cells and conidia closely resembled those of a fungal pathogen known to cause oak wilt disease in South Korea. Fungal hyphae were also found within the pits of the secondary cell walls. These observations suggest that the oak tree used for charcoal production in this study may have been diseased. These results indicate that wood structures are preserved through the white charcoal production process, allowing the observation of fungal structures within the host.