Non-destructive monitoring of early stages of white rot by Trametes versicolor in Fraxinus excelsior

Non-destructive detection of fungal decay in living trees is relevant for forest management of valuable species, hazard tree assessment, and research in forest pathology. A variety of tomographic methods, based on stress wave timing, radioactive radiation, or electrical resistivity have been used to detect decay in standing trees non-destructively. But apart from mobile gamma ray computed tomography (Habermehl and Ridder, 1993) which is virtually unavailable, the detection of incipient stages of decay is still not possible. Wood moisture and electrolyte content influence the electric resistivity of wood. Both are changed by fungal decay. Therefore electric resistivity tomography (ERT) should detect decay in its early stages. Then it could be used to monitor the spatial and temporal progress of degradation. We infected four Fraxinus excelsior trees with Trametes versicolor using wooden dowels and measured two-dimensional electric resistivity tomograms 3, 10, 13 and 21 months after infection. Immediately after the last electric resistivity measurement trees were felled for further analyses of stem cross-sections. Wood moisture content and raw density had significantly increased in infected areas, but dry density had not significantly changed after 21 months. Areas of very low electric resistivity around the infected wounds correlated very well with infected wood in the stem cross-sections. Increasing areas of low electric resistivity around the infected wounds during consecutive measurements indicate increasing areas of infected wood. We conclude that the growth of white rot by Trametes versicolor can be monitored with electric resistivity tomography (ERT) beginning from incipient stages, even before wood density decreases. ERT could therefore be a powerful research tool for decay dynamics as well as a method for diagnosing wood decay in forestry and arboriculture.