Eucalyptus globulus is a high-performance hardwood, but its low-permeability heartwood limits preservative penetration and protection against fungal biodeterioration. Microwave (MW) treatment can improve wood permeability, but the resulting microstructural changes may also alter its resistance to biological degradation and mechanical properties. This study investigated the effects of different MW treatment intensities (400W_3min, 400W_25min, and 700W_5min) on the biological durability of E. globulus heartwood exposed to the white-rot fungus Trametes versicolor, with and without copper-azole impregnation. The bending properties were also investigated. Resistance to fungal decay was evaluated according to American Wood Protection Association standard E10, using mass loss as the durability indicator. Mild MW treatment did not affect natural durability, with mass losses of 3.07% and 2.36% for the Reference and 400W_3min groups, respectively. Increasing treatment intensity intensified fungal degradation, raising mass loss to 9.54% after 400W_25min and 18.84% after 700W_5min. Preservative impregnation fundamentally changed this response: MW treatment increased preservative retention by up to 114.3%, while fungal mass loss decreased by up to 91.3% relative to the corresponding unpreserved MW-treated wood. Thus, the same anatomical modifications that increased susceptibility to fungal degradation also enhanced preservative uptake and protection against decay. Mild MW treatment preserved bending strength and stiffness, whereas severe conditions produced significant reductions. Overall, MW treatment exhibited a dual effect on biological durability; severe modification intensified fungal degradation in unprotected wood but markedly enhanced preservative efficacy when used as a pretreatment. Controlling MW intensity is therefore essential to balance protection against fungal decay, preservative performance, and mechanical integrity in refractory hardwoods.
