**5. Conclusions**

On the basis of biocidal efficacy tests conducted on wood blocks, more realistic than routine antimicrobial susceptibility tests conducted in agar plates, it was evidenced that the three protective agents assayed in the vacuum-pressure treatments (AgNPs, chitosan, and propolis) showed a protective effect on *Populus* spp. wood. However, the one based on AgNPs featured the highest effectiveness against *T. versicolor* over time: even at the lowest concentration (5 ppm), it minimized the biodeterioration of poplar wood for over 4 months, albeit higher concentrations (of up to 20 ppm) were more effective. SEM images confirmed the effectiveness of the impregnation with nanosilver and evidenced the severe cell wall degradation by the fungus on untreated samples. Chitosan oligomers and propolis-based treatments also showed results with significant differences vs. the non-treated wood, but a noticeable decrease in their effectiveness over the 16 weeks was observed, pointing to either biodegradation or adsorption issues (to which all organic bioactive agents are susceptible). Hence, AgNPs can be put forward as effective protective agents for poplar-wood-based engineering products.

**Author Contributions:** Conceptualization, M.M.C.-S., J.M.-G., and L.A.-R.; Formal analysis, P.M.-R. and L.A.-R.; Funding acquisition, M.M.C.-S.; Investigation, M.M.C.-S., I.S.-C., L.P.-H., and J.M.-G.; Methodology, M.M.C.-S., P.M.-R., J.M.-G., and L.A.-R.; Resources, M.M.C.-S., J.M.-G., and L.A.-R.; Supervision, M.M.C.-S., J.M.-G., and L.A.-R.; Validation, I.S.-C., L.P.-H., and L.A.-R.; Visualization, P.M.-R.; Writing—original draft, M.M.C.-S., I.S.-C., L.P.-H., and P.M.-R.; Writing—review & editing, P.M.-R. and L.A.-R.

**Funding:** This study was funded by JUNTA DE CASTILLA Y LEÓN under project VA258P18, with FEDER co-funding.

**Acknowledgments:** P.M.-R. acknowledges the support of Universidad de Zaragoza under project UZ2019-TEC-07. I.S.-C. would like to gratefully acknowledge the financial support of the National Council for Science and Technology (CONACYT) of Mexico, through PhD Scholarship ref. no. 329975. The authors also wish to thank Melissa Boyd for kindly revising the use of English language.

**Conflicts of Interest:** The authors declare no conflict of interest.

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