White-Rot Fungi Control on Populus spp. Wood by Pressure Treatments with Silver Nanoparticles, Chitosan Oligomers and Propolis

Abstract

There is growing interest in the development of non-toxic, natural wood preservation agents to replace conventional chemicals. In this paper, the antifungal activities of silver nanoparticles, chitosan oligomers, and propolis ethanolic extract were evaluated against white-rot fungus Trametes versicolor (L.) Lloyd, with a view to protecting Populus spp. wood. In order to create a more realistic in-service type environment, the biocidal products were assessed according to EN:113 European standard, instead of using routine in vitro antimicrobial susceptibility testing methods. Wood blocks were impregnated with the aforementioned antifungal agents by the vacuum-pressure method in an autoclave, and their biodeterioration was monitored over 16 weeks. The results showed that treatments based on silver nanoparticles, at concentrations ranging from 5 to 20 ppm, presented high antifungal activity, protecting the wood from fungal attack over time, with weight losses in the range of 8.49% to 8.94% after 16 weeks, versus 24.79% weight loss in the control (untreated) samples. This was confirmed by SEM and optical microscopy images, which showed a noticeably higher cell wall degradation in control samples than in samples treated with silver nanoparticles. On the other hand, the efficacy of the treatments based on chitosan oligomers and propolis gradually decreased over time, which would be a limiting factor for their application as wood preservatives. The nanometal-based approach is thus posed as the preferred choice for the industrial treatment of poplar wood aimed at wood-based engineering products (plywood, laminated veneer lumber, cross-laminated timber, etc.).

Natural Compounds for Wood Protection against Fungi—A Review

“…The results show that even though its effect on wood properties was mediocre, it could be a welcome addition to wood finishes based on natural ingredients [ 149 , 157 , 158 ]. Due to the proven antifungal properties, however, propolis has also been conceived as a potential natural and an environmentally-friendly wood preservative against moulds and wood-decaying fungi [ 150 , 159 , 160 , 161 , 162 ].…”

“…Budija et al [ 158 ] demonstrated that an ethanol extract of propolis 29% from Eastern Slovenia effectively protected Norway spruce wood against brown-rot fungi Antrodia vaillantii and G. trabeum , and a white-rot fungus T. versicolor , resulting in wood mass loss of 5.3%, 7.2% and 4.6%, respectively. Also, poplar wood treated with a propolis solution of 40 mg/mL was more resistant to T. versicolor than untreated wood (mass loss of about 11% vs 20%, respectively, after eight-weeks exposure) [ 162 ]. However, in this case, a gradual decrease of antifungal effects of propolis was observed over time during exposure to fungi.…”

“…The observed shortcomings of propolis extracts applied as wood preservatives, such as leachability from wood and a gradual decrease in antifungal activity over time [ 40 , 162 ], prompted researchers to search for stabilisers that would enhance the effectiveness of propolis. In wood preservation, application of some polymers, such as proteins or organosilicon compounds, proved effective in retaining fungicides in wood [ 14 ].…”

The Possibility of Propolis Extract Application in Wood Protection

“…The mass loss of pine wood treated with 7% propolis extract and exposed to T. versicolor and N. lepideus was 4.2% and 2.5%, respectively [13]. In turn, poplar wood protected with propolis extract in concentrations of 5-40 mg/mL after four weeks of exposure to T. versicolor showed the mass loss in the range of 13.7%-9.8%, compared to the 14% mass loss of the unprotected wood [32]. The weight loss of the wood treated with chitosan oligomers and propolis after 30 days of exposure to T. versicolor was 32.8% compared to 42.3% of the weight loss of the untreated wood [33].…”

“…In turn, Scots pine and paulownia wood treated with Turkish propolis extracts exposed to T. versicolor and Neolentinus lepideus showed a lower mass loss than that of untreated wood [13]. Propolis was also used as a constituent of the preparation containing chitosan, propolis extract and silver nanoparticles in preventing the decay of wood caused by T. versicolor [32,33]. The preparation based on propolis extract, caffeine and organosilanes was tested in protection of wood against brown-rot fungus C. puteana [34].…”