Treatment of wood with various physical and chemical factors can change the number of wood parameters, which can also lead to changes in resistance to wood-destroying fungi. This study evaluates the effects of hydrothermal treatments (additives Fe
2O
3 or FeCl
3 with and without commercial tannins, also without additives and fresh wood) on decay and mould fungi resistance of modified wood of Scots pine (
Pinus sylvestris), Norway spruce (
Picea abies), Douglas fir (
Pseudotsuga menziesii), walnut (
Juglans regia), and Norway maple (
Acer platanoides). For wood samples, the resistance against wood decay fungi
Trametes versicolor (white rot) and
Coniophora puteana (brown rot) and the resistance against mould fungi
Aspergillus niger and
Penicillium sp. were assessed. The study findings showed that wood modified with iron compounds could cause a higher resistance to wood-destroying fungi. The weight losses of the modified and control wood, caused by
T. versicolor and
C. puteana, differed for coniferous and deciduous: the average weight loss of treated pine, spruce, and fir wood caused by
C. puteana was higher than that caused by
T. versicolor, while these differences on maple and walnut wood were not significant. The wood hydrothermal treatment with Fe
2Cl
3 with and without tannins significantly reduced the weight loss caused by
T. versicolor and
C. puteana, and the treatment with Fe
2O
3 slightly improved the decay resistance. For the wood, hydrothermally modified with FeCl
3 and FeCl
3 + tannins, the mould area for both tested
Aspergillus niger and
Penicillium sp. was smallest for the wood of all tested tree species compared to other treatments. A different response was obtained for coniferous and deciduous tree species wood. The spruce wood, followed by fir wood, treated with FeCl
3 with and without tannins, was the most resistant against the mould fungi. Relatively low resistance against the mould fungi was fixed for the maple wood treated by various iron compounds, except the treatment with Fe
2O
3 + tannins, which gave a very positive response against the
Penicillium sp.
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