Effect of thermal modification on the durability and decay patterns of hardwoods and softwoods exposed to soft rot fungi

The durability and decay patterns of thermo-vacuum (Termovuoto process for 3-4 h at 160-220 °C) treated hardwoods (ash, beech) and softwoods (spruce, fir) TMWs exposed to three soft rot fungi (Chaetomium globosum, Phialophora malorum, P. mutabilis) were investigated using the soil-block test, light- and electron microscopy. Monitoring of mass loss over 1 year indicated that soft rot fungi do not attack softwood TMWs as rapidly or as extensively as hardwood TMWs. Decay resistance progressively increased in hardwood TMWs with increase in temperature but was unclear/or varied in softwood TMWs depending on fungal/wood species, particularly at lower temperatures (160-180 °C). Soft- and hardwood TMWs showed a major increase in decay resistance at 200-220 °C and 210-220 °C, respectively. Light microscopy of decayed hardwood TMWs showed formation of typical soft rot Type-I cavities in fibres at lower temperatures (190-200 °C). However, cavities were significantly inhibited or delayed at higher temperatures (210-220 °C). Cavity formation in vessels and parenchyma cells were only observed in beech TMW treated at 190 °C or references, indicating higher resistance than fibres. Transmission electron microscopy of decayed ash TMW treated at 200 °C showed a radial-like distribution of electron dense materials in cavities and lack of fibrillar-like materials within degraded fibre walls, which differed from reference.