Fungal growth and resultant discoloration are a result of complex interactions among RH, incubation period, and fungal species (Fig. 3). These interactions must be considered for the development of strategies to prevent fungal discoloration on WCHs in humid environments. RH was the key environmental factor determining fungal discoloration, and the degree of discoloration increased rapidly in high RH conditions. The lowest RH and shortest incubation period required for growth differed among the test fungi. The discoloration by Co. velutina was only observed at 100% RH, while G. chlorocephala began to discolor at RH values greater than 95%. A. pullulans and Cl. cladosporioides, discolored at RH values greater than 75%. Findings with A. pullulans and Cl. cladosporioides were similar to those of Viitanen [10] who found that the risk for fungal growth in pine and spruce sapwood exists in continual humidity exposures above 80% RH. Fungal discoloration by A. pullulans and Cl. cladosporioides was detected within 3 weeks to 4 weeks at 75% RH and within at least 2 weeks at RH values greater than 90%. However, these two fungi did not cause discoloration exceeding 1 (a little fungal discoloration) during incubation for 12 weeks at 75–90% RH, except in the case of Cl. Cladosporioides at a 90% RH. The degree of discoloration for Cl. cladosporioides increased to 2 (fungal discoloration of more than 10%) at 90% RH after 11 weeks of incubation. Maximum discoloration (fungal discoloration around 100%) at 100% RH was achieved, although the incubation period required to reach the maximum discoloration differed among the fungal species.
Prevention of fungal discoloration on wood surfaces is very crucial for the conservation of WCHs because of their heavy discoloration intensity, regardless of their degree of discoloration. The indoor RH of WCHs in Korea in the summer rainy season increases to above 75% and remains elevated for more than 4 weeks [4].
Thus, greater emphasis should be placed on discoloration by A. pullulans and Cl. cladosporioides, which start to grow at 75% RH, than on discoloration by Co. velutina and G. chlorocephala, which require at least 95% RH for growth. A. pullulans and Cl. cladosporioides were isolated at a low frequency [6], and the discoloration degree was only 1 at 75% RH. However, these taxa should not be overlooked because they are dark stain and mold fungus, respectively, and have a prominent effect on the appearance of wood. Also, Cl. cladosporioides has been reported to cause more severe discoloration in Japanese red pine than radiata pine [11]. Considering that the majority of domestic WCHs have been built using Japanese red pine, the discoloration problems caused by Cl. cladosporioides are a serious concern.
Therefore, the indoor RH of WCHs should be maintained at below 75% or even lower to prevent discoloration caused by A. pullulans and Cl. cladosporioides all year round, regardless of outdoor RH. If the outdoor RH is lower than 75%, the indoor RH of WCHs can be easily controlled by opening doors in the daytime. In contrast, if the outdoor RH exceeds 75% due to continuous rain especially in the summer rainy season, the indoor RH can rise to above 75% even if the doors are closed, in which case the dehumidifier can be used to control the indoor RH.
Because A. pullulans and Cl. cladosporioides are distributed worldwide and are common environmental fungi found in moisture-damaged buildings [12, 13], the results of this study can be applied to prevent mold and stain development in various wooden buildings located in humid climates.