Mould growth on wood under fluctuating humidity conditions

The growth of molds and other discoloring fungi on pine ( Pinus sylvestris ) sapwood was studied under fluctuating moisture conditions after inoculation with a conidial suspension. Fungal species were Aspergillus versicolor, Cladosporium sphaerospermum, Penicillium, Aureobasidium pullulans, Apergillus niger, Trichoderma , and Cladosporium . Experiments were performed in chambers in which the relative humidity (RH) was controlled by salt solutions or glycerol-water mixtures or in a humidity control chamber in which relative humidity was automatically controlled. The inoculated samples of wood were subjected to varying intermittent moisture conditions with several combinations of relative humidities from 35-100%. Growth halted during periods at RH below 80% but partly resumed after return to RH 97%. Growth rate was very slow when the fungi were subjected to rapid changes (6 h/6 h) in humidity between RH 97 and 75% or if the dry period was one week or longer. The fungi seemed to adjust to a high extent to moisture fluctuations according to a cycle with 24 h at RH 97% alternating with 24 h at RH 75%. The most likely reason for the slow growth rate at very rapid moisture changes is a slow increase of moisture at the surface of wood at these conditions. The results were analyzed for the effects of dry period durations and accumulated growth on wood in relation to the sum of the periods at humidity levels earlier found to support growth under static conditions. The possible significance of the results for development of mold problems in houses is briefly discussed.