Physiological needs of wood-decay fungi
The process of wood infestation by decay fungi can be divided into different phases. The authors suggest the following:
It is assumed that the requirements regarding moisture and other physio-chemical parameters (e.g. pH, temperature, nutrients) differ between the six phases. But the six phases of fungal infestation will overlap in the wood substrate because of spatial colonization, and the required physio-chemical factors can also overlap between phases of wood decomposition. Most relevant for wood in service especially in above-ground situations and therefore in the focus of wood pathologists are the phases of spore germination, mycelial growth and metabolization of wooden cell walls. Wood exposed in soil contact is often indirect contact with fully developed fungal mycelium, and the phases of spore arrival and spore germination is only relevant for the transition zone between soil and air. In the following, a chronological synopsis is given on methods, thresholds and experimental limitations regarding the moisture requirements for fungal growth and decay in wood based on a literature review by Brischke et al.(2018a).
Several authors starting in the 1850s performed experiments where wood specimens were subjected to different climatic conditions, and spore germination or mycelial growth were monitored (Zeller 1920). Moisture requirements were often in the focus (e.g Münch 1909; Wehmer 1914). Since then, thresholds for fungal growth and decay of wood were sought in numerous research works, where the experimental set-ups differed in external moisture supply and the way of infecting the wood specimens, and consequently, various minimum moisture thresholds (MMThr) were determined for different combinations of wood and fungal species.
Among the first, Zeller (1920) reported on the relationship between relative humidity (RH) and spore germination of wood-destroying fungi and found that the percentage of germinating spores of the brown-rot Lenzites saepiaria (syn.Gloeophyllum sepiarium) escalated above 90% RH, i.e. below the fibre saturation point (FSP), which he considered to be at 95% RH. It is important to keep in mind in the following that the terminology regarding the FSP is not consistent in literature and refers to different moisture states.
Butin (1962) reported about germination experiments with spores of the ascomycete Cryptodiaporthe populea at varying vapour pressure, and the results aligned with the basidiomycete findings by Zeller (1920). Ascospores on malt agar germinated at 20°C between 100-89% RH and conidia between 100-95.5% RH. However, the application of spores on wooden substrates at a given moisture content (MC) is challenging. Usually, for this purpose, spores are dispersed in water, and an aqueous spore suspension is sprayed or otherwise applied on the wood surface, which inevitably leads to a superficial increment in moisture. The latter can be reversed by rapid redrying. However, it is also challenging to produce viable spores in sterile laboratory conditions. Alternatively, spores can be allowed to drop from fruiting bodies directly on wood samples as reported by Zeller (1920), but the method bears a high risk of contamination by non-target organisms such as mould fungi and bacteria. It has been frequently shown that fungal spores were able to germinate at RH below 95% (Gottlieb 1950) corresponding to wood MC below fibre saturation. One might hypothesize that this also allows for the colonization of the wood substrate with fungal mycelium, but to the authors knowledge, evidence from experimental studies is still lacking. Other factors such as pH, oxygen content, volatile organic compounds and temperature are likely affecting both the germination of spores (Zeller1920; Gottlieb 1950; Merrill 1970; Viitanen 1994) and the formation of mycelium, but their effects are not necessarily the same.
Wood protection systems will obviously also alter the wood substrate by adding chemicals that are toxic for the fungi and/or by changing the wood-water properties.
Minimum moisture thresholds (MMThr)
An extensive chronology of experimental studies to determine the moisture requirements for mycelial growth and wood decomposition by different wood-destroying basidiomycetes has been provided by Brischke et al. (2018a). A brief and updated summary is provided below. Several experiments to determine MMThr for fungal growth and decay were performed using saturated salt solutions to establish well-defined climates and monocultures of wood destroying fungi.
Bavendamm and Reichelt (1938) conducted fungal growth tests on malt agar with wood sawdust and small wood blocks at different RH between 81.5 and 99% in small jars. Sodium chloride solutions of different concentration were used to obtain defined climates. Wood specimens were infested using pre-inoculated saw dust. After 4 months of exposure, more than 2% mass loss (ML) was detected on blocks stored at only 85.6% RH, but the MC after incubation was not determined.
The den (1941) determined the MMThr for new infection through mycelium, progress of decay in already incubated samples and reactivation of decay in infected, dried, and remoistened samples. The MMThr for onset of fungal decay was achieved at 98.2% RH for different test fungi. The higher the ML by fungal decay, the higher was the MC after incubation, which the den (1941) explained by the production of water during the biochemical degradation of wood. In summary, the den (1941) did not determine a MMThr below fibre saturation, even though decay started at RH below 100%.
Similar discrepancies between target MC and actual MC after incubation were reported by Ammer (1963), who used pre-inoculated specimens and stored the min screw-top jars above different saturated salt solutions. Ammer (1963) examined Norway spruce (Picea abies) sap-wood and determined at 85% RH an MMThr of 19% for fun-gal decay, which was approximately 7% points below its FSP.
In a similar set-up, Saito et al. (2012) exposed specimens made from Japanese red pine (Pinus densiflora) in small vessels with even smaller containers filled with different saturated salt solutions. In contrast to the above-cited studies, no decay was observed at MC below fibre saturation.
In a different approach, a wide range of wood MC was generated by piling wood specimens in Erlenmeyer flasks where the bottom of the piles was exposed to malt agar inoculated with fungal mycelium serving as nutrition and water source at the same time (Schmidt et al. 1996; Huckfeldt et al. 2005; Huck feldt and Schmidt 2006; Stienen et al. 2014; Meyer and Brischke 2015; Meyeret al. 2015b). The test fungus stopped growing upwards where moisture was insufficient, and ML decreased with the pile height. Within all the above-mentioned studies using the piling method, the MMThr were below FSP, partly remarkably far below FSP. For instance, Meyer et al. (2015b) found a lower moisture limit for decay (ML = 2.2%) of beech wood by the white-rot Trametes versicolor of only 15.4% MC. However, the malt agar at the bottom of the pile served as an external moisture and nutrition source. The fungus is able to transport water and likely nutrition from the agar pile upwards through mycelium and strains, which can barely reflect the real-life situation for decay fungi on wood exposed above ground.
In contrast, a permanent source of water and nutrients is available when wood is exposed in soil. Hence, Höpken (2015) modified the pile test method to examine the ability of decay fungi to transport water. Capillary water transport in the pile was interrupted by stainless steel washers between the wood specimens, and tests were conducted with and without malt agar. Höpken (2015) clearly showed that different fungi could actively transport water within the piles. Brischke et al. (2017) determined MMThr in different experiments without an external moisture source. These tests referred to the experimental setup suggested by Ammer (1963) using different saturated salt solutions and to the pile tests conducted by Meyer and Brischke (2015), but omitting malt agar as nutrition and moisture source. The MMThr for T. versicolor that caused significant ML on beech was achieved at 96% RH, i.e. at 25.3% MC, when specimens were conditioned above saturated salt solutions and deionized water, respectively, before inoculation with basidiomycete mycelium. Piled Norway spruce specimens showed significant ML already at 16.3% MC caused by T. versicolor without external supply of liquid water. Vanpachtenbeke (2019) abstained from the use of any pre-infection with decay fungi and exposed wood specimens at given climates for several months. In so-called fungal control units (FCU), wood samples were exposed to high humidity (25 °C, 97% RH). In a second setup, two modules (25 °C,97% RH and 5 °C/80% RH) were separated by mineral wool and a wind barrier. The vapour pressure gradient between the modules allowed for interstitial condensation and thus moistening of the wood specimens. However, in both FCU, no fungal decay occurred during 3, 9, and 12 months of exposure, respectively. Vanpachtenbeke (2019) also studied fungal decay in wood specimens with different initial MC at different RH compared to specimens incubated at 100% RH. The effect of RH on ML became evident, but it was also shown that within a few day seven at low RH (e.g. 43%) the MC increased rapidly above FSP which can be attributed to active moisture transport from the malt agar by the brown-rot fungus C. puteana.
Toward determine the moisture requirements of wood and decay fungi is challenging. Besides the various limitations with fungal experiments and the difficulties to determine wood MC accurately, it appeared that the most challenging task is the interpretation of the test results. Rather often, the origin and the exact location of water in wood stay unclear. The latter is closely related to the relationship between air humidity and the equilibrium moisture content (EMC) of wood.
However, different physico-chemical processes are involved in wetting and drying of wood. Hence, the moisture requirements of decay fungi cannot be reduced to static wood MC values but need to be seen in the context of dynamic processes including adsorption, diffusion, capillary condensation, desorption, and active moisture transport by the fungus itself. Usually, only an average wood MC (global MC) is measured, and MC gradients between different locations in wood (local MC) are barely accounted for (Meyer et al. 2015a).
Finally, fungal degradation of wood itself supplies moisture. Research often focussed on the question whether fungal decay can be initiated below fibre saturation or in other words whether capillary water in the cell lumens or other larger voids in the cell wall is needed for fungal decay. However, the definition of fibre saturation is somewhat diffuse and changed a lot during recent years and so did the understanding of wood-water relationships.