Effects of calcium-based materials and iron impurities on wood degradation by the brown rot fungus Serpula lacrymans

Calcium-containing materials have been implicated in promoting wood degradation by Serpula lacrymans, but mechanisms remain unresolved. In this study, S. lacrymans and Serpula himantioides degraded pine sapwood in agar-block microcosms with one of four treatments: calcium-free, 5 mM agar CaCl ₂, high-purity gypsum (CaSO₄), and gypsum amended with 1% FeSO₄. Calcium and iron availability were limited in minimal nutrient agar. At week 5, pine degradation was significantly higher for S. lacrymans in iron-amended gypsum treatments than other treatments, and the respective agar oxalate levels were also higher. Oxalate solubility was lowest in pure calcium microcosms. Scanning electron microscopy showed hyphae in contact with gypsum and precipitation of calcium oxalate. At week 15, wood degradation by S. lacrymans was severe (>60%) in both calcium-free and iron-amended treatments, but was significantly less in pure calcium treatments (≈45%). Cation analysis in week 15 wood revealed higher calcium and iron levels in treatments containing those element additions. Serpula himantioides had decayed wood equally among treatments at both harvests. Results demonstrate that calcium has an inhibiting effect - and not a promoting effect as hitherto believed - on wood degradation by S. lacrymans. It appears that oxalate and iron play a role in stimulating wood degradation by this destructive fungus.