Is Ischnoderma benzoinum a competitor or contributor to Heterobasidion annosum decomposition of pine and spruce wood? A comparison to Phlebiopsis gigantea.

The ecological role of Ischnoderma benzoinum in conifer stands remains poorly understood, particularly with regard to its potential to compete with the root rot pathogen Heterobasidion annosum. This study investigated the growth dynamics, wood decay capacity, and competitive interactions of two I. benzoinum isolates (originating from Pinus sylvestris and Picea abies) with two H. annosum isolates from the same host species, and with the biocontrol fungus Phlebiopsis gigantea. Laboratory assays involved growth on malt extract agar for 14 days, and single and dual inoculations on pine and spruce wood for 60 and 120 days. Both I. benzoinum isolates exhibited growth rates on medium comparable to those of H. annosum. On spruce wood, I. benzoinum caused the greatest mass loss, whereas H. annosum dominated on pine. In most dual cultures, wood mass loss was similar to that in single cultures; however, in the pairing of the spruce-derived I. benzoinum with P. gigantea, decay was significantly reduced, indicating antagonistic interaction. Competitive outcomes in dual cultures varied markedly. The pine-derived I. benzoinum strongly suppressed the pine-derived H. annosum, whereas the spruce-derived H. annosum consistently outcompeted both I. benzoinum isolates. P. gigantea strongly inhibited I. benzoinum mycelial growth and frequently reduced its decay activity. These findings demonstrate that I. benzoinum can function either as a strong competitor or as a co-colonizer with H. annosum, with interaction outcomes determined by isolate origin and host tree species. The capacity of I. benzoinum to match H. annosum in wood decay efficiency, particularly on spruce, suggests that it may influence disease progression and nutrient cycling in coniferous forests. This work advances understanding of fungal community dynamics in wood decomposition and highlights the need for further ecological and molecular studies to clarify the role of I. benzoinum in forest health and management.