Can Culture Imaging Implement Radial Growth Parameters to Disentangle Intraspecific Variability in Fomes fomentarius?

Fomes fomentarius (L.) Fr. sensu lato is a common, widespread polypore and a pathological decayer in many hosts such as poplar, beech, and birch. It is either regarded as a single species, a species complex, or displaying a significant intraspecific variability. Limits between populations are fuzzy, and local differences have been mainly related to the current distribution of preferred hosts. The aim of the work was to test an imaging technique (RGB profiling) of cultures’ macromorphology on Petri plates to implement the traditional growth profiles of pure cultures in order to point out differences between strains from different European regions, hosts, and climates. Growth rates at 24 ◦C and 30 ◦C poorly segregated strains based on the origin, whereas there is a marked difference at 15 ◦C between strains from oceanic climates and continental climates. K-means clustering of RGB profiles also marked a difference at 15 ◦C between Central/North European strains and the Italian strains, although this variability gradually attenuates by increasing temperature. The combined approach, including a radial growth measuring and RGB profiling, successfully pointed out the intraspecific diversity in F. fomentarius, suggesting local adaptations. This study contributes to establishing a methodology to investigate the ecotype concept in polypores.