Natural infection and colonization of pre-commercially cut stumps of Picea abies and Pinus sylvestris by Heterobasidion rot and its biocontrol fungus Phlebiopsis gigantea

Fungi from the genus Heterobasidion are among the most important pathogens of forest trees in Northern Hemisphere causing root rot and wood decay, while Phlebiopsis gigantea is a very common saprotrophic wood decay fungus. Both fungi are primary colonizers of freshly cut conifer stumps (through which Heterobasidion spp. accomplishes primary infections of tree root systems), thus both fungi are competing for the substrate. To date, P. gigantea is widely used as Heterobasidion spp. biocontrol agent. Hypothesis has been proposed that natural colonization of stumps by P. gigantea might also to some extent restrict infections by the pathogen. The main aim of the study was to assess the potential of natural infections of P. gigantea to restrict infection and spread of Heterobasidion spp. in Picea abies and Pinus sylvestris stumps. In total, 793 P. abies stumps and 1158 P. sylvestris stumps were examined in 24 sample plots located in the eastern part of Latvia. Of these, 325 (41.0%) P. abies stumps were infected by Heterobasidion spp., and 59 (7.4%) by P. gigantea, and 168 (14.5%) P. sylvestris stumps were infected by Heterobasidion spp., and 846 (73.1%) by P. gigantea. In P. abies, the observed Heterobasidion spp. infection frequencies were significantly (p < 0.05) higher than those of P. gigantea, while the respective situation in P. sylvestris was reverse and P. gigantea infections were more frequent (p < 0.05). The mean surface area colonized by Heterobasidion spp. in P. abies and P. sylvestris stumps was 5.7 and 5.3 cm² and did not differ significantly (mean coverage of stump surface area respectively 18% and 13%; p = 0.41). In contrast, the mean surface area colonized by P. gigantea was significantly different in the two tree species, respectively, 3.9 and 21.3 cm² (16% and 59%; p < 0.05). The mean surface area colonized by Heterobasidion spp. in P. abies stumps was significantly larger (p < 0.05) than the area colonized by P. gigantea, while conversely, the mean area colonized by P. gigantea in P. sylvestris stumps was significantly larger (p < 0.001) than that colonized by Heterobasidion spp. Both fungi were co-occurring in P. abies stumps in 33 cases (4.2% of all investigated stumps), and in P. sylvestris stumps in 138 cases (11.9%). There were no correlations between the sizes of colonized areas of Heterobasidion spp. and P. gigantea in P. abies stumps (r = 0.06; p = 0.76), or P. sylvestris stumps (r = 0.009; p = 0.27). In conclusion, the results of this study strongly suggest that even in stumps of P. sylvestris, that otherwise are much preferred for natural colonization by airborne spores of the biocontrol agent P. gigantea, natural colonization by P. gigantea is not able to restrict infections by Heterobasidion spp. This clearly indicates that for effective biocontrol of Heterobasidion spp. infections, the necessity for thorough treatment coverage of cut P. abies and P. sylvestris stumps at early stages of plantation management, during pre-commercial thinning.