Inoculation success of Inonotus obliquus in living birch (Betula spp.)

Abstract Inonotus obliquus is a pathogenic fungus––known as chaga in Asia and Russia and pakuri in Finland––that grows on broadleaves. Its sterile conks are used as an active ingredient in traditional healthcare products. Due to the higher value of pakuri than the tree it grows on, the cultivation of I. obliquus on living trees is expected to generate new and increased income opportunities for forest owners. So far, the success of large-scale inoculations of the fungus have not been studied. Therefore, the objectives of this study were to examine the probability of fungus infection and the appearance of sterile conks on birches, and to investigate the factors that affect the success of the inoculations. To this end, a total of 679 living birch trees (Betula pendula and B. pubescens) in 24 stands growing under different conditions in Finland were inoculated with two strains of I. obliquus, and re-examined after 4–5 years. The proportion of birches infected per stand was, on average, 79 ± 13%, whereas 6 ± 15% of birches per stand bore one or more conks. The size of the largest conk on a stem was, on average, 54 ± 64 cm3. Part of the infected trees (13% of all trees studied) also showed signs of bulging and/or cracked bark, indicating conk formation under the bark. The main factors that improved the probability of successful infection and the incidence of conk on birches were southerly location, mineral soil texture, low mean tree diameter, high stand basal area, and inoculation at the beginning of the thermal growing season. Also, the fungal strain used had a significant effect on the probability of infection and bulging. Despite successful inoculation, the formation of conks was found to be a slow process. Therefore, the cultivation of I. obliquus may be difficult to integrate into the management of commercial forests, although set-aside, low-productive birch, especially B. pubescens stands, can be recommended for utilization in its cultivation.