Birch tar oil (BTO), produced as by-product of the slow pyrolysis of birch wood and bark, is a phenol-rich liquid. The use of BTO as a fungicide is a new approach, in the development of novel plant protection products. In this study, potato late blight (Phytophthora infestans) and wood-rotting fungi (Cylindrobasidium evolvens, Libertella sp., Stereum hirsutum and Chondrostereum purpureum) were used as target pathogens to assess the anti-fungal and and-microbial effects of BTO. BTO was also assessed for the control of moss covering roof tiles. Results from our laboratory tests demonstrated that BTO is an effective fungicide and anti-rot agent. In greenhouse studies, BTO inhibited the infection and growth of Phytophthora infestans in potato. However, results from the field study suggest that under heavy disease pressure the effect of BTO treatment of potato was not equally good as that of a synthetic fungicide (mancozeb). BTO inhibited the growth of wood-rotting fungi in Petri dishes as well as on the cut surfaces of birch trees. This indicates that BTO has the potential to be used as horticultural grafting wax. Furthermore, preliminary results also highlighted the efficiency of BTO as an agent for cleaning moss from roof tiles and paved areas. However, new technologies for the application of pyrolysis oils are needed before the potential of BTO can be broadly implemented as a novel, bio-rational fungicide.