Survey Study Reveals High Prevalence of Heterobasidion Root Rot Infection in Scots Pine (Pinus sylvestris) Stands Established on Seemingly Low-Risk Sites

Heterobasidion spp. are among the most destructive root rot pathogens, causing severe economic losses to conifer forestry. High infection frequency has been observed in Scots pine stands growing on dry sandy soils with low organic matter or former agricultural soils. In this study, we investigated the incidence of Heterobasidion spp. infection in Scots pine forests established on low-risk sites where the trees looked healthy and unlikely to be infected. In total, 135 healthy-looking pine trees from nine different stands were examined for Heterobasidion spp. presence. Heterobasidion spp. was detected in six stands and infection frequency was 13%-33%. There was a significant correlation between site index and infection frequency, which was higher in pine stands established on more fertile soils. There was no correlation between disease incidence and defoliation level, diameter of tree at breast height, root diameter, tree volume, or stand age. Overall, our results showed that, regardless of the soil type, Scots pine can be intensively infected by Heterobasidion pathogens while showing no outward signs. Therefore, the risk of Heterobasidion disease should be taken into consideration in management of pine forests growing on both low- and high-risk sites for more productive and sustainable forests.