Persistence of Viable Heterobasidion parviporum Inoculum in Norway Spruce Root Fragments in Drained Peat Soils

Roots infected by the forest pathogen Heterobasidion parviporum that remain in the soil after tree harvesting may serve as a source of inoculum for root infection of new generations of trees, thereby perpetuating outbreaks over time. As drained peat soils are evolutionary novel yet common habitat for commercial Picea abies stands in Northern Europe, the experiment was conducted to assess the mid-term viability of H. parviporum mycelium in root deadwood. Persistence of viable mycelia of H. parviporum in relation to root fragment volume and exposure period was assessed over the seven-year period. Additionally, the potential of transmission of the pathogen from root fragments to conifer seedlings was assessed. The likelihood of finding viable H. parviporum inoculum in Norway spruce root fragments depended on the size of fragments, indicating a higher likelihood in larger fragments, and on the time since burial, showing a substantial reduction in viable inoculum after seven years. We also documented the low infection rate from the root fragments to nearby Picea abies seedlings. The obtained results indicate the necessity for the removal of larger root fragments during soil preparation in commercial Norway spruce stands on drained peat soils to reduce infection potential.