Spread of Heterobasidion genotypes in Norway spruce stands on drained peat soil in Latvia

According to several earlier studies, the prevalence of Heterobasidion in peat soils is generally lower compared to mineral soils. However, in some Norway spruce (Picea abies (L.) Karst.) stands on drained peat soils in Latvia, serious damage caused by Heterobasidion root rot has been observed. To determine the spread of Heterobasidion spp. on peat soil, we analyzed the structure of Heterobasidion genets in 20 study plots established in disease centres in 11 spruce-dominated peatland forest stands. A total of 381 standing spruce trees and 244 spruce stumps were examined for Heterobasidion infection. The fungus was isolated from 181 spruce trees (47.5%) and 43 stumps (17.6%). In total, 101 different Heterobasidion genotypes (genets) were identified (on average five genotypes per study plot). The average number of trees infected by a single Heterobasidion genotype was 2.2. Most of the genets (68.3%) had infected only one tree or stump while the rest of the genets (31.7%) had infected several trees and stumps. To the best of our knowledge, this is the first study to investigate the spread of Heterobasidion genotypes in peatland forest stands. To reduce losses caused by Heterobasidion root rot in spruce forests on drained peat soils, it is important to prevent primary spore infections as well as to avoid planting pure spruce stands with high density.