Storms are the main abiotic disturbance in European forests, effects of which are expected to intensify in the future, hence the importance of forest stand stability is increasing. The predisposition of Norway spruce to wind damage appears to be enhanced by pathogens such as Heterobasidion spp., which reduce stability of individual trees. However, detailed information about the effects of the root rot on the stability of individual trees across diverse soil types is still lacking. The aim of the study was to assess the effect of root rot on the individual tree stability of Norway spruce growing on drained peat and mineral soils. In total, 77 Norway spruce trees (age 50–80 years) growing in four stands were tested under static loading. The presence of Heterobasidion spp. had a significant negative effect on the bending moment at primary and secondary failure of the tested trees irrespectively of soil type. This suggests increased legacy effects (e.g., susceptibility to pathogens and pests due to fractured roots and altered water uptake) of storms. Damaged trees act as weak spots increasing the susceptibility of stands to wind damage, thus forming a negative feedback loop and contributing to an ongoing decline in vitality of Norway spruce stands following storms in the study region in the future. Accordingly, the results support the importance of timely identification of the decayed trees, lowering stand density and/or shortening rotation period as the measures to counteract the increasing effects of storms on Norway spruce stands.

Increasing resistance against biotic and abiotic factors is an important goal of forest tree breeding. The aim of the present study was to develop a root rot resistance index for Scots pine breeding and evaluate its effectiveness. The productivity, branch diameter, branchiness, stem straightness, spike knots, and damage from natural infection of root rot in 154 Scots pine open-pollinated families from Latvia were evaluated through a progeny field trial at the age of 38 years. Trees with decline symptoms were sampled for fungal isolations. Based on this information and kriging estimates of root rot, 35 affected areas (average size: 108 m²; total 28% from the 1.5 ha trial) were delineated. Resistance index of a single tree was formed based on family adjusted proportion of live to infected trees and distance to the center of affected area. Heritability for resistance to root rot based on the value of this index, was high (0.37) and comparable to indices of growth traits. Correlations of family breeding estimates between resistance to root rot and the other traits were not significant, except for a weak, yet significant, positive correlation with diameter at breast height and branch diameter. Selection index including only growth traits (height and stem volume) had a negligible effect on damage by root rot. We detected a maximum genetic gain in resistance index of 33.7% when incorporating it into the selection index with positive gains for growth traits (6.5–11.0%). Two-stage selection with prior selection of the most resistant families was not superior to the use of selection index with only rot resistance included. Overall; rot resistance index appeared to be an effective tool in tree breeding for the selection of more resistant families, using the existing trials with natural (uncontrolled) infection

This study investigated the efficacy of Rotstop^®, a native Latvian Phlebiopsis gigantea strain and 35% urea solution in combination with a stump cover treatment to control against natural spore infection by Heterobasidion spp. upon precommercial thinning of Norway spruce in three stands growing on former agricultural lands. The major findings were that (i) infection rates of Heterobasidion spp. on stumps treated with the native P. gigantea strain, Rotstop^® or urea are similar when stumps are uncovered, and (ii) stump cover promotes stump colonization by the Latvian P. gigantea strain and Rotstop^®, leading to a significantly smaller relative area colonized by Heterobasidion spp., as well greater efficiency against Heterobasidion in comparison with urea. Covering of stumps appears beneficial for controlling Heterobasidion stump colonization and may be valuable to forest owners if used in small-scale operations, but it is impractical in automatized thinnings, where managers should consider using regular Rotstop^® without covering the stumps.

Stumps play a pivotal role in the epidemiology of the fungal forest pathogens Heterobasidion spp. because they are the main courts of primary airborne infections. The aims of this study were (i) to determine the susceptibility of seven tree species (i.e., Larix sibirica, Picea abies, Picea sitchensis, Pinus contorta, Pinus strobus, Pinus sylvestris and Pseudotsuga menziesii) to primary infection by H. annosum and H. parviporum through comparative inoculation experiments of conidia on wood discs in controlled conditions; (ii) to compare the susceptibility of wood discs of the same tree species to natural airborne infections in two Latvian Norway spruce forest stands infested either by H. annosum or H. parviporum; (iii) to explore the rates of infection of wood discs at increasing distances from spore sources in these two forests to make inferences on the range of spores dispersal. Results obtained by spraying wood discs with conidial suspensions in controlled conditions are in agreement with those obtained by exposing wood discs to the natural airborne inoculum in the forests, as clearly supported by the significant correlation (r = 0.79; p < 0.05) between the two sets of data. Susceptibility was highest in Pinus species, followed by P. abies and P. sitchensis. Susceptibility was lowest for L. sibirica and P. menziesii. The area colonized by Heterobasidion spp. in the sapwood of wood discs was much greater than that colonized in the heartwood. A sharp decrease in the rate of infection of wood discs with distance from spore sources (i.e., fruiting bodies) was observed, further confirming the importance of local spore sources in the epidemiology of Heterobasidion spp. Taken together, these findings could help designing tactics to manage these fungal forest pathogens.