Impact of Heterobasidion spp. root rot in conifer trees and assessment of stump treatment : with emphasis on Picea abies, Pinus sylvestris and Larix × eurolepis

Abstract: In the thesis four studies were conducted to understand the impact of Heterobasidion spp. on Picea abies, Pinus sylvestris, and Larix × eurolepis in southern Sweden, and the efficacy of stump treatment against infection in order to economically assess control measures. The ability of secondary spread of H. parviporum inoculated on Norway spruce stumps created at precommercial thinning (PCT) was investigated. Stumps with a minimum diameter of 2.5 cm were able to transfer infection to adjacent trees, indicating the spread of infections early in a rotation cannot be neglected. Disease spread through PCT stumps was simulated and the economic benefits of stump treatment and winter thinning were compared. Decay frequency was affected by the intensity of PCT, and efficacy of control measures, but not by thinning age or the probability of stump infection. PCT with stump treatment was not profitable at a 3% discount rate regardless of the thinning age or the decay level at the previous rotation. The intensity and distribution of root infection in 36-year-old Scots pine trees nine years after thinning and the relationship between belowground infection and stem growth loss was investigated. Twenty-four Scots pine trees were extracted from two plots and whole root systems and tree volumes were measured. Mean incidence of Heterobasidion spp. infection was 87.5%, but no trees showed any symptoms aboveground. The proportion of infected root volume ranged between 0% and 32%, and negatively affected annual volume increment of individual tree. Both urea and Phlebiopsis gigantea were proved effective as stump treatment agents on hybrid larch stumps in two experiments by examining infection incidence and colony size on stump discs naturally infected by Heterobasidion basidiospores. Heterobasidion spp. infection results in losses in productivity of Norway spruce, Scots pine and hybrid larch. The results of these studies suggest that stump treatment can be economically justified for commercial thinnings of Scots pine and hybrid larch, but not for precommercial thinning of Norway spruce currently.

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