Establishment of Heterobasidion annosum s.l. infections in young Norway spruce dominated stands implications for silviculture

Abstract: One of the worst pathogens on Norway spruce, Heterobasidion annosum s.l., can establish in previously healthy trees by spores landing on freshly made stumps from where mycelia can grow via root grafts and contacts to the adjacent tree. The risk of spore infection as well as disease transfer has been considered to be negligible in young stands due to the small target size of stumps and their small root systems. Consequently, small stumps created during precommercial thinning and late precommercial thinning are usually not treated with protective agents against spore infections as is commonly done during commercial thinnings. The objective of this thesis was to increase the knowledge regarding establishment of H. annosum s.l. infections in young Norway spruce dominated stands, in order to provide a renewed knowledge-base for forest management decisions. The results from field studies showed that although the risk of spore infections increased with increasing stump size more than half of small Norway spruce stumps created during precommercial thinning and late precommercial thinning were infected with H. annosum s.l. (Paper I, II). Of the two Heterobasidion species present in Sweden H. parviporum had a competitive advantage over H. annosum s.s. during colonization of Norway spruce wood (III). Norway spruce stumps as small as 2.5 cm in diameter could transfer infection to trees and the risk of transfer increased with increasing stump size (IV). Stump treatment with the biocontrol agent Phlebiopsis gigantea decreased the amount of spore infections on small stumps (I). The reduction was within the lower range reported from commercial thinning stumps. Simulations of decay development after late precommercial thinning indicated that a substantial amount of the decay at final felling could be attributed to late precommercial thinning. Further, the simulations indicated that stump treatment could reduce the decay at final felling if removal intensities were high. However, the economic outcome of stump treatment varied (II). In conclusion, there is a risk of new H. annosum s.l. infections establishing during silvicultural operations in young Norway spruce dominated stands. Precautionary measures to reduce the risk such as early precommercial thinnings or, where applicable, winter fellings or stump treatment could be considered.