Persistence and infectivity of Heterobasidion parviporum in Norway spruce root residuals following stump harvesting

In boreal forests, stumps of Norway spruce are removed after clear cutting to gather wood for energy and, additionally, to reduce damage caused by the root rot fungus, Heterobasidion parviporum. In order to understand the behavior of H. parviporum at stump-harvested Norway spruce sites, the survival of Heterobasidion mycelia in buried root fragments and their ability to infect nearby spruce seedlings was investigated in southern Finland. The longevity of Heterobasidion mycelium in investigated root pieces exceeded the follow-up period of 83 months. After six years, 18% of the root fragments yielded living Heterobasidion mycelia. The survival of H. parviporum in root fragments was highest in sandy soils and increased with increasing volume of root fragment. The probability of occurrence of the fungus was highest when at least 60% of the root volume was originally colonized by the fungus, irrespective of the state of decay. It took 4.5 years until roots of the first seedling reached the buried root fragment and the seedling became infected by H. parviporum mycelia. After six years, 8% of the seedlings were found to be infected. In addition, the number of root residuals, i.e., the potential infection sources of the next tree generation, was determined at five stump-harvested Norway spruce sites. At these slightly infested spruce sites, on average 1500 potential disease sources ha−1 (in the form of decayed root fragments ⩾ 1.5 cm in diameter) were found in the upper soil layer after stump removal. Thus, our results indicate that stump harvesting procedure aimed at gathering wood for energy may not be an efficient way to remove infected wood material from forest soils. Furthermore, our survey on stored stumps next to clear-cut areas showed that the formation of Heterobasidion fruiting bodies may be abundant in decayed stumps in the lowest parts of the storage pile. In order to avoid increased risks of spore infection in nearby forests, decayed spruce stumps, including the lowest muddy stumps, should be transported within two years from forest storage to the end-user.