| Summary: | Stumps play a pivotal role in the epidemiology of the fungal forest pathogens <i>Heterobasidion</i> 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., <i>Larix sibirica</i>, <i>Picea abies</i>, <i>Picea sitchensis</i>, <i>Pinus contorta</i>, <i>Pinus strobus</i>, <i>Pinus sylvestris</i> and <i>Pseudotsuga menziesii</i>) to primary infection by <i>H. annosum</i> and <i>H. parviporum</i> 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 <i>H. annosum</i> or <i>H. parviporum</i>; (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 (<i>r</i> = 0.79; <i>p</i> < 0.05) between the two sets of data. Susceptibility was highest in <i>Pinus</i> species, followed by <i>P. abies</i> and <i>P. sitchensis</i>. Susceptibility was lowest for <i>L. sibirica</i> and <i>P. menziesii</i>. The area colonized by <i>Heterobasidion</i> 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 <i>Heterobasidion</i> spp. Taken together, these findings could help designing tactics to manage these fungal forest pathogens.
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