Susceptibility of Small-Diameter Norway Spruce Understory Stumps to Heterobasidion Spore Infection

Heterobasidion spp. cause economically important losses in conifer forests in the Northern Hemisphere, especially in Norway spruce stands. Freshly cut stumps are the main route for Heterobasidion spp. infection. Even small stumps of spruce seedlings are known to be susceptible to spore infection, however, very little is currently known about the susceptibility of small stumps of understory spruce to Heterobasidion spore infection. To determine the frequency of spore infections among stumps of understory trees, we analyzed 756 Norway spruce stumps in eight sample plots in eastern Latvia. Understory trees 35–68 years of age, with a stump diameter of 2–10 cm, were felled 33–48 weeks before sampling. In total, 514 (68%) of the spruce stumps were infected by Heterobasidion spores. The infection frequency among the plots varied from 56% to 86%. Both infection frequency and stump surface area occupied by Heterobasidion positively correlated with stump diameter, but neither correlated with the time span elapsed between felling and sampling. Colonization of trees by a competitor fungus, Phlebiopsis gigantea (Fr.) Jülich, was observed in 30 (4%) of investigated stumps, but did not have any effect on the frequency of Heterobasidion infections. Our data show that Norway spruce stumps can be highly susceptible to Heterobasidion spore infection. In order to control airborne Heterobasidion infections, stump treatment should be considered during the sporulation period of Heterobasidion spp.