Situation of Armillaria spp. and Heterobasidion spp. in Slovenia

Species of the genus Armillaria and Heterobasidion are among the most common causes of stem and root rot in Slovenia. Armillaria spp. infect deciduous and coniferous trees, while Heterobasidion spp. mainly threaten Picea abies, Pinus spp., and Abies alba. Based on the data from the sanitary felling of infected trees, we estimated the current state and calculated the proportions represented in total felling, total sanitary felling, total sanitary felling due to root rot diseases, in wood stock, and in increments from 2013 to 2017. Since 2014, there has been a constant increase in the sanitary felling of deciduous and coniferous trees due to infections by Armillaria spp. In 2017, 32,849 m(3) of timber were harvested due to Armillaria spp. Given the present situation, we assume that the amount of sanitary felling will continue to increase, but it will not account for large shares in wood stock or increment. In 2017, sanitary felling of Armillaria infected conifers represented 27.6% of all sanitary felling due to diseases. In the case of deciduous trees, the share was lower, i.e. 7.1%. Armillaria spp. was the main cause of sanitary felling due to disease (51.9%) in the Postojna forest management unit (FMU), while elsewhere shares of up to 10% were recorded. Regarding Heterobasidion spp., the amount of felling decreased over the years. In 2017, 33,922 m(3) of wood, accounting for 15.7% of the total sanitary felling due to disease, was felled due to Heterobasidion spp. Out of the total sanitary fellings due to these fungi, on average 79.5 12.9%, and 34.3% of Picea abies, Abies alba and Pinus sylvestris, respectively, were felled due to Heterobasidion infection. We predict that the volume of timber harvested due to Heterobasidion spp. will gradually decrease over the years due to the lower wood stocks of Norway spruce, which has recently been hit by numerous natural disasters and infestation of bark beetles. The trend of Heterobasidion incidence from our study conflicts with other research that expect higher incidence of root rots due to climate change effects.