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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.

The biological control agent Rotstop, composed of spores of Phlebiopsis gigantea, is used for treatment of conifer stumps to reduce the spread of Heterobasidion root rot in commercial forests. Two experiments were conducted to compare the antagonistic ability of the Rotstop isolate and nine Latvian isolates of P. gigantea against root rot fungus Heterobasidion, in wood of Scots pine and Norway spruce. Billets of conifer trees were first sprayed with a spore suspension of P. gigantea and then with Heterobasidion at different concentrations. The presence of fungi in billets was evaluated by morphological characteristics of mycelium. The Latvian isolates of P. gigantea showed similar or even higher values of efficacy against Heterobasidion (average efficacy 84% in spruce and 89% in pine) than the commercially manufactured Finnish preparation Rotstop (61% in spruce and 90% in pine). Latvian isolates of P. gigantea have a potential to be used for preparation of biological control agents in the future.

Promocijas darbs izstrādāts Latvijas Valsts mežzinātnes institūtā "Silava", doktorantūras studiju Latvijas Lauksaimniecības universitātes Meža fakultātē laika periodā no 2013. līdz 2018. gadam, LVMI Silava un a/s "Latvijas valsts meži'' 2011. gada 11. oktobra memoranda "Par sadarbību zinātniskajā izpētē" īstenotā pētījuma "Sakņu trupes izplatību ierobežojošo faktoru izpēte", ERAF līdzfinansētā pētījuma "Metodes un tehnoloģijas meža kapitālvērtības palielināšanai", Latvijas Zinātnes padomes grantu "Heterobasidion spp. izraisītās sakņu trupes ierobežošana, izmantojot Phlebiopsis gigantea-izolātu bioloģiskās aizsardzības efektivitāti ietekmējošo faktoru izvērtējums" un "Lielās pergamentsēnes nozīme sakņu piepes micēlija izplatības ierobežošanā egļu un priežu celmos", kā arī Valsts pētījumu programmas pētījuma "Meža un zemes dzīļu resursu izpēte, ilgtspējīga izmantošana-jauni produkti un tehnoloģijas (ResProd)-"Vienvecuma egļu mežu audzēšanas potenciāls auglīgajās meža ekosistēmās"" ietvaros./ The Doctoral thesis is designed at the Latvian State Forest Research Institute "Silava", PhD studies in Latvia University of Life Sciences and Technologies, Forest Faculty in period from 2013 to 2018 and has been developed within the Joint Stock Company "Latvijas valsts meži" project "Research of limiting factors of the spread of root rot", ERAF project "Methods and technologies for increasing the value of forest capital", the Latvian Science Council grant projects "Restrictions of Heterobasidion spp. root rot infection with Phlebiopsis gigantea-evaluation of factors affecting the biological protection of isolates" and "Importance of Phlebiopsis gigantea in controlling the prevalence of Heterobasidion spp. spawn in spruce and pine stumps", as well as in National research program project "Exploration of the resources of the forest and subterranean depths, sustainable use-new products and technologies (ResProd)-"The potential of growing one age spruce forests in fertile forest ecosystems"" Oficiālie recenzenti / Official reviewers: • Dr.silv. Olga MIEZĪTE, Latvijas Lauksaimniecības universitātes profesore, Latvijas Zinātnes padomes eksperte. /