R. Vasaitis’s research while affiliated with Swedish University of Agricultural Sciences and other places

Simple Summary This study explored the relationship between pine bark beetles, Tomicus spp. and fungi in forests of three European countries (Lithuania, Ukraine, Montenegro) with different climates. Bark beetles are tiny insects that attack trees, often carrying fungi that can harm forests. The study aimed to understand the types of fungi associated with these beetles and their potential impact on forest health. The results revealed significant differences in fungal communities across the regions, influenced by local environmental conditions and tree species. Some fungi, such as those causing blue-stain or shoot blight, are harmful to tree health and were found in high numbers in certain areas. Interestingly, some fungi that typically support beetle development were also discovered, showing how these fungi–beetle interactions can be both beneficial and harmful. The findings highlight the role of these beetles in spreading harmful fungi, especially as climate change weakens trees and creates favorable conditions for beetles. This research is valuable for understanding how beetles and fungi interact and showing the risks they pose to forests. It provides important information for forest management strategies to protect trees from these threats, especially in the face of environmental changes that could exacerbate the problem.

In 2019, the bark beetle Ips amitinus (native to central Europe) was identified in south-western Siberia at a distance exceeding 2500 km east of its previously known easternmost location in the European part of Russia. In Siberia, its invasive populations are characterised by high abundance and harmfulness. Here, I. amitinus accomplishes primary attacks on standing vital trees of Pinus sibirica with a lethal outcome. This invasion has already resulted in massive dieback in stands of pine over a large geographic territory. By, 2021, the invaded area was estimated to cover at least 31,200 km². The objectives of this study were to investigate fungi associated with/vectored by I. amitinus in its invasive area in south-western Siberia and wood decay fungi that cause root and butt rots to P. sibirica. This led to the following conclusions: (i) DNA analysis of sixty adult beetles of Ips amitinus collected from P. sibirica in south-west Siberia revealed the presence of 143 fungal taxa; (ii) species richness was significantly higher in beetles collected from dead branches than from (more recently infested) dying branches; (iii) fungal communities were >90% dominated by yeasts, among which the most common were Nakazawaea holstii, Kuraishia molischiana, and N. ambrosiae; (iv) entomopathogenic Beauveria bassiana s.l. was the most common fungus isolated from dead/mycosed beetles of I. amitinus, followed by Lophium arboricola and four Ophiostoma spp.; and (v) Heterobasidion parviporum was the most common decay fungus detected, which was causing heart rot in stems of P. sibirica.

Key message After 5–6 decades since inflicting resin tapping wounds, overmature (> 120 years old) Pinus sylvestris stems remain undecayed and vigorous. Abstract Overmature trees of Pinus sylvestris bearing large wounds made by resin tapping decades ago are still present in woodlands of south-eastern Baltic Sea region. The aim of the present study was to investigate health condition of those trees focusing on fungal infections and to estimate impact of the injury on radial stem growth. The study was conducted in Latvia in three overmature stands of P. sylvestris , resin-tapped in 1950–1970 s. On the studied ninety 120–167-year-old trees, exposed sapwood constituted from 1140 to 7755 cm ² per individual stem. Of the 127 wounds sampled, 52 (41%) showed wood discoloration. The discoloration in its extent was limited, expanding beyond wound margins approx. 1 (max 3) cm in radial, and 6–7 cm in longitudinal directions. Of the 127 wood samples/wounds subjected to fungal isolations, 96% resulted in fungal growth, yielding 236 fungal isolates that represented 47 fungal taxa. The most common among macro-fungi was basidiomycete Porodaedalea pini , which was isolated from 9% of stems. The fungus is currently classed not as a tree pathogen, but instead as an indicator species for woodland sites to be considered for nature conservation. Data from tree ring widths have revealed that tree reacted to the resin tapping injury by increasing radial increment of the un-affected part of the circumference of the stem. Current study demonstrated that even on the long term, resin tapping has little influence on health condition and vitality of P. sylvestris , even at the very old age. This should be considered as a supporting message in case resin taping practices in the region are to be revived.

Drought-induced stress and attacks by bark beetle Ips sexdentatus currently result in a massive dieback of Pinus sylvestris in eastern Ukraine. Limited and fragmented knowledge is available on fungi vectored by the beetle and their roles in tree dieback. The aim was to investigate the fungal community vectored by I. sexdentatus and to test the pathogenicity of potentially aggressive species to P. sylvestris. Analysis of the fungal community was accomplished by combining different methods using insect, plant, and fungal material. The material consisted of 576 beetles and 96 infested wood samples collected from six sample plots within a 300 km radius in eastern Ukraine and subjected to fungal isolations and (beetles only) direct sequencing of ITS rDNA. Pathogenicity tests were undertaken by artificially inoculating three-to-four-year-old pine saplings with fungi. For the vector test, pine logs were exposed to pre-inoculated beetles. In all, 56 fungal taxa were detected, 8 exclusively by isolation, and 13 exclusively by direct sequencing. Those included nine ophiostomatoids, five of which are newly reported as I. sexdentatus associates. Two ophiostomatoid fungi, which exhibited the highest pathogenicity, causing 100% dieback and mortality, represented genera Graphium and Leptographium. Exposure of logs to beetles resulted in ophiostomatoid infections. In conclusion, the study revealed numerous I. sexdentatus-vectored fungi, several of which include aggressive tree pathogens.

Hapalopilus croceus is a rare, wood-decay polypore associated with old coarse broadleaf trees, primarily Quercus. It is mainly distributed throughout the temperate zone of the Northern Hemisphere with its main distribution in Europe. Mating systems in fungi influence the probability of in- or outbreeding and may affect the species’ fitness. We found the mating system of H. croceus to be bipolar by pairing 18 single spore isolates from one sporocarp. We discuss this finding from the perspective of H. croceus as a threatened species with a very small and fragmented current population associated with a declining habitat and globally red-listed as Vulnerable (VU).

Fungi from the genus Heterobasidion are among the most important pathogens of forest trees in Northern Hemisphere causing root rot and wood decay, while Phlebiopsis gigantea is a very common saprotrophic wood decay fungus. Both fungi are primary colonizers of freshly cut conifer stumps (through which Heterobasidion spp. accomplishes primary infections of tree root systems), thus both fungi are competing for the substrate. To date, P. gigantea is widely used as Heterobasidion spp. biocontrol agent. Hypothesis has been proposed that natural colonization of stumps by P. gigantea might also to some extent restrict infections by the pathogen. The main aim of the study was to assess the potential of natural infections of P. gigantea to restrict infection and spread of Heterobasidion spp. in Picea abies and Pinus sylvestris stumps. In total, 793 P. abies stumps and 1158 P. sylvestris stumps were examined in 24 sample plots located in the eastern part of Latvia. Of these, 325 (41.0%) P. abies stumps were infected by Heterobasidion spp., and 59 (7.4%) by P. gigantea, and 168 (14.5%) P. sylvestris stumps were infected by Heterobasidion spp., and 846 (73.1%) by P. gigantea. In P. abies, the observed Heterobasidion spp. infection frequencies were significantly (p < 0.05) higher than those of P. gigantea, while the respective situation in P. sylvestris was reverse and P. gigantea infections were more frequent (p < 0.05). The mean surface area colonized by Heterobasidion spp. in P. abies and P. sylvestris stumps was 5.7 and 5.3 cm² and did not differ significantly (mean coverage of stump surface area respectively 18% and 13%; p = 0.41). In contrast, the mean surface area colonized by P. gigantea was significantly different in the two tree species, respectively, 3.9 and 21.3 cm² (16% and 59%; p < 0.05). The mean surface area colonized by Heterobasidion spp. in P. abies stumps was significantly larger (p < 0.05) than the area colonized by P. gigantea, while conversely, the mean area colonized by P. gigantea in P. sylvestris stumps was significantly larger (p < 0.001) than that colonized by Heterobasidion spp. Both fungi were co-occurring in P. abies stumps in 33 cases (4.2% of all investigated stumps), and in P. sylvestris stumps in 138 cases (11.9%). There were no correlations between the sizes of colonized areas of Heterobasidion spp. and P. gigantea in P. abies stumps (r = 0.06; p = 0.76), or P. sylvestris stumps (r = 0.009; p = 0.27). In conclusion, the results of this study strongly suggest that even in stumps of P. sylvestris, that otherwise are much preferred for natural colonization by airborne spores of the biocontrol agent P. gigantea, natural colonization by P. gigantea is not able to restrict infections by Heterobasidion spp. This clearly indicates that for effective biocontrol of Heterobasidion spp. infections, the necessity for thorough treatment coverage of cut P. abies and P. sylvestris stumps at early stages of plantation management, during pre-commercial thinning.

During recent years, a new disease of Siberian fir (A. sibirica) emerged in Central Siberia, exhibiting symptoms of stem/branch deformation, cambium necrosis, and dieback of branches and twigs, the causal agent remaining unknown. The aim was to identify agent of the disease and to investigate its pathogenicity to A. sibirica and Norway spruce (Picea abies). Symptomatic tissues of fir were subjected to pure culture isolation of anticipated pathogen(s). Obtained isolates were subjected to molecular identification, phylogenetic analyses, and pathogenicity tests with A. sibirica saplings, and seeds and seedlings of A. sibirica and P. abies. The study demonstrated that, (i) most commonly isolated fungus from canker wounds of A. sibirica exhibited Acremonium-like anamorphs; (ii) phylogeny demonstrated that investigated fungi belong to genus Corinectria, but are genetically well separated from other worldwide known Corinectria spp.; (iii) one species of isolated fungi has the capacity to cause the disease and kill A. sibirica saplings and seedlings, but also seedlings of P. abies. Guidelines for future research were defined in order to generate needed information on species description, its origin and ecology, and estimation of potential risks upon the eventual invasion of the pathogen to new geographic areas, in particular of Europe.

Ash dieback (ADB) caused by the pathogen Hymenoscyphus fraxineus is the cause of massive mortality of Fraxinus spp. in Europe. The aim of this work was to check for the presence of the molecular marker for ADB tolerance in mapped healthy‐looking F. excelsior trees, and to compare its occurrence in trees exhibiting severe ADB symptoms. Monitoring of 135 healthy‐looking F. excelsior on the island of Gotland, Sweden, showed that after 3–4 years 99.3% of these trees had 0%–10% crown damage, thus remaining in a similar health condition as when first mapped. After 5–6 years, 94.7% of these trees had 0%–10% crown damage. Molecular analysis of leaf tissues from 40 of those showed the presence of the molecular marker in 34 (85.0%) trees, while it was absent in 6 (15.0%) trees. Analysis of leaf tissues from 40 severely ADB‐diseased trees showed the presence of the molecular marker in 17 (42.5%) trees, but its absence in 23 (57.5%) trees (p < .0001). The results demonstrated that monitoring of healthy‐looking F. excelsior is a simple and straightforward approach for the selection of presumably ADB‐tolerant ash for future breeding. The cDNA‐based molecular marker revealed moderate capacity on its own to discriminate between presumably ADB‐tolerant and susceptible F. excelsior genotypes.

In boreal systems, soil profiles typically consist of distinct stratified horizons, with organic layers at the surface overlying deeper mineral horizons providing microhabitat variation along a depth gradient, and vertical stratification of fungal communities along such soil profiles is commonly observed. We studied fungal community structure in a coastal pine forest along a gradient of decreasing influence from the coast. In this system, the vertical stratification pattern of soil microhabitats (defined here as organic, mineral with roots, and mineral without roots; O, MR, and MN, respectively) is non-uniform; organic horizons are sometimes buried under drifting sand dunes. Our results show that soil microhabitats are distinct with respect to physiochemical characteristics, community composition, and OTU richness. While community composition was partly related to depth and distance from the coastal forest edge, microhabitat appeared to have the strongest influence. A closer inspection of the OTUs with the highest relative sequence abundance within each microhabitat revealed that microhabitats support functionally distinct fungal communities with respect to trophic mode and growth morphology. These results suggest that in coastal pine forests, variation in soil microhabitats contributes to the high fungal diversity found below-ground and may play an important role in optimizing nutrient cycling.