Heterobasidion Parviporum Research Articles

Linalool and 1,8-Cineole as Constitutive Disease-Resistant Factors of Norway Spruce Against Necrotrophic Pathogen Heterobasidion Parviporum.

Norway spruce is an important coniferous species in boreal forests. Root and stem rot diseases caused by the necrotrophic pathogen Heterobasidion parviporum threaten the wood production of Norway spruce which necessitates the search for durable control and management strategies. Breeding for resistant traits is considered a viable long-term strategy. However, identification of potential resistant traits and markers remains a major challenge. In this study, short-term disease resistance screening was conducted using 218 Norway spruce clones from 17 families. Disease resistance was evaluated based on the size of necrosis lesion length following infection with the pathogen. A subset of needles/branches from clones with small (partial resistant) or large (susceptible) lesions were used for terpene analysis and transcriptomic profiling. The results revealed that the content of monoterpene linalool and 1,8-cineole and their respective encoded genes were significantly more abundant and highly expressed in the partial resistant group. Furthermore, linalool and 1,8-cineole were demonstrated to have inhibitory effect on the growth of the pathogen H. parviporum, with morphological distortion of the hyphae. RNAseq analysis revealed that transcript of pathogen genes involved in the regulation of carbohydrate metabolism and stress responses were significantly decreased in presence of the terpenes. The results suggest the relevance of monoterpenes together with jasmonic acid precursor and some genes involved in phenylpropanoid biosynthesis, as constitutive tolerance factors for Norway spruce tolerance against necrotrophic pathogen. The high level of necrosis related cell death gene expression might be factors critical for host susceptibility and disease development.

PaLAR3 genotype variability for enhanced resistance against Heterobasidion parviporum in Norway spruce: Insights into allelic frequencies, disease response, and the role of mycoviruses

Heterobasidion parviporum is a destructive forest pathogen causing root and stem rot of Norway spruce (Picea abies) in northern hemisphere. Previous research has suggested that a gene form of PaLAR3 that encodes for the leucoanthocyanidin reductase enzyme, may increase resistance to H. parviporum in Norway spruce, whether heterozygous or homozygous. Our experiment was initially focused on testing this assertion with materials from the Finnish Norway spruce breeding program, comprising 52 full-sib families. Additionally, we investigated the influence of mycoviruses on the growth of H. parviporum. The frequencies of the three PaLAR3 genotypes AA, AB, and BB were 64%, 30%, and 6%, respectively. We found no significant differences in the necrotic response to H. parviporum between the homozygous (BB) and heterozygous (AB) genotypes vs. the homozygous (AA) genotype across the two fungal strains tested, suggesting the gene is rather ineffective as a marker for overall H. parviporum resistance. The two H. parviporum strains used in the study showed significantly different necrosis responses from the control. Mycovirus infection in H. parviporum strains led to a significant reduction in lesion width in phloem and sapwood. Furthermore, the growth of the fungus was notably restricted when hyphae hosted mycoviruses, indicating a high potential of mycoviruses to reduce fungal growth and necrosis development.

Leucoanthocyanidin Reductase 3 (PaLAR3) Locus in Norway Spruce (Picea abies) and Its Link to Resistance Against Heterobasidion parviporum

ABSTRACTHeterobasidion parviporum is a fungal pathogen that is drastically damaging Norway spruce (Picea abies) in Europe. The infections will result in root and stem rot, causing significant economic losses for forest owners. Previous studies have shown that the PaLAR3 gene, which encodes the leucoanthocyanidin reductase enzyme, can increase resistance to H. parviporum in Norway spruce. The presence of the B allele at the PaLAR3 locus has been associated with higher (+)‐catechin concentrations and increased enzyme production in inoculation experiments, resulting in inhibited pathogen growth. The control of H. parviporum involves a multifaceted approach, including silvicultural and sustainable forestry practices, genetic resistance and chemical/biological control. In this study, we determined the PaLAR3 genotypes in a representative sample of Norway spruce breeding materials from southern Finland and examined their effect on necrosis caused by H. parviporum in spruce stems. The results showed that the homozygous PaLAR3BB genotype was present in only 9% of the trees. However, the necrotic area interacted with homozygous PaLAR3BB under low‐water treatment. These findings support the idea that the PaLAR3 locus may be a valuable marker for identifying P. abies resistance to different strains of Heterobasidion parviporum.

Diversity and impact of single-stranded RNA viruses in Czech Heterobasidion populations.

Heterobasidion annosum sensu lato comprises some of the most devastating pathogens of conifers. Exploring virocontrol as a potential strategy to mitigate economic losses caused by these fungi holds promise for the future. In this study, we conducted a comprehensive screening for viruses in 98 H. annosum s.l. specimens from different regions of Czechia aiming to identify viruses inducing hypovirulence. Initial examination for dsRNA presence was followed by RNA-seq analyses using pooled RNA libraries constructed from H. annosum and Heterobasidion parviporum, with diverse bioinformatic pipelines employed for virus discovery. Our study uncovered 25 distinct ssRNA viruses, including two ourmia-like viruses, one mitovirus, one fusarivirus, one tobamo-like virus, one cogu-like virus, one bisegmented narna-like virus and one segment of another narna-like virus, and 17 ambi-like viruses, for which hairpin and hammerhead ribozymes were detected. Coinfections of up to 10 viruses were observed in six Heterobasidion isolates, whereas another six harbored a single virus. Seventy-three percent of the isolates analyzed by RNA-seq were virus-free. These findings show that the virome of Heterobasidion populations in Czechia is highly diverse and differs from that in the boreal region. We further investigated the host effects of certain identified viruses through comparisons of the mycelial growth rate and proteomic analyses and found that certain tested viruses caused growth reductions of up to 22% and significant alterations in the host proteome profile. Their intraspecific transmission rates ranged from 0% to 33%. Further studies are needed to fully understand the biocontrol potential of these viruses in planta.IMPORTANCEHeterobasidion annosum sensu lato is a major pathogen causing significant damage to conifer forests, resulting in substantial economic losses. This study is significant as it explores the potential of using viruses (virocontrol) to combat these fungal pathogens. By identifying and characterizing a diverse array of viruses in H. annosum populations from Czechia, the research opens new avenues for biocontrol strategies. The discovery of 25 distinct ssRNA viruses, some of which reduce fungal growth and alter proteome profiles, suggests that these viruses could be harnessed to mitigate the impact of Heterobasidion. Understanding the interactions between these viruses and their fungal hosts is crucial for developing effective, environmentally friendly methods to protect conifer forests and maintain ecosystem health. This study lays the groundwork for future research on the application of mycoviruses in forest disease management.

Beyond the surface: exploring the mycobiome of Norway spruce under drought stress and with Heterobasidion parviporum

The mycobiome, comprising fungi inhabiting plants, potentially plays a crucial role in tree health and survival amidst environmental stressors like climate change and pathogenic fungi. Understanding the intricate relationships between trees and their microbial communities is essential for developing effective strategies to bolster the resilience and well-being of forest ecosystems as we adopt more sustainable forest management practices. The mycobiome can be considered an integral aspect of a tree’s biology, closely linked to its genotype. To explore the influence of host genetics and environmental factors on fungal composition, we examined the mycobiome associated with phloem and roots of Norway spruce (Picea abies (L.) Karst.) cuttings under varying watering conditions. To test the “mycobiome-associated-fitness” hypothesis, we compared seedlings artificially inoculated with Heterobasidion parviporum and control plants to evaluate mycobiome interaction on necrosis development. We aimed to 1) identify specific mycobiome species for the Norway spruce genotypes/families within the phloem and root tissues and their interactions with H. parviporum and 2) assess stability in the mycobiome species composition under abiotic disturbances (reduced water availability). The mycobiome was analyzed by sequencing the ribosomal ITS2 region. Our results revealed significant variations in the diversity and prevalence of the phloem mycobiome among different Norway spruce genotypes, highlighting the considerable impact of genetic variation on the composition and diversity of the phloem mycobiome. Additionally, specific mycobiome genera in the phloem showed variations in response to water availability, indicating the influence of environmental conditions on the relative proportion of certain fungal genera in Norway spruce trees. In the root mycobiome, key fungi such as Phialocephala fortinii and Paraphaeosphaeria neglecta were identified as conferring inhibitory effects against H. parviporum growth in Norway spruce genotypes. Furthermore, certain endophytes demonstrated greater stability in root ecosystems under low water conditions than ectomycorrhizal fungi. This knowledge can contribute to developing sustainable forest management practices that enhance the well-being of trees and their ecosystems, ultimately bolstering forest resilience.

Species diversity of pathogenic wood-rotting fungi (Agaricomycetes, Basidiomycota) in China

ABSTRACT Wood-rotting basidiomycetes have been investigated in the Chinese forest ecosystem for the past 30 years. Two hundred and five pathogenic wood-decayers belonging to 9 orders, 30 families, and 74 genera have been found in Chinese native forests, plantations, and gardens. Seventy-two species (accounting for 35% of the total pathogenic species) are reported as pathogenic fungi in China for the first time. Among these pathogens, 184 species are polypores, nine are corticioid fungi, eight are agarics and five are hydnoid basidiomycetes. One hundred and seventy-seven species (accounting for 86%) cause white rot, while 28 species (accounting for 14%) result in brown rot; 157 species grow on angiosperm trees (accounting for 76.5%) and 44 species occur on gymnosperm trees (accounting for 21.5%), only four species inhabit both angiosperms and gymnosperms (accounting for 2%); 95 species are distributed in boreal to temperate forests and 110 in subtropical to tropical forests. In addition, 17 species, including Fomitopsis pinicola, Heterobasidion parviporum, and Phellinidium weirii etc. which were previously treated as pathogenic species in China, do not occur in China according to recent studies. In this paper, the host(s), type of forest, rot type, and distribution of each pathogenic species in China are given.

Genetic variation of Picea abies in response to the artificial inoculation of Heterobasidion parviporum

Norway spruce Picea abies is one of Europe’s most economically important tree species. However, it is highly susceptible to the root rot fungus Heterobasidion parviporum. Climate change will benefit the pathogen as the tree host is weakened by, e.g., extended drought. Breeding can improve forest health, and several root rot genetic markers are suggested to improve the resistance of Norway spruce. This study aimed to compare the resistance (here defined as necrosis length) of Norway spruce families and genotypes against two strains of H. parviporum under different water availabilities. Our results show that the family and the genotype within the family have an impact on the necrosis length that is related to the aggressiveness of the fungal strains. Under low water conditions, the necrosis increased only in horizontal directions in phloem and sapwood. Similarly, the growth (seedling height) was not disturbed by abiotic stress (less water), indicating that the stress level (drought) was too low in this setting. The knowledge gained in this study could improve forest health in the changing climate by understanding the response of Norway spruce to pathogenic attacks under additional stress at the family level. This knowledge could be strategically used in forest breeding to improve the resistance of Norway spruce trees against root rot.

STUDYING OF THE STS-MARKERS POLYMORPHISM IN HETEROBASIDION ISOLATES INFECTING NORWAY SPRUCE (PICEA ABIES (L.) H.KARST.) IN BELARUS

The article presents the results of studying the genetic polymorphism of Heterobasidion (root rot agent) isolates that
 infect Norway spruce (Picea abies) in various regions of Belarus. Based on the data obtained, a comparative assessment of the suitability of STS markers for species diagnostics of fungal isolates (Heterobasidion annosum (Fr.) Bref. sensu stricto (s. s.) and Heterobasidion parviporum Niemelä & Korhonen) was carried out. Samples of the sporocarp and infected wood were collected in Norway spruce stands on the territory of three geobotanical subzones — in seven forestry enterprises of the Ministry of Forestry of the Republic of Belarus, on the territory of the Negorelsky educational and experimental forestry and the National Park "Belovezhskaya Pushcha". Based on the comprehensive analysis of the samples, markers for specific molecular genetic diagnosis of H. annosum and H. parviporum were recommended.

Allele PaLAR3B in root rot resistance locus does not influence the infection pressure by Heterobasidion parviporum through root contacts

AbstractThe most destructive root rot pathogen of Norway spruce (Picea abies) in Finland is Heterobasidion parviporum. After primary infection, this pathogen spreads from tree to tree through root contacts and eventually causes high levels of root and stem rot. Locus PaLAR3 has been associated with root rot resistance of Norway spruce, and higher resistance in Norway spruce against H. parviporum has been noted when trees have allele PaLAR3B instead of the more common PaLAR3A. In this study we tested if trees with homo‐ or heterozygous PaLAR3B allele can better resist the infection through root contacts. The study was established in 2018 in a site where spruce trees had grown at a dense spacing for 10 years. The 72 trees were cut down and a mixture of four H. parviporum strains was inoculated on the stumps. After 3 years, 459 trees surrounding the inoculated stumps were harvested and analysed. Based on the presence of conidiospores, forty percent (40%) of the trees were infected by H. parviporum. The homozygous PaLAR3B genotype was found only in 1% of the trees. There were no differences in infection rates between the genotypes AA and AB (or BB). Similarly, the distance from inoculated stump or the diameter of the tree did not influence the infection rate.

The dark septate endophyte Phialocephala sphaeroides suppresses conifer pathogen transcripts and promotes root growth of Norway spruce.

Plant-associated microbes including dark septate endophytes (DSEs) of forest trees play diverse functional roles in host fitness including growth promotion and increased defence. However, little is known about the impact on the fungal transcriptome and metabolites during tripartite interaction involving plant host, endophyte and pathogen. To understand the transcriptional regulation of endophyte and pathogen during co-infection, Norway spruce (Picea abies) seedlings were infected with DSE Phialocephala sphaeroides, or conifer root-rot pathogen Heterobasidion parviporum, or both. Phialocephala sphaeroides showed low but stable transcripts abundance (a decrease of 40%) during interaction with Norway spruce and conifer pathogen. By contrast, H. parviporum transcripts were significantly reduced (92%) during co-infection. With RNA sequencing analysis, P. sphaeroides experienced a shift from cell growth to anti-stress and antagonistic responses, while it repressed the ability of H. parviporum to access carbohydrate nutrients by suppressing its carbohydrate/polysaccharide-degrading enzyme machinery. The pathogen on the other hand secreted cysteine peptidase to restrict free growth of P. sphaeroides. The expression of both DSE P. sphaeroides and pathogen H. parviporum genes encoding plant growth promotion products were equally detected in both dual and tripartite interaction systems. This was further supported by the presence of tryptophan-dependent indolic compound in liquid culture of P. sphaeroides. Norway spruce and Arabidopsis seedlings treated with P. sphaeroides culture filtrate exhibited auxin-like phenotypes, such as enhanced root hairs, and primary root elongation at low concentration but shortened primary root at high concentration. The results suggested that the presence of the endophyte had strong repressive or suppressive effect on H. parviporum transcripts encoding genes involved in nutrient acquisition.

Patterns and roles of lignan and terpenoid accumulation in the reaction zone compartmentalizing pathogen-infected heartwood of Norway spruce

Main conclusionLignan impregnation of the reaction zone wood protects against oxidative degradation by fungi. Traumatic resin canals may play roles in the underlying signal transduction, synthesis, and translocation of defense compounds.Tree defense against xylem pathogens involves both constitutive and induced phenylpropanoids and terpenoids. The induced defenses include compartmentalization of compromised wood with a reaction zone (RZ) characterized by polyphenol deposition, whereas the role of terpenoids has remained poorly understood. To further elucidate the tree–pathogen interaction, we profiled spatial patterns in lignan (low-molecular-weight polyphenols) and terpenoid content in Norway spruce (Picea abies) trees showing heartwood colonization by the pathogenic white-rot fungus Heterobasidion parviporum. There was pronounced variation in the amount and composition of lignans between different xylem tissue zones of diseased and healthy trees. Intact RZ at basal stem regions, where colonization is the oldest, showed the highest level and diversity of these compounds. The antioxidant properties of lignans obviously hinder oxidative degradation of wood: RZ with lignans removed by extraction showed significantly higher mass loss than unextracted RZ when subjected to Fenton degradation. The reduced diversity and amount of lignans in pathogen-compromised RZ and decaying heartwood in comparison to intact RZ and healthy heartwood suggest that α-conindendrin isomer is an intermediate metabolite in lignan decomposition by H. parviporum. Diterpenes and diterpene alcohols constituted above 90% of the terpenes detected in sapwood of healthy and diseased trees. A significant finding was that traumatic resin canals, predominated by monoterpenes, were commonly associated with RZ. The findings clarify the roles and fate of lignan during wood decay and raise questions about the potential roles of terpenoids in signal transduction, synthesis, and translocation of defense compounds upon wood compartmentalization against decay fungi.

Fungi Occurring in Norway Spruce Wood Decayed by Heterobasidion parviporum in Puszcza Borecka Stands (Northeastern Poland)

In many spruce stands, trees are frequently attacked by the pathogen Heterobasidion parviporum, albeit without visible symptoms in the crown. In the present work, the results of the presence of stem rot, assessed by PICUS Sonic Tomography, and the fungal biota on trees and stumps in eight plots in the Puszcza Borecka Forest are described. The plots were located in stands on original forest soil (4) and on post-agricultural soil (4), where around a stump with H. parviporum symptoms (signs of internal rot and basidiocarps), 30 trees were selected and examined for internal rot. Wood samples were collected from two selected trees for fungal molecular analysis. A total of 79 fungal taxa were found, including 57 taxa in plots on post-agricultural soil and 45 on forest soil. There were 395 fungal records on stumps and 22 records on trees, therein, from the inner parts of felled trunks. Significant differences in the Chao-1 diversity index indicate that the origin of the soil—post-agricultural or forest soil—influenced the alpha diversity of the fungal communities in the forests studied. The values of the Shannon and Simpson indices show that the two communities were similar in terms of species numbers. The presence of basidiomata of H. parviporum and two species of Armillaria (mainly A. cepistipes) in samples on all plots is striking, although Armillaria spp. was detected more frequently. Most of the species identified were typical saprotrophs, although rare species were also found, such as Entoloma byssisedum, Onnia tomentosa, Physisporinus vitreus, Postia ptychogaster, and Ramaria apiculata. The presence of H. parviporum in the inner woody parts was confirmed by PCR analysis, and decay was detected even up to a stem height of 6 m. Armillaria was the dominant genus in the studied stands and plays a significant and underestimated role in heartwood decay of old spruce trees in Puszcza Borecka Forest.

Mitonuclear Genetic Interactions in the Basidiomycete Heterobasidion parviporum Involve a Non-conserved Mitochondrial Open Reading Frame.

The mitochondrial and nuclear genomes of Eukaryotes are inherited separately and consequently follow distinct evolutionary paths. Nevertheless, the encoding of many mitochondrial proteins by the nuclear genome shows the high level of integration they have reached, which makes mitonuclear genetic interactions all the more conceivable. For each species, natural selection has fostered the evolution of coadapted alleles in both genomes, but a population-wise divergence of such alleles could lead to important phenotypic variation, and, ultimately, to speciation. In this study in the Basidiomycete Heterobasidion parviporum, we have investigated the genetic basis of phenotypic variation among laboratory-designed heterokaryons carrying the same pair of haploid nuclei, but a different mitochondrial genome. Radial growth rate data of thirteen unrelated homokaryotic parents and of their heterokaryotic offspring were combined with SNP data extracted from parental genome sequences to identify nuclear and mitochondrial loci involved in mitonuclear interactions. Two nuclear loci encoding mitochondrial proteins appeared as best candidates to engage in a genetic interaction affecting radial growth rate with a non-conserved mitochondrial open reading frame of unknown function and not reported apart from the Russulales order of Basidiomycete fungi. We believe our approach could be useful to investigate several important traits of fungal biology where mitonuclear interactions play a role, including virulence of fungal pathogens.

Discovery and Community Dynamics of Novel ssRNA Mycoviruses in the Conifer Pathogen Heterobasidion parviporum.

Heterobasidion species are highly destructive basidiomycetous conifer pathogens of the Boreal forest region. Earlier studies have revealed dsRNA virus infections of families Curvulaviridae and Partitiviridae in Heterobasidion strains, and small RNA deep sequencing has also identified infections of Mitoviridae members in these fungi. In this study, the virome of Heterobasidion parviporum was examined for the first time by RNA-Seq using total RNA depleted of rRNA. This method successfully revealed new viruses representing two established (+)ssRNA virus families not found earlier in Heterobasidion: Narnaviridae and Botourmiaviridae. In addition, we identified the presence of a recently described virus group tentatively named “ambiviruses” in H. parviporum. The H. parviporum isolates included in the study originated from experimental forest sites located within 0.7 km range from each other, and a population analysis including 43 isolates was conducted at one of the experimental plots to establish the prevalence of the newly identified viruses in clonally spreading H. parviporum individuals. Our results indicate that viral infections are considerably more diverse and common among Heterobasidion isolates than known earlier and include ssRNA viruses with high prevalence and interspecies variation.

The dark septate endophyte Phialocephala sphaeroides confers growth fitness benefits and mitigates pathogenic effects of Heterobasidion on Norway spruce.

Forest trees frequently interact with a diverse range of microorganisms including dark septate endophytes (DSEs) and fungal pathogens. Plant defense responses to either individual pathogens or endophytes have been widely studied, but very little is known on the effect of coinfection on host defenses. To study the impact of coinfection or tripartite interaction on plant growth and host defenses, Norway spruce (Picea abies (L.) Karst) seedlings were inoculated with a DSE Phialocephala sphaeroides or with a root pathogen Heterobasidion parviporum Niemela & Korhonen or coinfected with both fungi. The results showed that the DSE promoted the root growth of spruce seedlings. Control seedlings without any inoculum were subjected to sequencing and used as a baseline for identification of differentially expressed genes (DEGs). RNA-seq analysis of seedlings inoculated with P. sphaeroides, infected with H. parviporum or coinfected with both fungi resulted in a total of 5269 DEGs. The majority of DEGs were found in P. sphaeroides-inoculated seedlings. Lignin biosynthesis pathways were generally activated during fungal infections. The pattern was distinct with endophyte inoculation. The majority of the genes in the flavonoid biosynthesis pathway were generally suppressed during fungal infections. A specific transcriptional response to P. sphaeroides inoculation was the increased transcripts of genes involved in jasmonic acid biosynthesis, mitogen-activated protein kinases signaling pathway, plant hormone signal transduction and calcium-mediated signaling. This may have potentially contributed to promoting the root growth of seedlings. Although the coinfection suppressed the induction of numerous genes, no negative effect on the growth of the spruce seedlings occurred. We conclude that the subsequent H. parviporum infection triggered reprogramming of host metabolism. Conversely, the endophyte (P. sphaeroides), on the other hand, counteracted the negative effects of H. parviporum on the growth of the spruce seedlings.

Drought Stress Described by Transcriptional Responses of Picea abies (L.) H. Karst. under Pathogen Heterobasidion parviporum Attack

The major threats to the sustainable supply of forest tree products are adverse climate, pests and diseases. Climate change, exemplified by increased drought, poses a unique threat to global forest health. This is attributed to the unpredictable behavior of forest pathosystems, which can favor fungal pathogens over the host under persistent drought stress conditions in the future. Currently, the effects of drought on tree resistance against pathogens are hypothetical, thus research is needed to identify these correlations. Norway spruce (Picea abies (L.) H. Karst.) is one of the most economically important tree species in Europe and is considered highly vulnerable to changes in climate. Dedicated experiments to investigate how disturbances will affect the Norway spruce—Heterobasidion sp. pathosystem are important, in order to develop different strategies to limit the spread of H. annosum s.l. under the predicted climate change. Here, we report a transcriptional study to compare Norway spruce gene expressions to evaluate the effects of water availability and the infection of Heterobasidion parviporum. We performed inoculation studies of three-year-old saplings in a greenhouse (purchased from a nursery). Norway spruce saplings were treated in either high (+) or low (−) water groups: high water group received double the water amount than the low water group. RNA was extracted and sequenced. Similarly, we quantified gene expression levels of candidate genes in biotic stress and jasmonic acid (JA) signaling pathways using qRT-PCR, through which we discovered a unique preferential defense response of H. parviporum-infected Norway spruce under drought stress at the molecular level. Disturbances related to water availability, especially low water conditions can have negative effects on the tree host and benefit the infection ability of the pathogens in the host. From our RNA-seq analysis, 114 differentially expressed gene regions were identified between high (+) and low (−) water groups under pathogen attack. None of these gene pathways were identified to be differentially expressed from both non-treated and mock-control treatments between high (+) and low (−) water groups. Finally, only four genes were found to be associated with drought in all treatments.

Killing two enemies with one stone? Genomics of resistance to two sympatric pathogens in Norway spruce.

Trees must cope with the attack of multiple pathogens, often simultaneously during their long lifespan. Ironically, the genetic and molecular mechanisms controlling this process are poorly understood. The objective of this study was to compare the genetic component of resistance in Norway spruce to Heterobasidion annosum s.s. and its sympatric congener Heterobasidion parviporum. Heterobasidion root- and stem-rot is a major disease of Norway spruce caused by members of the Heterobasidion annosum species complex. Resistance to both pathogens was measured using artificial inoculations in half-sib families of Norway spruce trees originating from central to northern Europe. The genetic component of resistance was analysed using 63,760genome-wide exome-capture sequenced SNPs and multitrait genome-wide associations. No correlation was found for resistance to the two pathogens; however, associations were found between genomic variants and resistance traits with synergic or antagonist pleiotropic effects to both pathogens. Additionally, a latitudinal cline in resistance in the bark to H. annosum s.s. was found; trees from southern latitudes, with a later bud-set and thicker stem diameter, allowed longer lesions, but this was not the case for H. parviporum. In summary, this study detects genomic variants with pleiotropic effects which explain multiple disease resistance from a genic level and could be useful for selection of resistant trees to both pathogens. Furthermore, it highlights the need for additional research to understand the evolution of resistance traits to multiple pathogens in trees.

Carbon and nitrogen acquisition strategies by wood decay fungi influence their isotopic signatures in Picea abies forests

We examined whether sporocarp carbon and nitrogen isotope ratios (δ 13 C and δ 15 N values) reflected different functional strategies in 15 species of wood decay fungi. In Finnish Picea abies forests, we compared sporocarp δ 13 C and δ 15 N against log diameter, proximity to ground, and three wood decay types, specifically brown rot, nonselective white rot, and selective white rot (targeting hemicellulose and lignin preferentially). In regression analysis (adjusted r 2 = 0.576), species accounted for 31% of variability in δ 13 C, with factors influencing wood δ 13 C accounting for the remainder. Brown rot fungi and three white rot fungi that selectively attacked hemicellulose ( Heterobasidion parviporum , Phellopilus nigrolimitatus , and Trichaptum abietinum ) were higher in δ 13 C than nonselective white rot fungi. This was attributed to greater assimilation of 13 C-enriched pentoses from hemicellulose by these fungi. The pathogenic white rot fungus Heterobasidion parviporum had higher δ 15 N with proximity to ground and increasing log diameter. This suggested that 15 N-enriched soil N contributed to decomposing logs and that Heterobasidion growing from a bigger resource base had increased access to soil N. These isotopic patterns accordingly reflected both functional diversity of wood decay fungi and site-specific factors. • We investigated strategies of C and N use in wood decay fungi using stable isotopes. • Selective white rot fungi were higher in δ 13 C than nonselective white rot fungi. • Taxa of uncertain white rot type resembled nonselective white rot fungi in δ 13 C. • Spatial arrangements of hemicellulose and cellulose fibers influenced fungal δ 13 C. • If close to ground on big logs, Heterobasidion parviporum accessed soil N.

Infection and Spread of Root Rot Caused by Heterobasidion parviporum in Picea abies Stands after Thinning: Case Studies on Former Pasture and Meadow Lands

Afforestation of former agricultural lands is a common practice in several countries. This is beneficial for avoiding diseases carry-over from previous forest generations and to expand forest areas. However, several biotic and abiotic risks have been reported in such stands, including a higher risk of Heterobasidion root rot after thinning. Therefore, this study investigates the spread of Heterobasidion root rot in three Picea abies (L.) Karst. plantations in Latvia established on former pasture and meadow lands and subjected to forest management practices. Initially, to determine average infection rate, we sampled all standing trees (157 in total) along transects within sampling areas. On the transects, the proportion of trees suffering from Heterobasidion root rot varied from 16 to 33% among stands. Based on those data and observed dieback over all the stands, we established circular sample plots in disease centres where all trees and stumps were analysed. The average infection rate in the circular plots varied from 34 to 41%. Obtained Heterobasidion isolates were analysed with species-specific primers and were all determined to be Heterobasidion parviporum Niemelä & Korhonen. Isolates were paired to detect the number and size of genotypes. Of 141 genets examined, 99 were isolated from only one tree or stump, while 42 formed genets including two or more trees or stumps indicating spread of infection through root contacts. The total number of Heterobasidion genets per hectare in studied stands varied from 72 to 484. The following conclusions were reached: (i) stands on former non-forest lands (pastures and meadows) can be highly susceptible to Heterobasidion infection after thinning, (ii) the pathogen may form expanding territorial clones in both former pasture and meadows and (iii) stump treatment with biological or chemical control agents is recommended to prevent Heterobasidion infections.

Phenolic compound concentrations in Picea abies wood as an indicator of susceptibility towards root pathogens

AbstractQuantification of total phenolic compounds in the heartwood of Picea abies using the Folin–Ciocalteu methods showed that concentrations of phenolic compounds in the heart‐ and sapwood of Picea abies varied, depending on the decay class to which boles were allocated. In the heartwood, mean phenolic concentration was 3.28 mg/g, compared with 2.44 mg/g in sapwood. Experiments using different concentrations of phenolic compounds showed that Heterobasidion parviporum is more sensitive to the effects of phenolic compounds found in sapwood, and less sensitive to phenolic compounds found in heartwood. The high concentration of phenolic compounds extracted from spruce stimulated the decomposition of wood by Armillaria ostoyae. This demonstrates the great ability of A. ostoyae to metabolize phenolic compounds. This is confirmed by natural conditions, where A. ostoyae attacks mainly sapwood and H. parviporum attacks heartwood of P. abies. In heartwood, the average antioxidant activity was approximately 10% higher than in the sapwood. The extraction process and presence of amino acids and reducing sugars might influence on the results. The results suggest that timber quality monitoring in spruce stands could be partly based on measurements of phenol compound concentrations in sapwood, as the technique enables prediction of the potential impact of decay fungi early in succession.