Heterobasidion Parviporum Research Articles

Antifungal properties of terpenoids in Picea abies against Heterobasidion parviporum

SummaryThe antifungal properties of 14 major oleoresin‐constitutive terpenoids of Norway spruce (Picea abies) against Heterobasidion parviporum were evaluated in vitro at three gradient concentrations, 0.1, 0.2 and 0.4 μmol/cm2, on potato dextrose agar medium. Eight monoterpene hydrocarbons (+)‐ and (−)‐α‐pinene, (−)‐β‐pinene, (+)‐3‐carene, myrcene, (+)‐ and (−)‐limonene, terpinolene; four oxygenated monoterpenes α‐terpineol, terpinen‐4‐ol, 1,8‐cineole, bornyl acetate; and two resin acids abietic acid and dehydroabietic acid were selected. Abietic and dehydroabietic acids showed the highest antifungal activities at all concentrations. Among oxygenated monoterpenes, bornyl acetate and α‐terpineol showed antifungal activity at the highest concentration. Among monoterpene hydrocarbons, (+)‐α‐pinene showed similar activity to terpinen‐4‐ol and 1,8‐cineole at the highest concentration; however, it was lower than α‐terpineol and bornyl acetate. Other monoterpene hydrocarbons inhibited mycelial growth by <10%. Re‐extraction of medium surfaces after the test period revealed that most α‐terpineol and terpinen‐4‐ol remained unchanged on the surface but monoterpene hydrocarbons completely disappeared. However, notable fungal transformed products were observed on surfaces applied with 1,8‐cineole and bornyl acetate. Thus, mycelial growth inhibition of monoterpenoids might be caused by complex functions such as biodegradation and/or detoxification by interaction between mycelium and compound. These results provide a basis for future studies considering the role of chemodiversity in the comprehensive chemical defence of P. abies.

Species distribution, host affinity and genetic variability of Heterobasidion annosum sensu lato in the Czech Republic

SummaryThe distribution and host affinity of Heterobasidon annosum (Fr.) Bref. sensu lato (Basidiomycota, Bondarzewiaceae) in the Czech Republic was investigated. Sampling was undertaken in natural forests, in stands with former presence of fir and spruce and in forests on formerly agricultural lands. The identification and phylogenetic relationship of the species complex was studied by comparing DNA sequences of three gene regions: glyceraldehyde 3‐phosphate dehydrogenase (G3P), translation elongation factor 1‐alfa (EFA) and transcription factor (TF). The species relationships were demonstrated by haplotype network and Bayesian phylogram construction for the combined data set. The distribution of each species was demonstrated. Heterobasidion annosum s.s., which had the greatest host range, was found on thirteen genera (Pinus, Picea, Fraxinus, Betula, Corylus, Alnus, Abies, Acer, Salix, Ligustrum, Quercus, Larix and Prunus). Heterobasidion parviporum was found on spruce (Picea abies), fir (Abies alba), apple tree (Malus sylvestris), Vitis vinifera and Fagus sylvatica, and Heterobasidion abietinum was observed on fir (Abies), spruce (Picea), pine (Pinus) and maize (Zea mays). Sequences from different genes yielded conflicting results for six specimens, which were interpreted as belonging to interspecific hybrids. These were collected from A. alba, P. abies and Chamaecyparis lawsoniana. The occurrence of Heterobasidion annosum s.l. is strongly associated with the natural distribution of its hosts and the occurrence of H. abietinum appears to be related to the historical occurrence of A. alba in stands now reforested with P. abies. No statistically significant differences in the vertical distributions within the species complex were detected.

Predicting the activity of Heterobasidion parviporum on Norway spruce in warming climate from its respiration rate at different temperatures

SummaryWe studied the effect of climate warming on Heterobasidion root rot in boreal forests by measuring respiration activity of pure cultures of Heterobasidion parviporum in Norway spruce (Picea abies) sawdust and by linking these data to temperature data obtained from three spruce forests located along a north‐south transect stretching from northern Germany to northern Finland. The pure cultures applied in this investigation were homokaryotic, but in a separate investigation, we found no significant difference between the activity of homo‐ and heterokaryotic isolates. We also found that the temperature response curves of growth and respiration rates of this fungus were similar and propose that respiration reflects the general activity of H. parviporum. The respiration data were scaled up to annual cumulative respiration activity using daily temperature measurements from soil and air in the spruce forest sites. The annual respiration activity of H. parviporum showed a linear relationship with the average annual air temperature. An increase in the annual air temperature by 5°C would raise the annual activity of H. parviporum in spruce roots in northern Finland, southern Finland and northern Germany by 91%, 53% and 40%, respectively. This increase remains below the predicted increase in forest growth in northern Finland but exceeds considerably the predictions for southern Finland. According to the previous literature, a number of other decay fungi show a similar activity response to temperature as H. parviporum, suggesting that this result can be generalized to decay fungi with similar ecological habits.

Endophytic fungi of Norway spruce roots in boreal pristine mire, drained peatland and mineral soil and their inhibitory effect on Heterobasidion parviporum in vitro

Most of the spruce-dominated pristine mires in South Finland have been drained for forestry, and a large proportion of these forest stands are currently approaching regeneration maturity. We investigated the potential impact of the drainage of pristine mire on the species composition of root endophytes compared to undisturbed mire and mineral soil. Additionally, we further evaluated the inhibitory effect of a subset of the isolated endophytes on the root rot pathogen Heterobasidion parviporum. A total of 113 isolates of fungal root endophytes were obtained from non-mycorrhizal Picea abies roots, which were assigned to 15 different OTUs. Most of the isolates consisted of dark septate endophytes (77 %); the Phialocephala fortinii s.l.–Acephala applanata species complex was the most dominant group, comprising 52 % of all the isolates. No significant differences in the frequency of isolates were observed between the sites. All the diversity indices suggest relatively low or mid-level versatility of the endophyte community, with mineral soil sites having the lowest index and drained peatland the highest. Nineteen of the isolates (17 %) inhibited the growth of the conifer root rot pathogen in vitro. The potential of these endophytes to serve as alternative antagonistic agents to control and manage Heterobasidion root rot disease is discussed.

Tree‐ring isotope analysis of Norway spruce suffering from long‐term infection by the pathogenic white‐rot fungus Heterobasidion parviporum

SummaryWe report for the first time a tree‐ring isotopic analysis on host trees infected with Heterobasidion parviporum. By measuring carbon and oxygen stable isotope ratios in tree rings over ca. 150 years of forest growth, we obtained evidence that stomatal conductance increases in Picea abies affected by H. parviporum. We put forward this approach as a novel way of providing an insight into plant–pathogen relationships during tree life.

Incidence and spread of Heterobasidion root rot in uneven-aged Norway spruce stands

Notwithstanding Norway spruce (Picea abies (L.) Karst.) is highly prone to root rot caused by Heterobasidion parviporum Niemelä & Korhonen, but little is known about the epidemiology of Heterobasidion root rot in spruce stands applied to uneven-aged management. To get insight into the development of Heterobasidion infections in this type of forest, the size and spatial distribution of individual genets of H. parviporum were determined in five uneven-aged managed Norway spruce stands in southern Finland. In these stands, all tree size classes (regeneration, intermediate, and overstory trees) were infected by H. parviporum. The average number of trees and stumps infected by a single genet ranged from 3 to 6.3 (mean 4.4) among study plots. All Heterobasidion genets identified from overstory trees or stumps had spread to the younger tree generation. Secondary infection from overstory trees was the main way of infection (at least 85% of all infections) among the regeneration and intermediate trees. The results indicate that uneven-aged management strategies that maintain continuous spruce regeneration favour the secondary spread of H. parviporum between different tree size classes and may compromise the production of high-quality timber over successive generations.

Wood decay caused by Heterobasidion parviporum in juvenile wood specimens from normal- and narrow-crowned Norway spruce

Heterobasidion parviporum (Fr.) Niemelä & Korhonen is a serious forest pathogen affecting the quality of Norway spruce (Picea abies (L.) Karst), especially in northern hemisphere. Here, we studied the wood decay caused by this pathogen in small wood specimens of 19-year-old normal (P. abies) and narrow crowned (P. abies f. pendula) Norway spruce trees. After six months of incubation, wood specimens taken next to pith (inner) and bark (outer) at 1 m height from the stem base were measured to obtain the percent dry weight loss. The fungal development was also analyzed using Scanning Electron Microscopy (SEM). We found that the crown type (or genetic entry) had no effects on the weight loss. However, outer wood specimens lost, on average, more weight than inner ones. Similarly, wood density and content of lignans were lower at outer sections, opposite to the free glucose content. Wood density correlated negatively with the weight loss of wood specimens, regardless of crown type. Considering that narrow- and normal-crowned trees responded similarly to H. parviporum wood decay, our findings support the potential use of narrow-crowned Norway spruce in practical forestry since they can produce higher stem yield at very dense plantation and without thinning requirements.

Fungal root pathogen (Heterobasidion parviporum) increases drought stress in Norway spruce stand at low elevation in the Alps

Tree-ring patterns of Picea abies (L.) Karst. both unaffected and affected by Heterobasidion parviporum were analysed in three mature stands located at different elevations in the Eastern Alps. The main objectives were (1) to clarify the role of climatic conditions on infected trees; (2) to estimate indirect volume losses due to the prolonged presence of the fungus within the wood. The low elevation site showed the highest growth decline in the last decade, whereas all infected trees at medium and high elevation showed a slow growth decline over many decades. We hypothesise that infection could be dated over 80 years at the highest site. Fungal attack made P. abies more susceptible to drought stress at low elevation site. Both infected and healthy P. abies at medium and high elevation showed similar climate–growth relationships, suggesting that the same driving environmental factors influence their growth. At low elevation, H. parviporum was seemingly more aggressive, causing a more rapid decline, decreasing the ability of host trees to cope with drought and, in some cases, inducing cambial activity to stop. P. abies at higher elevation, however, exhibited a very slow decline and no sign of increasing water stress since the influence of climate on tree growth was the same for both infected and healthy trees.

Viruses ofHeterobasidion parviporumpersist within their fungal host during passage through the alimentary tract ofHylobius abietis

SummaryHeterobasidion annosum(Fr.) Bref.sensu latois an important fungal parasite of coniferous trees throughout the temperate regions of the world. Approximately 15% ofHeterobasidionisolates are infected by dsRNAviruses, which are considered as obligately intracellular and transmit vertically into both basidiospores and asexual conidia. Insects such asH. abietisand its larvae feeding on wood colonized byHeterobasidionfungi may carryHeterobasidionconidia and hyphae. In this study, we usedH. abietisas a model species to reveal whetherHeterobasidionviruses resist in their fungal host passing through the digestive tract of insects that may potentially serve as disseminators of fungal propagules.Pinus sylvestrisbranches were inoculated by three different strains ofHeterobasidion parviporum, hosting taxonomically diverse virus species:HeterobasidionRNAvirus 2, HetRV4 and HetRV6. Then, the inoculated branches were fed toH. abietisinsects, and beetle excrements were investigated forHeterobasidioninfections. All the inoculated fungal strains survived passage through the alimentary tract of the insects (survival rate 25–80%). The passage rates of the viruses inside their hosts varied considerably, ranging from 0 to 67%. Two different mycoviruses, HetRV2 and HetRV6, survived the intestinal passage of their fungal host, while the virus species HetRV4 was detected among none of the fiveH. parviporumisolates retrieved from insect faeces. The relative stability of fungi harbouring viruses suggests that if viruses are to be used for biological control againstHeterobasidionspecies, it is likely that insect feeding does not considerably decrease the virus effect, but might instead enhance short‐range dispersal of the viral biocontrol agent.

Heartwood stump colonisation by Heterobasidion parviporum and H. annosum s.s. in Norway spruce (Picea abies) stands

Stump-to-tree spread is a key step in the root and butt-rot disease caused by Heterobasidion annosum sensu lato. In Norway spruce stands, Heterobasidion parviporum is more likely to spread to neighbouring trees from stumps with a large diameter than from small stumps. However, H. annosum sensu stricto does not appear to share this pattern of infection. Given that the ratio of heartwood to sapwood varies depending on the size of the stump, we investigated the role of heartwood and sapwood in stump infection by H. parviporum and H. annosum s.s. To determine whether heartwood–sapwood susceptibility was different in different host species, both Norway spruce and Scots pine stumps were included in the study. Sapwood, heartwood and the border zone between sapwood and heartwood of spruce stumps and the heartwood and sapwood of pine stumps were artificially inoculated with different genotypes of H. annosum s.s. and H. parviporum. Only one zone was inoculated per stump. Infection was assessed 2 and 10months after inoculation at 5 and 30cm below the point of inoculation. On spruce stumps, H. parviporum and H. annosum s.s. were mostly confined to the heartwood. The area infected by H. annosum s.s. was smaller than that infected by H. parviporum. Both sapwood and border infections tended to grow towards the heartwood, whereas heartwood inoculations tended to remain in the heartwood, indicating that heartwood was more favourable for H. parviporum and H. annosum s.s. colonisation in spruce stumps. On pine, heartwood colonisation failed and only H. annosum s.s. was found in the sapwood. Our results suggest a possible association between the size of the stump and the spread of H. annosum s.s. to neighbouring trees mediated by the heartwood content, suggesting that screening for heartwood resistance could be a potentially interesting direction for future research. Treating stumps at final felling would be advisable owing to the large area of heartwood exposed by this operation.

Analysis of microtubule and microfilament distribution in Pinus sylvestris roots following infection by Heterobasidion species

SummaryCytoskeletal dynamics play a crucial role in pathogen recognition and cell defence during the initial interactions between an invader and plant host. The aim of the work reported here was to characterize how Heterobasidion annosum s.s., Heterobasidion parviporum, and Heterobasidion abietinum affect the microtubules and microfilaments of Pinus sylvestris root cells 12‐, 24‐, 48‐, and 96‐h post‐inoculation. Inoculation of P. sylvestris with H. parviporum or H. abietinum, which have a lower specificity for P. sylvestris than H. annosum s.s, resulted in greater reorganization of host microtubules during the early stages of interaction than inoculation with the more specific H. annosum s.s. In some infected cells, spots of actin aggregates were observed. Disruption of cytoskeletal components by the application of specific cytoskeletal inhibitors facilitated the entry of the H. parviporum and H. abietinum into roots. These results suggest that the P. sylvestris cytoskeleton plays a role in the host response in the initial stages of the host–pathogen interaction.

The primary module in Norway spruce defence signalling against H. annosum s.l. seems to be jasmonate-mediated signalling without antagonism of salicylate-mediated signalling

A key tree species for the forest industry in Europe is Norway spruce [Picea abies (L.) Karst.]. One of its major diseases is stem and butt rot caused by Heterobasidion parviporum (Fr.) Niemelä & Korhonen, which causes extensive revenue losses every year. In this study, we investigated the parallel induction of Norway spruce genes presumably associated with salicylic acid- and jasmonic acid/ethylene-mediated signalling pathways previously observed in response to H. parviporum. Relative gene expression levels in bark samples of genes involved in the salicylic acid- and jasmonic acid/ethylene-mediated signalling pathways after wounding and inoculation with either the saprotrophic biocontrol fungus Phlebiopsis gigantea or with H. parviporum were analysed with quantitative PCR at the site of the wound and at two distal locations from the wound/inoculation site to evaluate their roles in the induced defence response to H. parviporum in Norway spruce. Treatment of Norway spruce seedlings with methylsalicylate, methyljasmonate and inhibitors of the jasmonic acid/ethylene signalling pathway, as well as the Phenylalanine ammonia lyase inhibitor 2-aminoindan-2-phosphonic acid were conducted to determine the responsiveness of genes characteristic of the different pathways to different hormonal stimuli. The data suggest that jasmonic acid-mediated signalling plays a central role in the induction of the genes analysed in this study irrespective of their responsiveness to salicylic acid. This may suggest that jasmonic acid-mediated signalling is the prioritized module in the Norway spruce defence signalling network against H. parviporum and that there seems to be no immediate antagonism between the modules in this interaction.

The Pathogenic White-Rot Fungus Heterobasidion parviporum Responds to Spruce Xylem Defense by Enhanced Production of Oxalic Acid

Pathogen challenge of tree sapwood induces the formation of reaction zones with antimicrobial properties such as elevated pH and cation content. Many fungi lower substrate pH by secreting oxalic acid, its conjugate base oxalate being a reductant as well as a chelating agent for cations. To examine the role of oxalic acid in pathogenicity of white-rot fungi, we conducted spatial quantification of oxalate, transcript levels of related fungal genes, and element concentrations in heartwood of Norway spruce challenged naturally by Heterobasidion parviporum. In the pathogen-compromised reaction zone, upregulation of an oxaloacetase gene generating oxalic acid coincided with oxalate and cation accumulation and presence of calcium oxalate crystals. The colonized inner heartwood showed trace amounts of oxalate. Moreover, fungal exposure to the reaction zone under laboratory conditions induced oxaloacetase and oxalate accumulation, whereas heartwood induced a decarboxylase gene involved in degradation of oxalate. The excess level of cations in defense xylem inactivates pathogen-secreted oxalate through precipitation and, presumably, only after cation neutralization can oxalic acid participate in lignocellulose degradation. This necessitates enhanced production of oxalic acid by H. parviporum. This study is the first to determine the true influence of white-rot fungi on oxalate crystal formation in tree xylem.

Secondary spread of Heterobasidion parviporum from small Norway spruce stumps to adjacent trees

Root and butt rot from Heterobasidion parviporum on Norway spruce causes severe economic losses in northern Europe and stumps from commercial thinnings are regularly treated to prevent infection. Stumps created at precommercial thinnings are, due to their small size, believed to be unable to transfer the disease. The aims of this study were to investigate the ability of small-sized Norway spruce stumps to transfer infection to neighboring trees and to determine the lower size and age limit for disease transmission. Norway spruce trees, with diameters between 2 and 14cm at stump height, were felled at 14 sites in western Sweden. The resulting stumps were inoculated with H. parviporum and disease transfer recorded at stump height in the adjacent Norway spruce trees 5years later. Stumps covering the whole span of investigated sizes and ages could transfer infection of H. parviporum and consequently a lower limit under which transfer never occurred could not be determined. The probability of finding an inoculated infection in a tree increased with increasing diameter of both tree and stump. For a given stump size, the probability of infection in the nearby tree decreased with increasing age of the stump. It seems prudent to consider the risk of introducing H. parviporum during precommercial thinning in Norway spruce dominated stands.

Virus Community Dynamics in the Conifer Pathogenic Fungus Heterobasidion parviporum Following an Artificial Introduction of a Partitivirus

Viruses infecting the conifer pathogenic fungus Heterobasidion annosum sensu lato are intracellular and spread via anastomosis contacts. In the laboratory, these viruses transmit readily even between somatically incompatible isolates, but their dispersal capacity in natural conditions has not been previously studied. We introduced a mycovirus to a heavily diseased forest site by inoculating Norway spruce stumps with heartrot decay using a mycelial suspension of Heterobasidion parviporum strain RT3.49C hosting the partitivirus strain HetRV4-pa1. The Heterobasidion population at the sample plot was screened for mycoviruses prior to and after the inoculation. Based on sequence analysis, the resident H. parviporum strains harbored six different strains of the virus species Heterobasidion RNA virus 6 (HetRV6) and one strain of HetRV4 prior to the inoculation. After three growth seasons, the inoculated H. parviporum host strain was not detected, but the introduced virus had infected two resident H. parviporum genets. The presence of a preexisting HetRV6 infection did not hinder spread of the introduced partitivirus but resulted in coinfections instead. The resident HetRV6 virus population seemed to be highly stable during the incubation period, while the single indigenous HetRV4 infection was not detected after the inoculation. In laboratory infection experiments, the introduced virus could be transmitted successfully into all of the resident H. parviporum genets. This study shows for the first time transmission of a Heterobasidion virus between somatically incompatible hosts in natural conditions.

Indications of heightened constitutive or primed host response affecting the lignin pathway transcripts and phenolics in mature Norway spruce clones

Two mature clones of Norway spruce (Picea abies (L.) Karst.) that have previously been shown to have differential degrees of resistance towards the necrotrophic pathogen Heterobasidion parviporum (Niemelä & Korhonen) were compared with respect to the primed defense expression of transcripts related to biosynthesis of lignin, stilbenes and other phenolic compounds from one year to the next. The host's response to physical wounding and pathogen inoculation was examined in the initial year, whereas indications of heightened basal defense level or primed response, and responses to re-wounding, were examined the following year. The responses of the two clones to wounding and pathogen inoculation, examined in the initial year, differed; the increases in lignin and phenolics were more distinct in response to the pathogen than to wounding alone. The more resistant clone 589 had higher initial lignin concentrations in the cell walls when compared with clone 409, and these remained higher in clone 589 over both years and increased after the treatments. Both clones responded at the transcriptional and chemical levels to wounding; changes were evident both in the initial wounds and when re-wounded the following year. There were distinct differences in the basal transcript levels of the lignin pathway-related genes, phenolics and total lignin levels in healthy tissue from the initial year to the following year indicative of a primed host response or at least altered constitutive level of defense expression.

Xylem defense wood of Norway spruce compromised by the pathogenic white-rot fungus Heterobasidion parviporum shows a prolonged period of selective decay

Heterobasidion parviporum, a common pathogenic white-rot fungus in managed Norway spruce forests in northern and central Europe, causes extensive decay columns within stem heartwood of the host tree. Infected trees combat the lateral spread of decay by bordering the heartwood with a fungistatic reaction zone characterized by elevated pH and phenol content. To examine the mode of fungal feeding in the reaction zone of mature Norway spruce trees naturally infected by H. parviporum, we conducted spatial profiling of pectin and hemicellulose composition, and established transcript levels of candidate fungal genes encoding enzymes involved in degradation of the different cell wall components of wood. Colonized inner heartwood showed pectin and hemicellulose concentrations similar to those of healthy heartwood, whereas the carbohydrate profiles of compromised reaction zone, irrespective of the age of fungal activity in the tissue, indicated selective fungal utilization of galacturonic acid, arabinose, xylose and mannose. These data show that the rate of wood decay in the reaction zone is slow. While the up-regulation of genes encoding pectinases and hemicellulases preceded that of the endoglucanase gene during an early phase of fungal interaction with xylem defense, the manganese peroxidase gene showed similar transcript levels during different phases of wood colonization. It seems plausible that the reaction zone components of Norway spruce interfere with both lignin degradation and the associated co-hydrolysis of hemicelluloses and pectin, resulting in a prolonged phase of selective decay.

Comparison of colonization capacity by asexual spores of Heterobasidion species in Norway spruce wood

SummaryTwo species of the tree pathogenic fungus Heterobasidion spp. exist in Sweden, Heterobasidion annosum s.s. and Heterobasidion parviporum. Both species are known to infect Norway spruce (Picea abies). The aim of the study was to examine the interspecific competition between H. annosum s.s. and H. parviporum as well as their colonization rate in fresh Norway spruce wood. Equal amount of conidiospores from each species was sprayed together on 30 fresh, previously uninfected, Norway spruce billets. After incubation in a greenhouse, the proportion of Heterobasidion spp. colonies belonging to each species was recorded. Of the 196 colonies isolated from the upper part of billets, 195 were H. parviporum. All isolated colonies further down in the billets were H. parviporum. To study the colonization rate, H. annosum s.s. and H. parviporum were sprayed alone on 30 spruce billets each, incubated and growth recorded both vertically and horizontally. H. parviporum grew further down in the billets (p = 0.008) and covered a larger area (p < 0.001) than H. annosum s.s. While H. annosum s.s. and H. parviporum both infected fresh Norway spruce wood H. parviporum outgrew and outcompeted H. annosum s.s during the early colonization stage.

Susceptibility of pretreated wood sections of Norway spruce (Picea abies) clones to enzymatic hydrolysis

The structure of softwoods, which confers resistance to degradation through hydrolysis and decay, currently limits their use for the production of biofuels. However, since wood is very heterogeneous, it is possible that differences in wood properties within and between trees could differentially affect its processability. In this research, heartwood (inner) and sapwood (outer) from Norway spruce ( Picea abies (L.) Karst.) clones were enzymatically hydrolyzed by Trichoderma viride cellulases after concentrated acid pretreatment. Wood sections with two particle sizes were compared based on their susceptibility to enzymatic hydrolysis, evaluated by assaying the formation of hydrolysis products and measured as reducing sugar yield (RSY). We also studied the relationship between RSY and the susceptibility to Heterobasidion parviporum wood decay and whether these traits are reflected in wood density and yield. Wood from the outer section produced more RSY with higher glucan but lower lignin content than wood from the inner section. Furthermore, susceptibility to enzymatic hydrolysis was positively correlated with H. parviporum wood decay, while both processes were negatively correlated with wood density. Our results revealed the importance of clonal trials for identifying suitable lignocellulosic biomass when considering wood properties and indicate that potential genotypes for the production of biofuels are not necessarily the most productive.

Population structure of a novel putative mycovirus infecting the conifer root-rot fungus Heterobasidion annosum sensu lato

We describe a novel putative mycovirus infecting the conifer root-rot fungus Heterobasidion annosum sensu lato. This virus, designated as Heterobasidion RNA virus 6 (HetRV6), is taxonomically distant from all previously known viruses of Heterobasidion species, but somewhat related to the Curvularia thermal tolerance virus and the Fusarium graminearum virus 4. Based on a population analysis including 35 virus strains from Heterobasidion abietinum, Heterobasidion parviporum, Heterobasidion annosum sensu stricto and Heterobasidion occidentale, HetRV6 showed a considerable degree of geographical and host-related differentiation. The North American and Eurasian virus populations were clearly separated. In Eurasia, we observed cases of discrepancy between virus and host taxonomy, suggesting interspecies virus transfer. HetRV6 was also successfully transmitted between the three European species H. abietinum, H. annosum and H. parviporum. Based on growth rate tests on agar plates and spruce stem pieces, HetRV6 seemed to be cryptic or slightly mutualistic to its host.