Early selection for resistance to <Emphasis Type="Italic">Heterobasidion parviporum</Emphasis> in Norway spruce is not likely to adversely affect growth and wood quality traits in late-age performance

Infections with Heterobasidion parviporum devalue the Norway spruce timber as the decayed wood does not meet the necessary quality requirements for sawing. To evaluate the incorporation of disease resistance in the Norway spruce breeding strategy, an inoculation experiment with H. parviporum on 2-year-old progenies of 466 open-pollinated families was conducted under greenhouse (nursery) conditions. Lesion length in the phloem (LL), fungal growth in sapwood (FG) and growth (D) were measured on an average of 10 seedlings for each family. The genetic variation and genetic correlations between both LL, FG and growth in the nursery trial and wood quality traits measured previously from 21-year old trees in two progeny trials, including solid-wood quality traits (wood density, and modulus of elasticity) and fiber properties traits (radial fiber width, tangential fiber width, fiber wall thickness, fiber coarseness, microfibril angle and fiber length). For both LL and FG, large coefficients of phenotypic variation (>?26%) and genetic variation (>?46%) were detected. Heritabilities of LL and FG were 0.33 and 0.42, respectively. We found no significant correlations between wood quality traits and growth in the field progeny trials with neither LL nor FG in the nursery trial. Our data suggest that the genetic gains may reach 41 and 52% from mass selection by LL and FG, respectively. Early selection for resistance to H. parviporum based on assessments of fungal spread in the sapwood in nursery material, FG, will not adversely affect growth and wood quality traits in late-age performance.     

Intraspecific variation in Heterobasidion annosum for growth in sapwood of Picea abies and Pinus sylvestris

Two greenhouse experiments were conducted to study intraspecific variation in growth of the root rot fungus Heterobasidion annosum in living host sapwood. In experiment 1, Scots pine (Pinus sylvestris) and Norway spruce (Picea abies) seedlings were inoculated with H. annosum isolates, 14 each of the S-and P-intersterility groups, collected from various parts of Sweden. In pine, the P-group isolates were more virulent than the S-group isolates both in terms of infection frequency, induced mortality rate (p < 0.05), and fungal growth in sapwood (p < 0.05). In spruce, the P-group isolates were also more virulent on average, but the difference was not statistically significant. Both S and P isolates had a higher infection frequency and a significantly longer sapwood growth on spruce than on pine. The P-group caused higher mortality on pine than on spruce. The length of the lesion in the inner bark was strongly correlated with fungal growth in spruce, but not in pine where the lesions were short or absent. In experiment 2, ten Norway spruce clones were inoculated with 18 S-isolates, originating from nine live-decayed trees and from nine spore-infected stumps in a single Norway spruce stand. The objective was to test whether any selection for growth rate in sapwood was detectable among individuals of H. annosum originating either from stumps or trees. The results gave no support for such selection since no difference in sapwood growth between the two groups of isolates was found.     

Effect of Thinning and Phlebiopsis gigantea Stump Treatment on the Growth of Heterobasidion parviporum Inoculated in Picea abies

The spread of Heterobasidion parviporum Niemelä & Korhonen in roots of Norway spruce was studied in three unthinned first rotation stands of Norway spruce [Picea abies (L.) Karst.] on former agricultural land in south-western Sweden. Heterobasidion parviporum was inoculated at stump height into the trunk of 135 standing trees in a randomized block design. One year after inoculation, two-thirds of the trees were thinned out and one-third was left standing. Half of the stumps left by thinning were treated with spores of Phlebiopsis gigantea (Fr.) Jül and half were left untreated. The spread of H. parviporum was examined both 3 and 5 yrs after inoculation. The rate of spread of H. parviporum and the proportion of infected roots were found to be significantly higher in the root systems of the stumps than in those of the standing trees. Treatment with P. gigantea had no significant effect on the development of H. parviporum in the stumps. There was a tendency 5 yrs after inoculation, however, for a lower proportion of H. parviporum-infected roots in the stumps treated with P. gigantea than in the untreated stumps. In conclusion, thinning of infected Norway spruce was found to increase the rate of spread of H. parviporum in the root systems of the infected trees, which could increase the risk of a rapid build-up of infection in the remaining stand.     

Susceptibility to Heterobasidion parviporum in Picea abies clones grown in different environments

Thirty-five Norway spruce, Picea abies, clones from Sweden were tested for resistance to Heterobasidion parviporum. Rooted cuttings of the clones were planted in Italy and Greece and cultivated for two growing seasons before inoculation with H. parviporum. Extent of infection was determined 6 weeks later. The results were compared to those of earlier inoculations in Sweden. Plant growth traits were under strong genetic control in all locations with broad sense heritability estimates between 0.14 and 0.54. Lesion and fungal extension heritabilities were moderate, H² ranged from 0.09 to 0.20, and exhibited rather large genetic variation. There was significant genotypic correlation between Italy and Greece with respect to both lesions and fungal extension. No such correlations were found between Sweden and the two other countries. The lack of repeatability in testing susceptibility is unsatisfactory. It could be explained by C-effects associated with propagation of the host plants. Height growth correlated significantly and positively among all countries.     

Biological control of Heterobasidion annosum (Fr.) Bref. (Fomes annosus) in Finland

Fresh stumps of Norway spruce (Picea abies) were inoculated with Peniophora gigantea (Fr.) Massee, Botrytis cinerea Pers. ex Fr., Gliocladium deliquescens Sopp, Trichoderma viride Pers. ex Fr. ex S. F. Gray aggr. and Verticicladiella procera Kendrick in order to study their effect on infection by Heterobasidion annosum (Fr.) Bref. The organisms colonizing spruce stumps under natural conditions were identified by collecting samples in the field. In addition, the effect of season of the year, air temperature, precipitation and relative air temperature on fungal colonization of the stumps was investigated.     

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

Two 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.     

Initial reactions in sapwood of Norway spruce and Scots pine after wounding and infection by Heterobasidion parviporum and H. annosum

The xylem surface of seedlings, stem material and roots of Norway spruce (Picea abies) and Scots pine (Pinus sylvestris) were inoculated with strains of Heterobasidion annosum s. str. and H. parviporum s. str. The depth of necrosis in wounded spruce increased at a linear rate for at least seven weeks of incubation, but the rate of necrotic spread was significantly faster in infected wounds. In wounded pine the necrosis was maintained at a more superficial level for several weeks. Both spruce and pine sapwood were initially infected by hyphae of both species. In spruce, the hyphae advanced at a constant rate behind the necrotic front. On the contrary, after 1–2 weeks living H. parviporum hyphae were rare in pine rays. Heterobasidion annosum hyphae survived in pine rays, phloem and tracheids, despite a heavy accumulation of phenolics and resins and were able to penetrate into the sapwood at a linear rate although slower than infections in spruce. Histochemistry and quantitative estimates demonstrated that peroxidase activity was initially higher in spruce sapwood than in pine. Within three days of incubation, the activity in spruce sapwood disappeared concurrently with deepening necrosis. However, in pine, in both control and infected samples, there was a significant increase in peroxidase activity in the area surrounding the superficial necrosis, up to the wound surface and in the cambium and phloem around the wound. After wounding and infection, the content of soluble protein increased significantly in wood of older trees but not in seedlings. Infection resulted in an increased formation of lipophilic extractives in both spruce and pine but to a significantly greater degree in the latter, whereas the amount of hydrophilic compounds decreased in both. High‐performance liquid chromatography (HPLC) analyses of lipophilic extracts showed that inoculation of pine with the two species of Heterobasidion increased the amounts of pinosylvin, its monomethylether and several other phenolics as also resinous compounds. The results obtained may be relevant in explaining the known higher resistance of Scots pine to H. parviporum.     

Swiss stone pine trees and spruce stumps represent an important habitat for Heterobasidion spp. in subalpine forests

In the Western Italian Alps (WIA), the three European species of the forest pathogen Heterobasidion spp. can coexist in the same area. Heterobasidion parviporum Niemelä & Korhonen and Heterobasidion abietinum Niemelä & Korhonen are normally found in areas with a significant presence of their respective primary hosts, spruce (Picea spp.) and fir (Abies spp.). The host/niche occupied by Heterobasidion annosum (Fr.) Bref. in the region still remains unclear. Although Scots pine (Pinus sylvestris), a major host for this fungal species in other parts of Europe, is abundant in the region, little or no evidence of disease caused by H. annosum is visible in this tree species. Two different, but not mutually exclusive, hypotheses can explain the presence of H. annosum: (1) Scots pines are infected but largely asymptomatic and (2) H. annosum has adapted to different hosts. An analysis of Heterobasidion species was performed in two natural, mixed‐conifer forests using traditional isolation techniques and novel direct molecular diagnosis from wood. In a subalpine stand of mixed spruce (Picea abies), larch (Larix spp.), and Swiss stone pine (Pinus cembra), 18 naturally infected spruces and larches only yielded H. parviporum. A Swiss stone pine in the same stand was extensively colonized by both H. parviporum and H. annosum. In a second subalpine stand, an analysis of 18 spruce stumps and nine Swiss stone pine stumps yielded both H. parviporum and H. annosum isolates. Pine stumps had been mostly colonized by H. parviporum prior to tree felling, suggesting that this species may be secondarily infected by the locally predominant Heterobasidion species (i.e. H. parviporum). Results of our analysis also indicated that primary colonization of spruce stumps (e.g. through basidiospores) was caused by both H. parviporum and H. annosum, while secondary infection of such stumps was mostly because of H. parviporum.     

Heterobasidion annosum root rot in Picea abies: Modelling economic outcomes of stump treatment in Scandinavian coniferous forests

Abstract

The economic outcomes of stump treatment against spore infections of the root rot pathogen Heterobasiodion annosum s. l. were analysed based on simulations in four stands typical of Swedish forestry and forest management: (A) Norway spruce [Picea abies (L.) Karst.] stand on former agricultural soil (SI?=?32), (B) Norway spruce stand (SI?=?26) on forest land; (C) Mixed stand of Norway spruce and Scots pine (Pinus sylvestris L.) (SI?=?24) with only H. parviporum present, i.e. no infection of Scots pine and no possibility of interspecies spread of disease between hosts; and (D) same as C, but H. annosum s. str. (Fr.) Bref and H. parviporum Niemelä & Korhonen present, i.e. interspecies spread of disease possible. Models for disease development, growth and yield and cross-cutting were used in the simulations. The simulated decay frequency in Norway spruce trees ranged between 2 and 90%. Stump treatment at the previous final felling and in all thinning operations was profitable at interest rates 1 and 3% in stands A, B and D, but not in stand C. In stand C, no stump treatment at all or treatment in thinnings only gave the highest net future value. Implications for stump treatment in practical forestry are discussed.     

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

We 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.     

Multiplex real-time PCR detection of pathogen colonization in the bark and wood of Picea sitchensis clones differing in resistance to Heterobasidion annosum

A quantitative multiplex real-time polymerase chain reaction (PCR) procedure was developed to assess the extent of Heterobasidion annosum (Fr.) Bref. growth in Sitka spruce (Picea sitchensis (Bong.) Carr.) bark and wood and to determine correlations between lesion length and fungal colonization. Based on lesion length and real-time PCR, the responses of four 3-year-old Sitka spruce clones to inoculation with H. annosum were characterized as showing either resistance or susceptibility to the pathogen. In susceptible clones, the extent of bark colonization did not differ from the visible length of the bark lesion, whereas lesions were longer than the extent of fungal colonization in resistant clones. The resistant clones contained considerably less fungal DNA than the susceptible clones, relative to the amount of host DNA in both the bark and the wood, indicating less resistance and more host cell death in the susceptible clones following inoculation. In both resistant and susceptible clones, fungal colonization in the wood extended beyond the visible necrotic lesion in the bark, indicating that host defense responses are weaker in wood than in bark. The spread of the pathogen in both bark and wood was less in the resistant clones than in the susceptible clones, indicating that defenses in both bark and wood of the resistant clones were superior to those in the susceptible clones.     

Eine Methode zur Prüfung der Anfälligkeit und Ausbreitungsresistenz der Fichte (Picea abies Karst.) gegenüber dem Wurzelschwamm Fomes annosus (Fr.) Cooke1

A method for examining Norway spruce (Picea abies Karst.) for susceptibility and resistance to the spread of Fomes annosus (Fr.) Cooke . An easily applied method is described for determining the results of infections and the reactions of young Norway spruce to Fomes annosus. By the use of genetically homogeneous material, e. g. clonal plants, the importance of environmental factors such as nutrient supply and also the varying virulence of different fungal strains can be studied.     

Growth of Phlebiopsis gigantea in wood of seven conifer species

Heterobasidion parviporum and Heterobasidion annosum are widely distributed root‐rot fungi that infect conifers throughout Europe. Infection of conifer stumps by spores of these pathogens can be controlled by treating fresh stumps with a competing non‐pathogenic fungus, Phlebiopsis gigantea. In this study, growth of three Latvian strains of P. gigantea and the biological control agent ‘Rotstop’ strain was evaluated in stem pieces of Norway spruce, Scots pine, lodgepole pine, Douglas‐fir, Weymouth pine, Siberian larch and Sitka spruce. The growth rates of one H. parviporum and one H. annosum isolate were also measured in the same stem pieces. The growth rate of P. gigantea varied greatly in wood of different conifer species. It was higher in the three pine species, lower in Norway spruce and lowest in Sitka spruce and Siberian larch, and in Douglas‐fir, this fungus did not grow. The largest area of wood occupied by P. gigantea was in lodgepole pine. Growth of Latvian isolates of P. gigantea in the wood of Pinus and Picea species was comparable to that of the Rotstop isolate. Consequently, stump treatment with local P. gigantea isolates should be recommended. However, our results suggest that Douglas‐fir stump treatment against Heterobasidion by P. gigantea may be ineffective and other stump treatment methods should be considered.     

Turkish Heterobasidion abietinum is pathogenic to inoculated Abies nordmanniana ssp. nordmanniana and ssp. bornmülleriana

Four‐year‐old seedlings of Abies nordmanniana ssp. bornmülleriana and 5‐year‐old seedlings of Abies nordmanniana ssp. nordmanniana were inoculated on the lower stem with 28 Heterobasidion abietinum isolates originating from four different regions of Turkey. Replicate seedlings were incubated in greenhouse and growth chamber. After 7 weeks, infection incidence, mortality, lesion length in the inner bark and fungal growth in the sapwood were examined. Infection incidence in different host–incubation combinations ranged between 70.5 and 79.5% and the average mortality rate was 4.2%. Average lesion lengths varied between 8.4 and 33.9 mm, and average fungal growth between 18.7 and 34.8 mm. There was a significant positive correlation between lesion length in the inner bark and fungal growth in sapwood on both hosts and conditions. Clear differences in virulence between H. abietinum isolates originating from different regions were not found. The results indicate that H. abietinum is pathogenic on both investigated subspecies of A. nordmanniana. Both lesion length and fungal growth were greater on ssp. bornmülleriana, indicating that it would be more susceptible than ssp. nordmanniana.     

Spatial distribution and persistence of Heterobasidion parviporum genets on a Norway spruce site

Spatial distribution of Heterobasidion genets over a period of ca 50 years in two successive generations of Norway spruce (Picea abies) was unravelled. The genets were first identified in 1993 in a naturally regenerated 43‐year‐old spruce stand that had been thinned the previous winter. Heterobasidion parviporum was found in 17.5% of the old stumps of the previous spruce generation. Nine genets were identified on the study plot; seven of them were present in old stumps of the previous tree generation and two only in the new spruce generation. Eighteen spruce trees of the new generation were infected, 15 of them by vegetative growth of genets originating from the old stumps. The study plot was investigated again in 2005. No new genets had been established after thinning, and three old genets had died out. The remaining genets had infected five new trees, most likely from the thinning stumps of diseased trees. At the age of 56 years, 16.1% of the residual spruces were infected by Heterobasidion. The results of this study suggest that if spore infection to stumps of spruce can be prevented, the decay frequency caused by H. parviporum will not necessarily increase in successive generations.     

Mycelial growth and exclusion of Heterobasidion parviporum inoculated in branches of 15‐year‐old Picea abies clones

Twigs of 22 clones of 15‐year‐old Picea abies grafts were inoculated with Heterobasidion parviporum in July and in September 2003. Fungal growth varied between clones and exclusion of the pathogen varied significantly among clones inoculated in July. The results indicate that twigs exhibit the same infection pattern as cuttings when inoculated with H. parviporum and that exclusion of the pathogen in twigs is a clone‐dependent character that occurs during the vegetation period.     

Incidence of Root and Butt Rot in Consecutive Rotations of Picea abies

The incidence of butt rot in two consecutive rotations of Norway spruce [Picea abies (L.) Karst.] in 28 permanent sample plots at four different sites in Denmark was evaluated. Incidence of butt rot was estimated by visual examination of stumps at final felling of the previous rotation and by examination of bore cores taken at the butt from a random sample of trees before first thinning of the subsequent rotation. There was no correlation between the incidence of butt rot at final felling of the previous rotation of Norway spruce and the incidence of butt rot at first thinning of the subsequent rotation of Norway spruce. The incidence of butt rot at final felling was between 19 and 100%, and at first thinning between 0 and 20%. The S-form of Heterobasidion annosum (Fr.) Bref. was the most commonly found decay-causing organism at all sites. Root systems of 28 trees without decay at stump height in the present rotation were excavated to estimate the incidence of root rot. Heterobasidion annosum was found in only one root. Resinicium bicolor (Alb. & Schw. ex Fr.) Parm. was found in 25% of the excavated root systems. The result of the study shows that the incidence of butt rot at first thinning of Norway spruce is not necessarily higher on sites where the previous rotation was heavily infected than on sites where infection in the previous rotation was low.     

Infection of roots of Norway spruce (Picea abies) by Heterobasidion annosum

Wounding of excised roots of Norway spruce [Picea abies (L.) Karst.] caused a rapid accumulation of phenolic compounds, decrease of starch content and of certain enzyme activities in the ray cells. Infection of the wounds by Heterobasidion annosum (Fr.) Bref. intensified these processes, caused oxidation of the phenols, and induced activity of acid phosphatase and β-glucosidase in the ray cells of the infected area. The reactions were oxygen-dependent. Intra- and extracellular enzymes were detected in the wood-infecting hyphae. Other wood-growing fungi caused similar reactions in the wood as H. annosum.     

Viruses of Heterobasidion parviporum persist within their fungal host during passage through the alimentary tract of Hylobius abietis

Heterobasidion annosum (Fr.) Bref. sensu lato is an important fungal parasite of coniferous trees throughout the temperate regions of the world. Approximately 15% of Heterobasidion isolates are infected by dsRNA viruses, which are considered as obligately intracellular and transmit vertically into both basidiospores and asexual conidia. Insects such as H. abietis and its larvae feeding on wood colonized by Heterobasidion fungi may carry Heterobasidion conidia and hyphae. In this study, we used H. abietis as a model species to reveal whether Heterobasidion viruses resist in their fungal host passing through the digestive tract of insects that may potentially serve as disseminators of fungal propagules. Pinus sylvestris branches were inoculated by three different strains of Heterobasidion parviporum, hosting taxonomically diverse virus species: Heterobasidion RNA virus 2, HetRV4 and HetRV6. Then, the inoculated branches were fed to H. abietis insects, and beetle excrements were investigated for Heterobasidion infections. 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 five H. parviporum isolates retrieved from insect faeces. The relative stability of fungi harbouring viruses suggests that if viruses are to be used for biological control against Heterobasidion species, 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.     

Outdoor exposure of untreated Scots pine (Pinus sylvestris L.) and Norway spruce [Picea abies (L.) Karst.] wood samples

Abstract

Untreated Scots pine (Pinus sylvestris) and Norway spruce (Picea abies) samples were exposed above ground in a durability test for 6 years. The samples consisted of three pieces of wood, 22×95×500 mm, screwed together; two pieces lengthwise with a third piece overlapping. Weight was measured, to calculate moisture content (MC), and samples checked regularly for cracks and fungal growth. Parameters investigated were heartwood/sapwood (pine), annual ring orientation (spruce), stand site, annual ring width and density. Stand site, annual ring width and density had no influence on MC or fungal growth for either pine or spruce. Spruce samples with vertical annual rings had fewer cracks than samples with horizontal annual rings. Pine sapwood samples had a high MC and a large amount of rot fungi, while heartwood had a lower MC and no rot. Most spruce samples were similar to pine heartwood, except from a few samples that had high MC and fungal growth. Those were all sawn from the outer part of the log. Therefore, it can be stated that spruce sawn from the inner part has almost the same properties as pine heartwood, while spruce from the outer part of the log has similar properties to pine sapwood.