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.     

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 planting Scots pine around Norway spruce stumps on the spread of Heterobasidion coll.

Management of a Norway spruce stand planted on a site infected by Heterobasidion coll. is problematic because the fungus spreads vegetatively from the colonized stumps of the previous generation to the new seedlings. Growing of mixed stand with more resistant tree species has been suggested to decrease the economic losses caused by butt rot in Norway spruce trees. The mechanistic simulation model Rotstand describing the spread of Heterobasidion coll. in coniferous stands of southern Finland was used to study the effect of planting Scots pines around colonized clear‐felling stumps of Norway spruce of the previous generation. Planting of Scots pines in clusters around colonized stumps markedly decreased the butt rot of Norway spruce trees at the age of 20 years and at clear felling. If the same number of Scots pines were planted randomly, the effect was weak. When the average diameter of colonized clear‐felling stumps was 30 cm, a Scots pine circle with a radius of 3 m resulted in the highest soil expectation value (SEV) at 2% discounting rate, whereas with 40‐cm stump diameter, a 4‐m radius produced the highest SEV. When the proportion of Heterobasidion parviporum in the old colonized stumps was 50% instead of 95%, planting pines around colonized stumps still clearly decreased the butt rot at the age of 20 years and in final felling.     

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.     

Variation in growth of Heterobasidion parviporum in a full-sib family of Picea abies

Abstract

Heterobasidion parviporum (Fr.) Niemelä & Korhonen and Heterobasidion annosum (Fr.) Bref. sensu lato are some of the major forest pathogens in the northern hemisphere causing root and butt rot to conifers. The relative susceptibility to H. parviporum was investigated in a full-sib family of Norway spruce [Picea abies (L.) Karst.] by inoculating a set of 252 cloned progenies from a controlled cross. Four ramets of each progeny were used and the 2-year-old rooted cuttings were incubated for 6 weeks under greenhouse conditions. The condition of the cuttings was assessed visually and all the plants were in excellent vigour with no mortality recorded during the experiment. To score the relative susceptibility, lesion length in the inner bark and fungal growth in the sapwood were measured. Among the progenies, significant differences were found for fungal growth in the sapwood (p<0.0005). There was no significant difference for lesion length; however, there was a significant positive correlation between fungal growth and lesion length. The broad-sense heritability was 0.11 for fungal growth. This shows that the genetic component for susceptibility to H. parviporum can be detected even within a full-sib family of Norway spruce and that there is a potential for mapping quantitative trait loci for this trait in Norway spruce.     

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.     

Comparative susceptibility of pine,spruce and larch to sapstain

Summary This study compared the susceptibility of five UK‐grown conifer species to colonization by sapstain fungi in two trials carried out in consecutive years. The conifers consisted of Sitka spruce (Picea sitchensis), Japanese larch (Larix kaempferi), Norway spruce (Picea abies), Scots pine (Pinus sylvestris) and lodgepole pine (Pinus contorta). Freshly cut 1‐m logs were exposed to the available inoculum of sapstain fungi from April to August in a woodland environment in the south east of England. Logs of each species were removed after 1‐, 2‐ and 4‐month exposure and sampled destructively to assess the amount of sapstain. In the second trial, per cent moisture content and concentrations of nitrogen, carbohydrate and phenolic compounds in the sapwood were also measured at the start and end of the trial. After 2 months, only the sapwood of both pine species had significant levels of sapstain; mean values of 37% and 19% for lodgepole pine (year 1 and year 2 respectively) and 12% and 1% for Scots pine. After 4 months, the levels of sapstain in both pine species exceeded 60% in both years. By contrast, very little sapstain developed in the other conifer species with maximum mean values of 10% for Norway spruce, 3.5% for larch and less than 1% for Sitka spruce. Overall, the moisture content of the logs decreased progressively in all species over the length of the trial. However, pine logs tended to retain higher levels of moisture throughout the trial compared with spruce or larch. The relatively higher moisture content of pine sapwood may be closer to the optimal moisture content that sapstain fungi require for infection and colonization, thereby contributing to the increased susceptibility of pine compared with the other conifer species. The pine logs also suffered from some colonization by bark beetles (Ips sexdentatus), which increased the inoculum potential and the opportunity for colonization by sapstain fungi. In addition, particular phenolic compounds in conifer sapwood may play a role in determining the resistance of some species to sapstain. Notably the most resistant species, Sitka spruce, was the only softwood that still retained detectable levels of phenolics in the sapwood to the end of the trial.     

The spreading of the S type of Heterobasidion annosum from Norway spruce stumps to the subsequent tree stand

The occurrence of Heterobasidion annosum in stumps and growing trees was investigated on 15 forest sites in southern Finland where the previous tree stand had been Norway spruce (Picea abies) infected by H. annosum, and the present stand was either Scots pine (Pinus sylvestris), lodgepole pine (Pinus contorta), Siberian larch (Larix siberica), silver birch (Betula pendula) or Norway spruce 8–53 years old. Out of 712 spruce stumps investigated of the previous tree stand, 26.3% were infected by the S group and 0.3% by the P group of H. annosum. The fungus was alive and the fruit bodies were active even in stumps cut 46 years ago. In the subsequent stand, the proportion of trees with root rot increased in spruce stands and decreased in stands of other tree species. On average, one S type genet spreading from an old spruce stump had infected 3.0 trees in the following spruce stand, 0.5 trees in lodgepole pine, 0.3 trees in Siberian larch, 0.05 trees in Scots pine and 0.03 trees in silver birch stand. Although silver birch generally was highly resistant to the S type of H. annosum, infected trees were found on one site that was planted with birch of a very northern provenance.     

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.     

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.     

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.     

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.     

Infection experiments with Gremmeniella abietina on seedlings of Norway spruce and Scots pine

Conidia of Gremmeniella abietina infected and caused disease symptoms in annual shoots of both Scots pine (Pinus sylvestris) and Norway spruce (Picea abies) seedlings. In Norway spruce shoots the infection remained largely latent, with only a few seedlings showing symptoms. Mycelial growth inside the shoots was faster in Scots pine than in Norway spruce and was favoured by low temperature in both hosts. The shoots of Norway spruce seedlings had higher endophyte populations than those of Scots pine, and the populations were decreased by low temperatures. Reductions in the normal epiphytic or endophytic flora by acid mist treatments seemed to favour the development of G. abietina.     

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.     

Surface mould growth on wooden claddings – effects of transient wetting,relative humidity,temperature and material properties

ABSTRACT

Effects of climatic factors and material properties on the development of surface mould growth on wooden claddings were investigated in a laboratory experiment. Specimens of aspen (Populus tremula), Siberian larch (Larix Sibirica), American white oak (Querqus alba), Scots pine (Pinus sylvestris), Norway spruce (Picea abies) and thermally modified pine were incubated in eight climatic chambers at specified wetting periods (2 or 4?h per day), relative humidity (58–86%) and temperature conditions (10–27°C). Surface mould growth was assessed weekly for 13 weeks, and the results were evaluated statistically using Generalized Estimating Equations logistic regression models. All tested climatic factors had significant effects on the mould growth, and there were significant differences between the materials. The ranking of the materials varied with temperature and over time. Aspen, pine sapwood and oak were overall most susceptible to mould growth, and thermally modified pine least susceptible. There were significant differences between sapwood and heartwood for pine and spruce. The effect of density was tested on the spruce heartwood material, but was not found to be significant. The results can be used to further develop prediction models for mould growth on wooden claddings.     

Persistence of a biocontrol strain of Phlebiopsis gigantea in conifer stumps and its effects on within‐species genetic diversity

Fungal isolations and genetic fingerprinting were used to determine whether Phlebiopsis gigantea stump treatment against Heterobasidion annosum sl. using a single genotype (Rotstop) would affect the genetic diversity of P. gigantea populations. The survival time of P. gigantea was longer in Norway spruce (Picea abies) stumps compared to Scots pine (Pinus sylvestris) as no isolates were obtained from pine stumps 6 years after treatment, whereas in about half of the spruce stumps the fungus was still present. The usage of Rotstop did not seem to increase the occurrence of the fungus 5 years after the treatment in fresh (1‐year‐old) untreated stumps within the same forest stands. All the isolates from the 6‐year‐old treated spruce stumps were identical in genotype with the Rotstop‐strain, whereas all isolates from the fresh untreated spruce and pine stumps differed from it. Within the treated pine stand, the biocontrol usage seemed to have caused a slight reduction in genetic markers not related to Rotstop, but there were no statistically significant differences between the marker frequencies and the local natural population. Thus, Rotstop is not likely to cause any immediate threat to the genetic diversity of P. gigantea.     

Two studies to assess the risk to Pinus sylvestris from Heterobasidion spp. in southern Sweden

Abstract

Pine stumps are not being treated against Heterobasidion spp. in Sweden. To determine whether they should be, the frequency of stump infections and the species of Heterobasidion involved were investigated in nine newly thinned pure Scots pine (Pinus sylvestris L.) stands in southern Sweden. The incidence of Heterobasidion was measured in roots of standing Scots pine in another 15 stands. Infections by both H. annosum (Fr.) Bref. s.s. and H. parviporum Niemelä & Korhonen were numerous in stumps six months after a summer thinning. The pathogen, mostly H. annosum, was found in 44 of 60 sampled root systems, from 14 of the 15 stands. Twenty of the infected pines were assessed as healthy on the basis of crown symptoms while 24 trees had defoliated crowns. Infected root systems were most frequent among trees with thin crowns in stands on former agricultural land, where previous thinnings had been carried out during the growing season when airborne spores are plentiful. The study suggests that stump treatment may be a profitable way to reduce disease development in Scots pine monocultures on sandy soils as well as in mixtures with Norway spruce [Picea abies (L.) Karst.] on any soil.     

Responses of Sitka spruce of different genetic origins to inoculation with Heterobasidion annosum: lesion lengths,fungal growth and development of the lignosuberized boundary zone

Three genetically distinct groups of Sitka spruce, open‐pollinated Queen Charlotte Island provenance, A13 selected seed and M0044 half‐sib mixture, were wounded alone or wounded and inoculated with Heterobasidion annosum sensu stricto on the lower stems. Growth of the pathogen and lesion formation was compared in the three genetic groups after treatment. No differences in the rate of colonization of the three genetic groups were observed over a 40 day period; lesion lengths in the bark and sapwood correlated closely. Moreover, lesions were considerably longer in inoculated plants than in those which were wounded only. No correlations were found within or between host genetic groups in the numbers or total areas of resin canals present in the first 18 mm from the wound in bark tissues for the three host genetic groups. Formation of the ligno‐suberized boundary zone (LSZ), however, was inhibited in the bark of inoculated plants, being first detected later and at a greater distance from the wound/inoculation point in the presence of H. annosum than in plants that were wounded only. Thickness of the suberin cell layers within the LSZ of M0044 plants was greater in wounded and inoculated, than in wounded only plants. The significance of these findings in relation to detecting spruce genotypes potentially resistant to H. annosum is discussed.