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.     

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.     

Effectiveness of treatment of Norway spruce stumps with Phlebiopsis gigantea at different rates of coverage for the control of Heterobasidion

The natural establishment of the root and butt rot causing fungus Heterobasidion annosum s.l. on Norway spruce (Picea abies) thinning stumps treated with Phlebiopsis gigantea was investigated on seven sites in southern Sweden. The trees were cut during summertime and the stumps were treated with different patterns simulating the effect of mechanical stump treatment with a single‐grip harvester. Sampling was conducted 3 and 12 months after treatment. At both samplings, the best control was obtained when 100% of the stump surface was covered by P. gigantea: in contrast, untreated control stumps showed the highest incidences of H. annosum s.l. infection at both sampling times. However, 30 and 26% of the fully covered stumps at the first and second samplings, respectively, were diseased, and question the efficacy of treating Norway spruce stumps with this biological control agent in Sweden.     

Optimizing the management of a Picea abies stand under risk of butt rot

A simulation model was developed to predict the growth of a Norway spruce stand under risk of butt rot caused by Heterobasidion annosum stump infection and logging injuries. The simulation model was distance‐dependent; tree growth was predicted with a distance‐dependent model, and the spread of butt rot through root contacts depended on tree location. Infection of stumps and injured trees, and the spread of butt rot in the stand were stochastic processes whereas tree growth and mortality were treated as deterministic processes. The simulation model was used with the nonlinear optimization algorithm of ooke and J eeves (J. Assoc. Comput. Mach, 8, 212–229, 1961) to find the most profitable management schedule for an even‐aged, young stand. Optimization used four different stump infection rates and two spreading capacities from infected stumps. The profitability was evaluated by the expected soil expectation value (SEV) at a 3% interest rate. Two thinnings, both in winter‐time, and hence without H. annosum infections, resulted in the highest SEV. If any stump infection by H. annosum occurred, only one thinning and a shortened rotation were suggested. The optimal thinning rate tended to decrease but also large trees were removed with the increasing infection rate. With one thinning during a rotation, stump treatment was profitable above a stump infection rate of 10%     

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.     

Urea treatment reduced Heterobasidion annosum s.l. root rot in Picea abies after 15 years

Stump protection using chemical or biological agents is the main control measure against root and butt rot caused by Heterobasidion annosum s.l. in northern and temperate conifer forests. Long-term effects of urea treatment of stumps are poorly known and here we describe a 15-year study of urea treatment on the rot incidence on Norway spruce (Picea abies). We also tested the effect of urea protection on tree growth and on the resistance of stands against strong winds. Four treatments were made in two replications in two first-rotation P. abies stands in southern Sweden; after first-thinning stumps were (i) treated with urea 35% (w/v), (ii) artificially infected with H. annosum conidia, (iii) half urea treated and half artificially conidia infected, (iv) untreated, therefore naturally infected. After 15 years, the trees were sampled at 20 cm above ground using an increment borer and observed for presence of rot and, following incubation, presence of H. annosum conidia. Tree growth was calculated by measuring the diameter before and after the treatment. Urea treated plots showed the lowest incidence of rot (3%) as well as of H. annosum incidence (0%). Conidia treatment showed the highest incidence of rot (68%), its incidence was higher than that observed in natural infection treatment (43%), but did not differ from that of the 50% conidia treatment (47%). On about 30% of the rotted trees we observed conspicuous H. annosum fruiting. We did not observe growth reduction associated with tree rot. H. annosum was the only fungus observed associated with rotted trees which suggest that it was responsible for most of the rot observed in the investigated plots. Urea treated plots showed the lowest incidence of windthrown trees, and 59% of the windthrown tree incidence among the plots was explained by the incidence of H. annosum. Urea can be regarded as a reliable long-term protection method against root and butt rot of Norway spruce.     

In vitro interactions between bacteria isolated from Sitka spruce stumps and Heterobasidion annosum

Culture medium composition affected antagonism by bacterial isolates from Sitka spruce (Picea sitchensis) stumps against Heterobasidion annosum. Fifty percent of bacterial isolates inhibited H. annosum growth on sporulation agar or yeast–dextrose–peptone agar; only 10% of isolates caused inhibition on both media. Proportions of isolates inhibiting H. annosum varied with stump age; fewer isolates from 4‐ or 6‐year‐old stumps exhibited antagonism than isolates from older or younger stumps. Fifteen isolates showing antagonism on sporulation agar were tested against H. annosum in spruce wood cubes. None of the bacterial isolates alone caused a significant weight reduction in inoculated cubes. Relative inoculation times of bacterial isolates and H. annosum had an effect on weight loss in interactions; simultaneous inoculation with isolates and H. annosum inhibited weight loss caused by H. annosum compared with bacteria‐free controls. Inoculation with bacterial isolates 10 days before H. annosum had no effect on the decay rate. In contrast, inoculation with H. annosum 10 days before bacteria increased weight loss of cubes by 200% relative to cultures lacking bacteria. The effect of a mixed bacterial inoculum on weight change in 0.2‐mm spruce wood slips co‐inoculated with H. annosum, Resinicium bicolor, Hypholoma fasciculare, Stereum sanguinolentum or Melanotus proteus differed between different fungi.     

Reaction zone and periodic increment decrease in Picea abies trees infected by Heterobasidion annosum s.l.

Norway spruce (Picea abies) trees infected by Heterobasidion annosum s.l. decrease their periodic increment after a long period of time. Periodic increment decrease hypothetically relates to the formation of a reaction zone in order to prevent fungal colonisation. We studied 11 stands in Sweden, where we compared the periodic increment of healthy, rotten- and H. annosum-infected trees growing on plots thinned in winter, unthinned or thinned in summer, with and without urea or P. gigantea treatment of the stumps. Based on the rot incidence and the population structure of H. annosum of the plots, two phases of infection were considered: > 13 years and < 13 years. The presence of reaction zone and decay was observed on wood cores extracted with an increment borer. Rotten and H. annosum-infected trees with reaction zones exhibited a lower periodic increment than healthy trees (13.0% and 12.5% losses in terms of diameter, respectively), while no differences were observed between healthy trees and rotten and H. annosum-infected trees without reaction zone. Our results support the hypothesis of a periodic increment decrease in individual trees due to photosynthate re-allocation resulting from decay compartmentalization. Periodic increment decrease was only evident in trees that had been infected for more than 13 years. Trees in urea-treated plots registered a higher periodic increment, suggesting a possible response of trees to the nitrogen addition of the urea treatment of the stumps.     

Modelling the incidence of Heterobasidion annosum butt rots and related economic losses in alpine mixed naturally regenerated forests of northern Italy

A total of 2257 recently felled stumps were sampled from 22 mixed naturally regenerated forest stands in the Aosta Valley, western Italian Alps, and examined for Heterobasidion annosum butt rots. Disease incidence ranged from 6% to 71% depending on sites, and H. annosum accounted for 92% of the total number of diseased stumps. H. annosum incidence was significantly higher (Chi‐squared multiple comparison tests, p < 0.05) on Norway spruce (44%) than on the other tree species (silver fir = 18%, larch = 12% and Scots pine = 6%). Based on the information on the airborne inoculum composition of the fungus, all the three European species of H. annosum were present, with a variable frequency, depending on site. A partial least squares regression analysis showed that the relative abundance of Norway spruce and four variables describing the size of trees were the best predictors for the incidence of H. annosum butt rots. A model having these variables as predictors was developed (r² = 0.75; p < 0.001) and successfully validated on five additional forest stands. An estimate of the losses in yield and value of timber associated with the disease, i.e. direct losses, was also carried out in the forests included in the survey. The percentage of direct financial losses was either comparable or lower than the average level of disease incidence.     

Susceptibility of Sitka spruce and grand fir trees to decay by Heterobasidion annosum

The susceptibility of Sitka spruce (Picea sitchensis) and grand fir (Abies grandis) to decay by Heterobasidion annosum was compared on a site with a previous history of serious disease. The incidence of decay 15–16 years after planting was 1.5% in grand fir, compared with 34.8% in Sitka spruce. These results are broadly similar to those reported from other trials. More information is required for older trees in Britain but the evidence so far available suggests that grand fir is more resistant to H. annosum than Sitka spruce. Planting grand fir on severely infested sites may therefore significantly reduce the serious losses that could be expected with Sitka spruce. In 14 out of 271 decayed Sitka spruce H. annosum was absent from the middle or base of the decay column but present at the top. Approximately 3% of infected spruce showed features suggesting some degree of resistance. Resin bleeding was observed at the base of 23.2% of the infected spruce, and the mean height of the decay column in these trees was significantly greater (0.96 m) than in those without this feature (0.66 m). The inoculum for infection was provided by stumps of the previous crop, and the genets of H. annosum that were present in some infected trees were also identified in stumps. The majority of infected trees contained only a single genet but three trees contained two genets. In this young crop individual genets were smaller than those found elsewhere; the largest extended to include six trees.     

Resinicium bicolor; a potential biological control agent for Heterobasidion annosum

After introductory competition tests in the laboratory, Resinicium bicolor was used as a potential control agent for the conifer root rot fungus Heterobasidion annosum. Greenhouse pathogenicity tests with R. bicolor on 4-year-old seedlings of Norway spruce (Picea abies) and Scots pine (Pinus sylvestris) showed moderate incidence of infection. In three places in Sweden, four test areas were chosen for field experiments in first-rotation plantations and also in old forest sites of Norway spruce. Wood blocks, pre-inoculated with one strain of R. bicolor, were buried in the soil beside stumps at 0, 1, 2, 3 and 4 months after thinning using various spatial designs. In two of the test areas, half of the stumps were treated with a suspension of H. annosum conidia from one strain by surface spraying. After 2–3 years, stump roots were investigated and the length of growth of both species were noted. The identity of mycelia reisolated from and wood debris in the test areas were confirmed by somatic compatibility tests with the original strains. The strain of R. bicolor released was recovered from all over the test area; the released H. annosum strain was only reisolated from the conidia sprayed stump roots. R. bicolor had little effect on the growth and occurrence of H. annosum. Potential control of disease spread may arise, however, from occlusion of the pathogen from outer parts of roots.     

The effect of nitrogen fertilization on fungistatic phenolic compounds in roots of beech (Fagus sylvatica) and Norway spruce (Picea abies)

The effect of nitrogen fertilization on fungistatic phenolic compounds in fine roots of beech and Norway spruce growing in afforestation plots was analysed. The plots were situated at two sites in Switzerland on acidic soil with low base saturation. For 9 years, the trees have been treated with dry ammonium nitrate to give 0, 10, 20, 40, 80, 160 kg N ha^?1 year^?1, respectively. The phenolic compounds responded differently to fertilization. Fine roots of beech showed a significant decrease of (?)‐epicatechin and piceatannol with increasing nitrogen fertilization. The concentration of protocatechuic acid was increased with fertilization. Roots of fertilized Norway spruce showed significantly decreased concentrations of 4‐hydroxyacetophenone and piceatannol. The mycelial growth of three isolates each of Heterobasidion annosum s.l. and Cylindrocarpon destructans was tested on agar media containing various phenolic compounds in concentrations found in fine roots of Norway spruce (Picea abies) and beech (Fagus sylvatica). All three H. annosum isolates were inhibited by p‐coumaric acid and (?)‐epicatechin. Two isolates were inhibited by another four phenolic compounds (p‐hydroxybenzoic acid, 4‐hydroxyacetophenone, piceatannol and protocatechuic acid), one by (+)‐catechin. Two of three C. destructans isolates were inhibited by all phenolic compounds except for (+)‐catechin which affected only one isolate, one isolate did not respond at all.     

Early infection by Heterobasidion annosum in Larix x eurolepis seedlings planted on infested sites

The incidence of Heterobasidion annosum was examined in three young hybrid larch (Larix × eurolepis) plantations in southern Sweden that were established after felling Norway spruce (Picea abies) stands heavily infected by the fungus. The incidence of H. annosum was 7 %, 33 %, and 70 %, respectively, in the 2-, 3-, and 5-year-old plantations. In all plantations, H. annosum was the most frequent rot-causing fungus (81 %) and all isolates tested belonged to the P-intersterility group. No visible external differences between infected and healthy seedlings were found on any site. Both tap and lateral roots were infected by H. annosum. The results suggest that when hybrid larch is planted on sandy soils after clear felling of Norway spruce heavily infected by H. annosum, it is very susceptible to infection by H. annosum. Furthermore, transfer of H. annosum from infected stumps to hybrid larch occurs during the first few years after planting on scarified sites. In this study, scarification may have exacerbated the root disease situation and actually led to more early infections by distributing pieces inoculum across the sites.     

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.     

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.     

Induced resistance of Norway spruce,variation of phenolic compounds and their effects on fungal pathogens

Three clones of Norway spruce (Picea abies) were studied for their response to mass‐inoculation with the blue‐stain fungus Ceratocystis polonica. The effect of different pretreatments (fungal inoculation and wounding) before mass‐inoculation was investigated for their possible role in an acquired resistance reaction. Pretreated trees showed enhanced resistance to the subsequent mass‐inoculation relative to control trees that received no pretreatment. Furthermore, the fungal colonization of inoculated trees was less than that of wounded trees. The phenolic content of the bark, analysed by RP‐HPLC, was compared in trees receiving different treatments. Trees inoculated with C. polonica had higher average concentration of (+)‐catechin, taxifolin and trans‐resveratrol than wounded trees. Both inoculated and wounded trees had higher average concentrations of these compounds than control trees. The effect of the phenolic extract of Norway spruce bark on the growth of the root rot fungus Heterobasidion annosum and the blue‐stain fungi C. polonica and Ophiostoma penicillatum were investigated in vitro. Heterobasidion annosum was not negatively affected, and the extracts had fungistatic effects on the blue‐stain fungi. The growth of O. penicillatum was more inhibited than the growth of the more aggressive C. polonica.     

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.     

Intersterilitätsgruppen und Klone von Heterobasidion annosum-Isolaten aus Koniferen- Wurzel- und -Stammfäulen

Intersterility groups and clones of Heterobasidion annosum isolates from root and butt rots of conifers 69 H. annousum heterokaryons from scots pine, Douglas fir, Norway spruce and larch belonged to the P group, three heterokaryons from Norway spruce to the S group. The results of a study with clones of H. annosum isolates from roots of Scotch pine trees in close neighbourhood suggest colonisation by H. annosum (partly) via root contacts.     

Intersterilitätsgruppen und Klone von Heterobasidion annosum in einem 31jährigen Fichtenbestand1

Intersterility groups and clones of Heterobasidion annosum in a 31 years old Norway spruce stand . H. annosum isolates from roots of 13 spruce trees growing in close neighbourhood be longed to the P group (from five roots) and to the S group (from 44 roots). The clone identifications revealed colonisation via root contacts.     

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.