North American P type of Heterobasidion annosum shows pathogenicity towards Pinus halepensis in Italy

During a survey on the distribution of Heterobasidion root rot, the North American P group of H. annosum was recorded causing root rot on Aleppo pine, Pinus halepensis, in the gardens of a historical villa in Rome. This is the first report of Heterobasidion root rot on P. halepensis in its natural distribution area, the Mediterranean Basin.     

Distribution of Heterobasidion annosum intersterility groups in Poland

Specimens of Heterobasidion annosum were collected in 104 different stands in 43 regions of Poland. Pure cultures originating from 439 collections were identified in mating tests. Three intersterility groups, P, S and F, of H. annosum were found. Their occurrence in Poland was connected with the natural distribution of the main hosts: Pinus sylvestris, Picea abies and Abies alba, respectively. P was the most common intersterility group of H. annosum in Poland, causing mortality in Scots pine plantations and root rot in older stands. It was also isolated from Betula pendula, P. abies, Larix decidua, Fagus sylvatica and Carpinus betulus. The S group was present in the southern and north‐eastern parts of the country, causing root and butt rot mostly in spruce stands. The F group occurred in the south of Poland, in the mountains, highlands and lowland up to the northern border of the distribution of fir. It was found only on stumps, old dead trees and logs. There was no evidence of damage caused by the F group on A. alba trees.     

Heterobasidion annosum group S identified in north-eastern China

Samples of Heterobasidion annosum were collected in two stands of mixed forest in Changbaishan Nature Reserve in north-eastern China. Pure cultures were isolated from 15 specimens growing on Abies, Picea, Pinus and Populus. Mating tests showed that all these collections belong to the S inter-sterility group. The compatibility of Chinese homokaryons with S homokaryons from Europe was 100%. Clamp connections appeared almost equally in both strains of each pairing, and no distinct demarcation line formed in the pairings. The mating frequency of Chinese isolates with the European F group was also high, approximately 94%. These matings, however, were usually unilateral and a more or less distinct demarcation line was present in most of the pairings. The Chinese isolates were completely intersterile with the European P group. The S group of H. annosum in north-eastern China appears to have similar morphology and ecology as in Europe. It is common on Abies and Picea, but is not a serious pathogen in the pristine forests of north-eastern China.     

Liberation of Heterobasidion annosum conidia by airflow

The effect of speed and humidity of the airflow and the míst sprayed with airflow on the liberation of conidia of Heterobasidion annosum from pure cultures was studied in a small windtunnel. The conidia were liberated more easily in humid than in dry airflow. Liberation was most abundant when mist was added to the airflow. Threshold speed for the liberation of conidia was calculated to vary between 1.8 m/s and 3.9 m/s. According to the results, conidia of H. annosum may be liberated into the air by wind gusts associated with high humidity or mist and thus the conidia may have a role in aerial dispersal, although this is probably small compared with basidiospores.     

F group of Heterobasidion annosum found in Poland

The F intersterility group of Heterobasidion annosum, specialized to Abies species and earlier reported from southern parts of Europe, was identified from Abies alba and Picea abies in the Carpathian Mountains in southern Poland, close to northern limit of the natural distribution of A. alba. However, the fungus is not common in this area and seems to be primarily a saprophyte, growing on stumps and dead trees.     

Presence of double‐stranded RNA in Heterobasidion annosum

A search for double‐stranded RNA (dsRNA) was conducted among 204 European isolates of the pathogenic fungus Heterobasidion annosum. Nucleic acids were extracted and purified by cellulose CF‐11 chromatography or lithium chloride precipitation. dsRNA was present in eight of the isolates and was confirmed by nuclease digestion. The dsRNA elements ranged between 1.8 and 2.4 kbp and were found in two H. annosum intersterility groups, S and P. Partial amino acid sequence information from one dsRNA element showed significant homology to RNA‐dependent RNA polymerases from several fungal partitiviruses. This is the first report of the presence of dsRNAs in H. annosum. Possible implications of dsRNA for the biology of the fungus and the potential for biological control are discussed.     

Variation in Heterobasidion annosum detected by Random Amplified Polymorphic DNAs

The production of Random Amplified Polymorphic DNAs (RAPDs) by Polymerase Chain Reaction (PCR) was used to detect variation in the isolates of Heterobasidion annosum with various geographical origins. Specific RAPD products were detected for each of the intersterility groups: the European S, F, P and the North American S group. There was considerably more polymorphism found among European S than P isolates. The analysis of RAPDs was shown to be a simple and a fast way to generate DNA markers specific to the previously established intersterility groups and thus useful for diagnostic purposes.     

Intersterility groups of Heterobasidion annosum and their host specificity in Bulgaria

The occurrence of 46 strains of Heterobasidion annosum from four forest areas in Bulgaria is reported. Three intersterility groups, P, S and F, were identified for the first time in populations of this pathogen in Bulgaria. A strict host specificity of different intersterility groups was observed even in mixed stands. The P group was isolated from Pinus sylvestris and Pinus nigra and occasionally from stumps of Abies alba. It was rare in artificial pine plantations on dry sites, whereas it was more common on more moist sites. The F group caused severe damage on Abies alba but was not found, even as a saprotroph, on admixed pines. This is the first report of the F group on Abies alba in the Balkans. The S group was found only on Picea abies.     

Growth inhibitors produced by the in vitro culture of Heterobasidion annosum

The optimum conditions of in vitro production of growth inhibitory substances by Heterobasidion annosum are determined. Some terms and derivation of some functions are described in connection with the phases of in vitro production of growth inhibitory substances.     

In vitro inhibition of Heterobasidion annosum by Phaeotheca dimorphospora

The antagonistic fungus Phaeotheca dimorphospora has been tested in vitro on agar plates and wood discs to evaluate its potential use against P (Eastern Canada) and S (Finland) intersterility groups strains of Heterobasidion annosum using two methods. Production of antifungal metabolites by P. dimorphospora was demonstrated on agar plates as well as on Pinus resinosa and Picea abies discs using the bi‐layer technique. Diffusible metabolites inhibited the growth of the pathogen on both substrates. Direct confrontation on wood discs showed that a 7‐day period of advanced growth by P. dimorphospora completely prevented the colonization of the tissue by H. annosum. Control was partial when the antagonist had only one day of advanced growth. Growth inhibition was superior against S group strains with less than 1% of the samples colonized by H. annosum compared with 22% for P group strains. Moreover, P. dimorphospora was isolated in more than 83% of the samples in the absence of the pathogen in both wood species. In vitro tests demonstrated that P. dimorphospora is able to colonize red pine as well as Norway spruce tissue and to use wood components for the production of antifungal metabolites.     

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.     

Response of Compensatory-Fertilized Pinus sylvestris to Infection by Heterobasidion annosum

The effects of compensatory fertilization on the infection and growth rate of Heterobasidion annosum were studied in a 40-yr-old, naturally regenerated Scots pine (Pinus sylvestris L.) stand in south-eastern Finland. The treatments were: (1) unfertilized control, (2) a compound fertilizer containing P, K, Ca, Mg, S, Cu, Zn and B, (3) as treatment 2 but with nitrogen, (4) as treatment 3 but with limestone, (5) a mixture of nitrogen and limestone composed on the basis of needle analysis, and (6) wood ash. Pine roots were inoculated on two occasions four and five growing seasons after fertilization using four different strains of the P type of H. annosum. Two years after inoculation, the fungus was alive in 28.9% of the 135 inoculated roots. Fungus survival was highest in the control trees (50%) and poorest in the treatments 2 and 3 (10.5% and 16.7%, respectively). These two treatments differed significantly from the control. Total mycelial extension during the 2-yr study period was 16.4, 2.1, 4.5, 18.1, 15.2 and 5.3 cm in treatments 1, 2, 3, 4, 5 and 6, respectively. Only treatment 2 differed significantly from the control (p<0.05, df=23). The result suggests that the application of slow-release compound fertilizers without supplementary limestone may increase the resistance of Scots pine to infection by H. annosum. The nitrogen-free compound fertilizer may also retard the spread of annosum root rot in infected pine stands.     

An intersterility group of Heterobasidion annosum specialized to Abies alba

A third intersterility group of H. annosum in Europe has been identified from Italy. It is specialized to Abies alba., and hence is called the F (fir) group. It is closely related to the European S group, but is sexually incompatible with the geographically adjacent S population occurring in the Alps.     

Intersterility group differentiation in Heterobasidion annosum using ribosomal IGS1 region polymorphism

Summary The intergenic spacer region 1 (IGS1) of the nuclear ribosomal RNA gene repeat was examined in three European intersterility groups of the wood‐rot fungus Heterobasidion annosum. The IGS1 elements were informative for restriction fragment length polymorphism (RFLP) fingerprinting the fungal intersterility groups following digestion with two different four‐cutter restriction endonucleases. The fingerprints of isolates from the same intersterility group from different localities shared some fragments in common, but also showed specific fragments suggesting that RFLP of the IGS1 region might be used in population studies. RFLP patterns between isolates from different intersterility groups differed more markedly. RFLP were also detected in the IGS1 region of North American strains of H. annosum and in the related species Heterobasidion araucariae. The results indicate that ribosomal IGS1 region fingerprinting could be a useful tool to identify the fungal intersterility group in Heterobasidion annosum.     

Experiments on the susceptibility of conifers to Heterobasidion annosum in Great Britain

During the period 1960–71, experimental plantings were established at three sites in western Britain that were infested with Heterobasidion annosum: Ceri in mid‐Wales, Lael in north‐west Scotland and Red Marley in the West Midlands of England. At each site a randomized block experiment involving at least four species was supplemented with an ancillary trial of other species. In two of the experiments various treatments were applied to the previous stand of trees before or at felling, but only stump removal reduced the amount of disease in the succeeding crops. At Ceri, the incidence of H. annosum in stems removed at first thinning was: Picea sitchensis, 14%; Pseudotsuga menziesii 11%; Pinus contorta 3% and Abies procera 1%. At Lael, the figures were Larix decidua 59%; P. menziesii 51%; Chamaecyparis lawsoniana 37%; Abies amabilis 33% and Tsuga heterophylla 21%. There was negligible disease in A. procera;Abies grandis and Pinus sylvestris. At Red Marley, the incidence of disease was: P. menziesii 28%; T. heterophylla 18%; A. grandis 7%; Picea abies 1% and Pinus nigra var. maritima 0%. In the ancillary trial at Lael, the incidence of H. annosum in P. sitchensis was 55% and in P. abies 16%. The mean height of colonization by H. annosum within the diseased stems removed at first thinning at Lael (age 21–22 years) was 2.1 m for L. decidua, 1.4 m for P. sitchensis and 1.3 m for P. abies. Armillaria sp. caused mortality and decay in two of the experiments and these data are also presented. The results are discussed in relation to other information on the susceptibility of these species to H. annosum in the UK and elsewhere.     

Compartmentalization of decay associated with Heterobasidion annosum in roots of Pinus resinosa

Decay associated with Heterobasidion annosum was compartmentalized in roots of Pinus resinosa.     

Estimation of dispersal gradients of S- and P-type basidiospores of Heterobasidion annosum

The number of airborne deposits of the root pathogen Heterobasidion annosum was counted with a wood disc method at distances 5–1000 m from one S-type and one P-type infection centre in stands of Picea abies in June, July and August, 1995. The level of background deposition in the area was determined in four healthy control stands. Based on intersterility group determination of deposits, only measurements within 100 m of the infection centres could be used in estimations of dispersal gradients owing to confusion with background deposition. A power law gradient model adequately described the reduction of deposits with increasing distance from the infection centres. Gradients were fairly similar in four directions around the infection centres as the wind speeds were low (3.2m/s) during the collection periods. The gradients were steeper around the P-type centre, probably due to its lower spore production capacity. The background deposition level (2.8 colonies/m²× h) was reached at distances 1255 m and 98 m from the S-type and P-type centre, respectively.     

The mating system of Fomes annosus (Heterobasidion annosum)

Conditions are described which allow reproducible formation of fruiting bodies of Fomes annosus under sterile conditions, suitable for genetic studies. Analysis of the progeny of a fruiting body isolated from woods revealed a bipolar mechanism of genetic control of fruiting body formation as well as “Grenz-haut”-formation.     

Early infection of Fagus sylvatica by Heterobasidion annosum sensu stricto

Heterobasidion annosum sensu stricto is the most important damaging agent in Scots pine stands planted on the former agricultural lands in Poland. The routine action in pine stands which have lost stability because of H. annosum root rot is to change stand management, including species conversion. In many cases, the Fagus sylvatica is used for this purpose. This study was the first assessment of widespread infection by H. annosum in young F. sylvatica plantations. Disease symptoms included atrophy and yellowing of leaves, wilting and the presence of pathogen sporocarps around the root collars of young trees. Heterobasidion annosum s. s. was observed on both 4‐ and 17‐year‐old beech. Based on annual increments, the disease could be present for 3–4 years before tree death. A high incidence of H. annosum in pine stumps of previous stands (80–100%) and dry periods in recent years may be the main reasons for such common infection of F. sylvatica. This work also showed that mice and frost were not the main killing factors F. sylvatica in plantations.