Summary 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.
We describe a novel putative mycovirus infecting the conifer root-rot fungus Heterobasidion annosum sensu lato. This virus, designated as Heterobasidion RNA virus 6 (HetRV6), is taxonomically distant from all previously known viruses of Heterobasidion species, but somewhat related to the Curvularia thermal tolerance virus and the Fusarium graminearum virus 4. Based on a population analysis including 35 virus strains from Heterobasidion abietinum, Heterobasidion parviporum, Heterobasidion annosum sensu stricto and Heterobasidion occidentale, HetRV6 showed a considerable degree of geographical and host-related differentiation. The North American and Eurasian virus populations were clearly separated. In Eurasia, we observed cases of discrepancy between virus and host taxonomy, suggesting interspecies virus transfer. HetRV6 was also successfully transmitted between the three European species H. abietinum, H. annosum and H. parviporum. Based on growth rate tests on agar plates and spruce stem pieces, HetRV6 seemed to be cryptic or slightly mutualistic to its host.
The fungal genus includes some of the most devastating conifer pathogens in the boreal forest region. In this study, we showed that the alphapartitivirus Heterobasidion partitivirus 13 from (HetPV13-an1) is the main causal agent of severe phenotypic debilitation in the host fungus. Based on RNA sequencing using isogenic virus-infected and cured fungal strains, HetPV13-an1 affected the transcription of 683 genes, of which 60% were downregulated and 40% upregulated. Alterations observed in carbohydrate and amino acid metabolism suggest that the virus causes a state of starvation, which is compensated for by alternative synthesis routes. We used dual cultures to transmit HetPV13-an1 into new strains of and The three strains of that acquired the virus showed noticeable growth reduction on rich culturing medium, while only two of six isolates tested showed significant debilitation. Based on reverse transcription-quantitative PCR (RT-qPCR) analysis, the response toward HetPV13-an1 infection was somewhat different in and We assessed the effects of HetPV13-an1 on the wood colonization efficacy of in a field experiment where 46 Norway spruce trees were inoculated with isogenic strains with or without the virus. The virus-infected strain showed considerably less growth within living trees than the isolate without HetPV13-an1, indicating that the virus also causes growth debilitation in natural substrates. A biocontrol method restricting the spread of species would be highly beneficial to forestry, as these fungi are difficult to eradicate from diseased forest stands and cause approximate annual losses of €800 million in Europe. We used virus curing and reintroduction experiments and RNA sequencing to show that the alphapartitivirus HetPV13-an1 affects many basic cellular functions of the white rot wood decay fungus, which results in aberrant hyphal morphology and a low growth rate. Dual fungal cultures were used to introduce HetPV13-an1 into a new host species, , and field experiments confirmed the capability of the virus to reduce the growth of in living spruce wood. Taken together, our results suggest that HetPV13-an1 shows potential for the development of a future biocontrol agent against fungi.
Fungal viruses (mycoviruses) with RNA genomes are believed to lack extracellular infective particles. These viruses are transmitted laterally among fungal strains through mycelial anastomoses or vertically via their infected spores, but little is known regarding their prevalence and patterns of dispersal under natural conditions. Here, we examined, in detail, the spatial and temporal changes in a mycovirus community and its host fungus Heterobasidion parviporum, the most devastating fungal pathogen of conifers in the Boreal forest region. During the 7-year sampling period, viruses accumulated in clonal host individuals as a result of indigenous viruses spreading within and between clones as well as novel strains arriving via airborne spores. Viral community changes produced pockets of heterogeneity within large H. parviporum clones. The appearance of novel viral infections in aging clones indicated that transient cell-to-cell contacts between Heterobasidion strains are likely to occur more frequently than what was inferred from genotypic analyses. Intraspecific variation was low among the three partitivirus species at the study site, whereas the unassigned viral species HetRV6 was highly polymorphic. The accumulation of point mutations during persistent infections resulted in viral diversification, that is, the presence of nearly identical viral sequence variants within single clones. Our results also suggest that co-infections by distantly related viral species are more stable than those between conspecific strains, and mutual exclusion may play a role in determining mycoviral communities.
Root rot infections in Norway spruce (Picea abies (L.) Karst.) regeneration, planted after the clear-cutting of spruce on sites infested by Heterobasidion, were investigated on 21 experimental plots in eight 2-to 23-year-old plantations. Heterobasidion root rot became evident about 10 years after planting and the proportion of infected spruces increased steadily with plantation age. The average number of planted spruces infected per old decayed stump was 0.2 trees in 2-to 9-year-old plantations, 0.8 trees in 11-to 15-year-old plantations, and 1.8 trees in 20-to 23-year-old plantations. About 10 and 20 years after planting, 7 and 23% of the planted spruces in the disease centers were infected by Heterobasidion. Heterobasidion parviporum Niemelä & Korhonen, and Heterobasidion annosum (Fr.) Bref. s. str. caused 98 and 2% of the Heterobasidion infections in the previous spruce rotation, and 96 and 4% in the spruce regeneration, respectively. In all, 71% of the infected regeneration trees were attacked by a Heterobasidion genet that was also isolated from the stumps of the previous tree stand. Armillaria species (Armillaria borealis Marxmüller & Korhonen and Armillaria cepistipes Velenovský) were isolated from 7% of the planted spruces.Résumé : Les infections causées par les champignons de carie de racines chez la régénération d'épicéa commun (Picea abies (L.) Karst.), plantée après que l'épicéa ait été coupé à blanc dans des sites infestés par Heterobasidion, ont été étudiées dans 21 parcelles expérimentales établies dans huit plantations âgées de 2 à 23 ans. La carie de racines causée par Heterobasidion est devenue évidente environ 10 ans après la plantation et la proportion d'épicéas infectés a augmenté régulièrement avec l'âge de la plantation. Le nombre moyen de plants d'épicéa infectés par vieille souche cariée est de 0,2 arbres dans les plantations âgées de 2 à 9 ans, 0,8 arbres dans les plantations âgées de 11 à 15 ans et 1,8 arbres dans les plantations âgées de 20 à 23 ans. Environ 10 et 20 ans après la plantation, 7 et 23 % des arbres plantés dans les centres d'infection étaient infectés par Heterobasidion. Heterobasidion parviporum Niemelä & Korhonen et Heterobasidion annosum (Fr.) Bref. s. str. ont causé respectivement 98 et 2 % des infections chez les épicéas de la ré-volution précédente et 96 et 4 % dans la régénération d'épicéa. En tout, 71 % des plants infectés parmi la régénération ont été attaqués par un genêt de Heterobasidion qui était aussi isolé des souches des arbres du peuplement précédent. Des espèces d'Armillaria (Armillaria borealis Marxmüller & Korhonen et Armillaria cepistipes Velenovský) ont été isolées de 7 % des plants d'épicéa parmi la régénération.[Traduit par la Rédaction] Piri 611
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