Iben Margrete Thomsen

Occurrence and pathogenicity of fungi in necrotic and non-symptomatic shoots of declining common ash (Fraxinus excelsior) in Sweden

Currently, massive dieback of Fraxinus excelsior is observed in countries of eastern, northern and central Europe, and the reasons for it are unclear. The aims of the present work were (a) to study fungal communities in declining F. excelsior crowns; (b) to clarify role of fungi in the decline. Shoots from symptomatic crowns were collected in four localities in central Sweden, and distributed into the following categories: (a) visually healthy; (b) initial necroses; (c) advanced necroses; (c) dead tops. The most frequently isolated fungi were Gibberella avenacea, Alternaria alternata, Epicoccum nigrum, Botryosphaeria stevensii, Valsa sp., Lewia sp., Aureobasidium pullulans and Phomopsis sp., and these taxa were consistently found in shoots of all four symptomatic categories. Forty-eight taxa of other fungi were isolated, and fungal diversity was not exhausted by the sampling effort. The same taxa of fungi were dominant in F. excelsior shoots of different symptomatic categories, and moderate to high similarity of fungal communities was observed in shoots despite the symptoms. Forty-four isolates from 24 fungal taxa were used for artificial inoculations of 277 1-year-old F. excelsior seedlings in bare root nursery. After 2 years, only four fungi caused symptomatic necroses of bark and cambium: A. alternata, E. nigrum, Chalara fraxinea and Phomopsis sp. The most pathogenic was C. fraxinea, inducing symptoms on 50% of inoculated trees, while three other fungi caused necroses on 3–17% of inoculated trees. Infection biology of C. fraxinea and environmental factors determining susceptibility of F. excelsior to decline deserve future investigations.

Somatic compatibility in Amylostereum areolatum and A. chailletii as a consequence of symbiosis with siricid woodwasps

Somatic compatibility is used to identify clones or vegetative compatibility groups (VCG) in fungal populations. For basidiomycetes, VCGs have normally been associated with either homothallic, non-outcrossing fungi or root decay fungi spreading by vegetative growth. A special instance is provided by the symbiotic relationships between woodwasps and the fungi Amylostereum areolatum and A. chailletii. This association results in clonal propagation, as the wasps (Sirex sp. and Urocerus sp.) transfer arthrospores of A. areolatum or A. chailletii during ovipositing in conifers. Identical isolates of the symbiont are normally carried by all the female offspring of one wasp. The presence of clones of A. areolatum and A. chailletii was tested through somatic compatibility shown in pairings of heterokaryons obtained from wasps, basidiocarps and wood. Isolates found more than 100 km apart within Denmark belonged to the same vegetative compatibility groups. In addition, A. areolatum isolates from Sweden and Lithuania were compatible with a Danish clone, in spite of the barrier of the Baltic Sea. The association between woodwasps and fungi thus creates dispersive clones or VCGs which are stable across time and space. The dispersal through basidiospores may be considered of less importance for A. areolatum, but common in A. chailletii.

Incidence of butt rot in a tree species experiment in northern Denmark.

The susceptibility to infections by Heterobasidion annosum (Fr.) Bref. was examined in a 28 - year - old tree species experiment in northern Jutland, Denmark. Silver fir (Abies alba Mill.), Caucasian fir (Abies nordmanniana (Stev.) Spach), grand fir (Abies grandis (Dougl) Lindl.), noble fir (Abies procera Rehd.), Japanese larch (Larix koempferi (Lamb.) Carr.), Norway spruce (Picea abies (L.) Karst.), Sitka spruce (Picea sitchensis (Bong.) Carr.), and Douglas fir (Pseudotsuga menziesii (Mirb.) Franco) were planted after shelterwood felling of a mountain pine (Pinus uncinata Mill. ex Mirb.) stand that had been infected by H. annosum. Douglas fir and noble fir showed the greatest mortality due to H. annosum during the first 5 years after planting. At first thinning the highest incidences of butt rot were recorded in noble fir, Japanese larch and Sitka spruce, with 44%, 43% and 36% of the thinned trees infected, respectively. Silver fir and Caucasian fir were almost free from infections. Maximum extension of ...

The Woodwasp Sirex noctilio and Its Associated Fungus Amylostereum areolatum in Europe

The current knowledge about the biology and ecology of siricid woodwasps and their Amylostereum mutualistic fungi, with a specific focus on Sirex noctilio and Amylostereum areolatum, are summarized from a European perspective. The woodwasp females deposit their eggs together with fungal spores and toxic mucus in single or multiple drills. The symbiotic fungus supports the development of the larvae, which usually takes 2 years. The fungus propagates vegetatively through the formation of asexual spores in the teneral females and is stored in the wasps’ mycangia. Both A. areolatum and the related species A. chailletii are known to have vegetative compatibility groups as a consequence of the symbiosis with woodwasps. Among the natural enemies of the woodwasps, woodpeckers have a minor significance in Sirex mortality compared to parasitoids. Particularly the parasitic Ibalia species can account for 40–70% larval mortality. Like all European woodwasps, S. noctilio preferably colonizes newly dead, damaged or greatly weakened trees with considerable needle loss and low moisture content. Thus, unlike in the countries where it has been accidentally introduced, S. noctilio causes hardly any pine mortality in Europe and is economically irrelevant.

Multilocus genotyping of Amylostereum spp. associated with Sirex noctilio and other woodwasps from Europe reveal clonal lineage introduced to the US

Sirex noctilio is a woodwasp of Eurasian origin that was inadvertently introduced to the southern hemisphere in the 1900s and to North America over a decade ago. Its larvae bore in Pinus spp. and can cause significant mortality in pine plantations. S noctilio is associated with a symbiotic white rot fungus, Amylostereum areolatum, which females inject into trees when they oviposit and which is required for survival of developing larvae. We compared the genetic diversity of A. areolatum isolated from S. noctilio and other woodwasps collected from Europe and from northeastern North America to determine the origin of introduction(s) into the United States. Multilocus genotyping of nuclear ribosomal regions and protein coding genes revealed two widespread multilocus genotypes (MLGs) among the European samples, one of which is present in the US. The other two MLGs associated with S. noctilio in the US represented unique haplotypes. These latter two haplotypes were likely from unrepresented source populations, and together with the introduced widespread haplotype reveal multiple A. areolatum MLGs introduced by S. noctilio and indicate possible multiple S. noctilio introductions to North America from Europe. Our results also showed a lack of fidelity between woodwasp hosts and Amylostereum species.

Species variation in susceptibility to the fungus Neonectria neomacrospora in the genus Abies

ABSTRACT The fungus Neonectria neomacrospora has recently caused an epidemic outbreak in conifer species within the genus Abies in Denmark and Norway. Christmas tree producers in Europe and North America rely, to a large extent, on Abies species. The damage caused by N. neomacrospora, including dead shoot tips, red flagging of branches and potentially dead trees, have therefore caused concern about reduced quality and loss of trees, and thereby of revenue. Field observations of natural infection of 39 taxa, from 32 species, within the genus Abies in the Hørsholm Arboretum, Denmark, were evaluated; significant differences were seen between taxa, that is, species, and between some species and their subspecies. The Greek fir, Abies cephalonica, was the only species without damage. An inoculation experiment on detached twigs with mycelium plugs from a N. neomacrospora culture showed that all species could be infected. The damage observed in the inoculation experiment could explain 30% of the variation in the field observations based on species mean values. The epidemic outbreak and the high number of species susceptible to this fungus indicate that N. neomacrospora requires attention in the cultivation and conservation of Abies species.

Environment and host as large-scale controls of ectomycorrhizal fungi

Explaining the large-scale diversity of soil organisms that drive biogeochemical processes—and their responses to environmental change—is critical. However, identifying consistent drivers of belowground diversity and abundance for some soil organisms at large spatial scales remains problematic. Here we investigate a major guild, the ectomycorrhizal fungi, across European forests at a spatial scale and resolution that is—to our knowledge—unprecedented, to explore key biotic and abiotic predictors of ectomycorrhizal diversity and to identify dominant responses and thresholds for change across complex environmental gradients. We show the effect of 38 host, environment, climate and geographical variables on ectomycorrhizal diversity, and define thresholds of community change for key variables. We quantify host specificity and reveal plasticity in functional traits involved in soil foraging across gradients. We conclude that environmental and host factors explain most of the variation in ectomycorrhizal diversity, that the environmental thresholds used as major ecosystem assessment tools need adjustment and that the importance of belowground specificity and plasticity has previously been underappreciated.Analyses of data from 137 forest plots across 20 European countries show that ectomycorrhizal fungal diversity is strongly influenced by environmental and host species factors and provide thresholds to inform ecosystem assessment tools

Author Correction: Environment and host as large-scale controls of ectomycorrhizal fungi

Change history: In the HTML version of this Article, author ‘Filipa Cox’ had no affiliation in the author list, although she was correctly associated with affiliation 3 in the PDF. In addition, the blue circles for ‘oak’ were missing from Extended Data Fig. 1. These errors have been corrected online.

Genetic variation and genotype by environment interaction in the susceptibility of Abies nordmanniana (Steven) Spach to the fungus Neonectria neomacrospora (Booth & Samuels) Mantiri & Samuels

Key messagePronounced clonal variation and moderate to high broad-sense heritability estimates of susceptibility toNeonectria neomacrosporawere found inAbies nordmannianain three sites. Significant genotype by environment (G × E) interaction was detected across sites.ContextNordmann fir, a widely used Christmas tree species in Europe, has, since 2011, been increasingly damaged by a canker disease caused by Neonectria neomacrospora.AimsThe objective was to study the genetic variation and genotype by environment interaction in the susceptibility of Nordmann fir to N. neomacrospora.MethodsDamage caused by N. neomacrospora was evaluated using a visual scale in three Nordmann fir clonal seed orchards in Denmark, partly containing the same clones.ResultsDamage due to N. neomacrospora was substantial at all three sites, and no clone was completely resistant to N. neomacrospora, but a large genetic variation in the susceptibility was detected among clones. Estimates of single-site individual broad-sense heritability for susceptibility varied between 0.38 and 0.47. The average type-B genetic correlation for damage score across sites was 0.34.ConclusionGenetic variation was very pronounced, and significant G × E interactions were detected for susceptibility. Further investigations of narrow-sense heritability, expression of the trait in younger material, and identification of the cause of G × E for N. neomacrospora susceptibility in Nordmann fir across different sites are recommended.