Ottmar Holdenrieder

Wood-decaying fungi in the forest: conservation needs and management options

Wood-decaying fungi are essential for the functioning of forest ecosystems. They provide habitat for many other organisms and enable the regeneration of forests throughout the world. Since wood decomposition is a decisive process in nutrient recycling, soil formation and the carbon budget of forest ecosystems, it is receiving increasing attention from forest ecologists, pathologists and managers. Research has focussed on the factors driving the species-richness of wood-decomposing organisms and is moving on to analyse the effects of this species-richness on ecosystem functioning. Coarse woody debris (CWD) and its associated wood-decaying organisms have been drastically reduced in abundance and diversity by forestry and so these features often have potential as conservation indicators. Protective measures at a landscape level are needed for threatened wood-inhabiting fungi. These include restricting salvage operations in windthrow stands, actively encouraging the accumulation of deadwood in forests, and facilitating decay in standing trees by inoculating them with fungi. Here, we aim to collect and summarize recently produced work on deadwood ecology, pointing out research gaps and perspectives.

Hymenoscyphus pseudoalbidus, the causal agent of European ash dieback

UNLABELLED The ascomycete Hymenoscyphus pseudoalbidus (anamorph Chalara fraxinea) causes a lethal disease known as ash dieback on Fraxinus excelsior and Fraxinus angustifolia in Europe. The pathogen was probably introduced from East Asia and the disease emerged in Poland in the early 1990s; the subsequent epidemic is spreading to the entire native distribution range of the host trees. This pathogen profile represents a comprehensive review of the state of research from the discovery of the pathogen and points out knowledge gaps and research needs. TAXONOMY Members of the genus Hymenoscyphus (Helotiales, Leotiomycetidae, Leotiomycetes, Ascomycota) are small discomycetes which form their ascomata on dead plant material. A phylogeny based on the internal transcribed spacers (ITSs) of the rDNA indicated the avirulent Hymenoscyphus albidus, a species native to Europe, as the closest relative of H. pseudoalbidus. SYMPTOMS Hymenoscyphus pseudoalbidus causes necrotic lesions on leaves, twigs and stems, eventually leading to wilting and dieback of girdled shoots. Bark lesions are characterized by a typical dark- to cinnamon-brown discoloration. LIFE CYCLE Hymenoscyphus pseudoalbidus is heterothallic and reproduces sexually on ash petioles in the litter once a year. Ascospores are wind dispersed and infect ash leaves during the summer. The asexual spores only serve as spermatia. TOOLS AND TECHNIQUES The most important techniques for fungal handling, such as detection, isolation, culturing, storage, crossing and ascocarp production, are briefly described. MANAGEMENT Once the disease is established, management is hardly possible. The occurrence of a small fraction of partially tolerant trees constitutes hope for resistance breeding in the future. Healthy-looking trees should be preserved.

Plant health and global change – some implications for landscape management

Global change (climate change together with other worldwide anthropogenic processes such as increasing trade, air pollution and urbanization) will affect plant health at the genetic, individual, population and landscape level. Direct effects include ecosystem stress due to natural resources shortage or imbalance. Indirect effects include (i) an increased frequency of natural detrimental phenomena, (ii) an increased pressure due to already present pests and diseases, (iii) the introduction of new invasive species either as a result of an improved suitability of the climatic conditions or as a result of increased trade, and (iv) the human response to global change. In this review, we provide an overview of recent studies on terrestrial plant health in the presence of global change factors. We summarize the links between climate change and some key issues in plant health, including tree mortality, changes in wildfire regimes, biological invasions and the role of genetic diversity for ecosystem resilience. Prediction and management of global change effects are complicated by interactions between globalization, climate and invasive plants and/or pathogens. We summarize practical guidelines for landscape management and draw general conclusions from an expanding body of literature.

Fungal associations of serially washed healthy non-mycorrhizal roots of Picea abies

The mycoflora of serially washed healthy fine roots of Norway spruce ( Picea abies ) from six climatically and edaphically different sites in Southern Bavaria was examined. More than 120 taxa were detected. Mycelium radicis atrovirens α was most frequently isolated and occurred in up to 70% of roots. Three main types of fungal associations were identified among sites. Soil pH and altitude had a decisive influence on the type of association. Fungal associations on roots in alkaline soil at low altitudes were dominated by Cylindrocarpon destructans with Cryptosporiopsis sp., Idriella lunata, Mortierella alpina, M. minutissima and Mucor hiemalis f. hiemalis as the other major components. A second type of fungal association formed on roots in peat. Trichoderma viride was the most typical representative of this association, although M. radicis atrovirens α was more frequent. M. radicis atrovirens α was also the dominant fungus of the third association, which formed in both acidic soil and alkaline soil at high altitude. The possible relationships between M. radicis atrovirens α or C. destructans and Norway spruce are discussed.

Networks in plant epidemiology: from genes to landscapes, countries, and continents

There is increasing use of networks in ecology and epidemiology, but still relatively little application in phytopathology. Networks are sets of elements (nodes) connected in various ways by links (edges). Network analysis aims to understand system dynamics and outcomes in relation to network characteristics. Many existing natural, social, and technological networks have been shown to have small-world (local connectivity with short-cuts) and scale-free (presence of super-connected nodes) properties. In this review, we discuss how network concepts can be applied in plant pathology from the molecular to the landscape and global level. Wherever disease spread occurs not just because of passive/natural dispersion but also due to artificial movements, it makes sense to superimpose realistic models of the trade in plants on spatially explicit models of epidemic development. We provide an example of an emerging pathosystem (Phytophthora ramorum) where a theoretical network approach has proven particularly fruitful in analyzing the spread of disease in the UK plant trade. These studies can help in assessing the future threat posed by similar emerging pathogens. Networks have much potential in plant epidemiology and should become part of the standard curriculum.

Characterisation of dark septate endophytic fungi (DSE) using inter-simple-sequence-repeat-anchored polymerase chain reaction (ISSR-PCR) amplification

Suitability and reproducibility of ISSR-PCR to find strain-specific and taxon specific markers for strains of the tree-root endophytes Phialocephala fortinii and ’Type 1’, a non-sporulating dematiaceous mycelium, were examined. The results were compared with data previously generated by isozyme analyses. P. fortinii and ’Type 1’ are two DSE taxa and are abundant colonisers of coniferous forest-tree roots in the North Temperate zone. ’Type 1’ was never observed to sporulate in pure culture but is well defined by its cultural characteristics. DNA of 14 strains per taxon was amplified with three short, 17-18-nucleotide-long, tandemly-repeated primers (CCA, CGA, ACA) with two (CCA) or three degenerated bases at their 5’-ends. The resulting DNA products were separated by agarose gel electrophoresis. The bands were scored for absence/presence, respectively, and the binary matrix subjected to multiple correspondence analysis (MCA). ISSR-PCR was found to be highly reproducible since amplification of DNA from several single-hyphal-tip cultures of the same strain resulted in identical banding patterns. Eighty-five (92.4%) of the 92 DNA fragments were polymorphic. The fragments ranged from 320 to 4100 bp. ISSR-PCR was found to be a powerful tool to find strain-specific and taxon-specific markers. Each strain showed a unique banding pattern and diagnostic bands for the two taxa could be identified. ISSR-PCR data correlated neither with the geographical nor the host origin of the strains. The strains grouped into similar clusters independently of whether MCA was performed with ISSR-PCR or isozyme data. However, ISSR-PCR allowed the differentiation of strains with the same allozyme phenotype.

Genetic structure of newly established populations of Cryphonectria parasitica.

The genetic structure of three newly established Cryphonectria parasitica populations (Choex, Weggis, Murg) was analysed and compared to an older, post-epidemic population (Claro). Vegetative compatibility (vc) type and DNA fingerprint analysis revealed an almost clonal C. parasitica population in Choex, Weggis and Murg. These stands are all situated in northern Switzerland, outside the main range of European chestnut. Only one vc type and one dominant DNA fingerprint was found in Choex and Murg. In Weggis, two vc types and two dominating DNA fingerprints were found. The European Cryphonectria hypovirus (CHV1) was not detected in these three populations. In contrast, the population in Claro, situated within the main range of European chestnut in southern Switzerland, had much higher vc type and DNA fingerprint diversity. DNA fingerprints were correlated to vc types in Claro and in Weggis. Mating type determination revealed one strongly dominating mating type in each of the three northern populations, but not in Claro. From these results we conclude that C. parasitica disseminated almost exclusively by means of asexual reproduction in Choex, Weggis and Murg, whereas in Claro sexual reproduction also played an important role. Additionally, founder effects and restricted gene flow were dominating factors in shaping the genetic structure of the three northern populations.

Forest health in a changing world.

Forest pathology, the science of forest health and tree diseases, is operating in a rapidly developing environment. Most importantly, global trade and climate change are increasing the threat to forest ecosystems posed by new diseases. Various studies relevant to forest pathology in a changing world are accumulating, thus making it necessary to provide an update of recent literature. In this contribution, we summarize research at the interface between forest pathology and landscape ecology, biogeography, global change science and research on tree endophytes. Regional outbreaks of tree diseases are requiring interdisciplinary collaboration, e.g. between forest pathologists and landscape ecologists. When tree pathogens are widely distributed, the factors determining their broad-scale distribution can be studied using a biogeographic approach. Global change, the combination of climate and land use change, increased pollution, trade and urbanization, as well as invasive species, will influence the effects of forest disturbances such as wildfires, droughts, storms, diseases and insect outbreaks, thus affecting the health and resilience of forest ecosystems worldwide. Tree endophytes can contribute to biological control of infectious diseases, enhance tolerance to environmental stress or behave as opportunistic weak pathogens potentially competing with more harmful ones. New molecular techniques are available for studying the complete tree endobiome under the influence of global change stressors from the landscape to the intercontinental level. Given that exotic tree diseases have both ecologic and economic consequences, we call for increased interdisciplinary collaboration in the coming decades between forest pathologists and researchers studying endophytes with tree geneticists, evolutionary and landscape ecologists, biogeographers, conservation biologists and global change scientists and outline interdisciplinary research gaps.

Spatial distribution of dark septate endophytes in a confined forest plot

In the present study we investigated the abundance and spatial distribution of dark septate root endophytes (DSE) in a 3 x 3 m plot in a spruce stand ( Picea abies ). A total of 144 DSE isolates were obtained by means of a hierarchical sampling design. Most roots were colonised, as DSE were isolated from 81.7% of root segments. ISSR-PCR fingerprinting was used to identify 21 unique ISSR types. Dominant types were isolated from adjacent points that covered an area of up to 6.8 m 2 of the study plot, and ISSR types were intermingled extensively. Frequency of isolation of the different ISSR types was uneven with two dominant types that accounted for 38% and 28% of all DSE isolates, respectively. Seven DSE strains representing six different ISSR types were identified as Phialocephala fortinii based on the morphology of fertile conidiophores and/or ITS 1 and 2 sequence comparisons.

Characterization of Guignardia mangiferae isolated from tropical plants based on morphology, ISSR-PCR amplifications and ITS1-5.8S-ITS2 sequences

Phenotypic and genotypic variability of 18 endophytic Guignardia strains from different host plants (Anacardiaceae: Anacardium giganteum, Myracrodruon urundeuva, Spondias mombin; Apocynaceae: Aspidosperma polyneuron; Ericaceae: Rhododendron sp.; Fabaceae: Bowdichia nitida; Leguminosae: Cassia occidentalis; Rutaceae: Citrus aurantium) growing in different sites in Brazil was assessed by means of morphometric measurements and inter-single-sequence-repeat-anchored polymerase chain reaction (ISSR-PCR) amplifications of the DNA. Morphology of conidia and ascospores and growth rates of the Brazilian isolates corresponded well with those of G. mangiferae. Multiple correspondence analysis (MCA) of the ISSR-PCR data yielded three groups of strains, which did, however, not correspond either to the host or to the geographic origin. The same individual tree hosted genotypically different strains indicating multiple infections. Phylogeny based on ITS1-5.8S-ITS2 sequence analyses confirmed conspecificity of the Brazilian isolates with G. mangiferae (syn. G. endophyllicola).