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Ash dieback is a recent widespread disease on ash (Fraxinus sp.) that is causing important economic and ecological losses throughout Europe. The disease is initiated by the ascomycetous fungus Hymenoscyphus pseudoalbidus (anamorph Chalara fraxinea). The main aim of this study was to investigate seasonal pattern of lesion development associated with ash dieback. We present data on the spread of 324 natural lesions in ash shoots, branches and stems surveyed over a 32 month period. Most lesions were active and showed the greatest rate of growth during the summer; however, lesions were active throughout the year. Tree mortality was high, with more than a third of the surveyed trees dying during the study. Although many lesions permanently ceased to develop, the rate at which new lesions emerged was greater than the rate at which lesions entered a resting phase. The most common cause for a lesion going into a permanent state of rest was that it had encountered a branch-base. Genotype analysis showed that multiple infections can occur in a single tree given that different genotypes were identified in different lesions as well as in single lesions. A weak positive correlation was noted between tree health and tree size and a weak negative correlation was noted between tree overall health and lesion activity. The lower limit for H. pseudoalbidus growth in culture was between 4.0°C and 0.5°C.

Summary1. Edges in landscapes have an effect on the abundance of many species but the underlying ecological mechanisms are poorly known for most taxonomic groups. One way to gain insight into the mechanisms is to examine how key demographic or metapopulation parameters are affected by proximity to edge. The main objective of this study was to investigate the effects of edge creation through clearcutting on the dynamics of forest species' metapopulations. 2. We used the red-listed epiphytic moss Neckera pennata as a model species. Based on data from repeated surveys of a metapopulation and its host tree network in a hemiboreal forest, we tested the effect of edge creation on key metapopulation parameters: rates of local colonization and extinction, local abundance growth and patch destruction through the fall of host trees. We predicted the long-term consequences of the edge effects using simulations with Bayesian statistical models. We also explored the potential effects of the pathogen Chalara fraxinea causing ash dieback, a tree disease currently spreading in Europe. 3. The colonization probability on host trees unoccupied by the moss increased with increasing connectivity to occupied trees. The growth of local populations on occupied trees decreased with increasing proximity to edge, and with initial local abundance. Stochastic extinctions of the epiphyte from standing trees were very rare and only occurred near the edge; most of the observed extinctions were deterministic due to tree fall. Tree fall decreased with increasing distance from the edge into the forest, and with increasing tree diameter. 4. Under edge conditions, simulations predicted decreases in the total number of host trees, number of occupied host trees, and in the total abundance of the epiphyte over a 30-year period. We suggest that ash dieback increases the tree fall rate and thereby the local extinction rate, leading to increased metapopulation extinction risk. 5. Synthesis and applications. The results show that small protected forest areas such as woodland key habitats may not allow long-term persistence of red-listed epiphytes if they are influenced by edge creation through clearcutting. Delineating uncut buffers of 50-100 m around the protected areas may alleviate such effects.

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Two viridin-related B-norsteroids, B-norviridiol lactone (1) and B-norviridin enol (2), both possessing distinct unprecedented carbon skeletons, were isolated from a liquid culture of the ash dieback-causing fungus Hymenoscyphus pseudoalbidus. Compound 2 was found to degrade to a third B-norsteroidal compound, 1β-hydroxy-2α-hydro-asterogynin A (3), which was later detected in the original culture. The proposed structure of 1 is, regarding connectivity, identical to the original erroneous structure for TAEMC161, which was later reassigned as viridiol. Compound 2 showed an unprecedented 1H–13C HMBC correlation through an intramolecular hydrogen bond. The five-membered B-ring of compounds 1-3 was proposed to be formed by a benzilic acid rearrangement. The known compound asterogynin A was found to be formed from 3 by a β-elimination of water. All compounds were characterized by NMR spectroscopy, LC-HRMS and polarimetry.

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