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Understanding the genetic diversity and structure of invasive pathogens in source and in introduced areas is crucial to the revelation of hidden biological features of an organism, to the reconstruction of the course of invasions and to the establishment of effective control measures. Hymenoscyphus pseudoalbidus (anamorph: Chalara fraxinea) is an invasive and highly destructive fungal pathogen found on common ash Fraxinus excelsior in Europe and is native to East Asia. To gain insights into its dispersal mechanisms and history of invasion, we used microsatellite markers and characterized the genetic structure and diversity of H. pseudoalbidus populations at three spatial levels: (i) between Europe and Japan, (ii) in Europe and (iii) at the epidemic's front in Switzerland. Phylogenetic and network analysis demonstrated that individuals from both regions are conspecific. However, populations from Japan harboured a higher genetic diversity and were genetically differentiated from European ones. No evident population structure was found among the 1208 European strains using Bayesian and multivariate clustering analysis. Only the distribution of genetic diversity in space, pairwise population differentiation ( GST) and the spatial analysis of principal components revealed a faint geographical pattern around Europe. A significant allele deficiency in most European populations pointed to a recent genetic bottleneck, whereas no pattern of isolation by distance was found. Our data suggest that H. pseudoalbidus was introduced just once by at least two individuals. The potential source region of H. pseudoalbidus is vast, and further investigations are required for a more accurate localization of the source population. [ABSTRACT FROM AUTHOR]

We assessed the effects of ambient tropospheric ozone on annual tree-ring growth, δ13C in the rings, leaf gas exchange and visible injury in three ozone-sensitive woody plant species in southern Switzerland. Seedlings of Populus nigra L., Viburnum lantana L. and Fraxinus excelsior L. were exposed to charcoal-filtered air (CF) and non-filtered air (NF) in open-top chambers, and to ambient air (AA) in open plots during the 2001 and 2002 growing seasons. Ambient ozone exposures in the region were sufficient to cause visible foliar injury, early leaf senescence and premature leaf loss in all species. Ozone had significant negative effects on net photosynthesis and stomatal conductance in all species in 2002 and in V. lantana and F. excelsior in 2001. Water-use efficiency decreased and intercellular CO2 concentrations increased in all species in response to ozone in 2002 only. The width and δ13C of the 2001 and 2002 growth rings were measured for all species at the end of the 2002 growing season. Compared with CF seedlings, mean ring width in theAAandNFP. nigra seedlings was reduced by 52 and 46%, respectively, in 2002, whereas in V. lantana and F. excelsior, ring width showed no significant reductions in either year. Although δ13C was usually more negative in CF seedlings than in AA and NF seedlings, with the exception of F. excelsior in 2001, ozone effects on δ13C were significant only for V. lantana and P. nigra in 2001. Among species, P. nigra exhibited the greatest response to ozone for the measured parameters as well as the most severe foliar injury andwas the only species to showa significant reduction in ring width in response to ozone exposure, despite significant negative ozone effects on leaf gas exchange and the development of visible foliar injury in V. lantana and F. excelsior. Thus, significant ozone-induced effects at the leaf level did not correspond to reduced tree-ring growth or increased δ13C in all species, indicating that the timing of ozone exposure and severity of leaf-level responses may be important in determining the sensitivity of tree productivity to ozone exposure. [ABSTRACT FROM AUTHOR]