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Pines are major components of native forests and plantations in Europe, where they have both economic significance and an important ecological role. Diseases of pines are mainly caused by fungal and oomycete pathogens, and can significantly reduce the survival, vigor, and yield of both individual trees and entire stands or plantations. Pine pitch canker (PPC), caused by Fusarium circinatum (Nirenberg and O’Donnell), is among the most devastating pine diseases in the world, and is an example of an emergent invasive disease in Europe. The effects of microbial interactions on plant health, as well as the possible roles plant microbiomes may have in disease expression, have been the focus of several recent studies. Here, we describe the possible effects of co-infection with pathogenic fungi and oomycetes with F. circinatum on the health of pine seedlings and mature plants, in an attempt to expand our understanding of the role that biotic interactions may play in the future of PPC disease in European nurseries and forests. The available information on pine pathogens that are able to co-occur with F. circinatum in Europe is here reviewed and interpreted to theoretically predict the effects of such co-occurrences on pine survival, growth, and yield. Beside the awareness that F. circinatum may co-occurr on pines with other pathogens, an additional outcome from this review is an updating of the literature, including the so-called grey literature, to document the geographical distribution of the relevant pathogens and to facilitate differential diagnoses, particularly in nurseries, where some of them may cause symptoms similar to those induced by F. circinatum. An early and accurate diagnosis of F. circinatum, a pathogen that has been recently introduced and that is currently regulated in Europe, is essential to prevent its introduction and spread in plantings and forests. [ABSTRACT FROM AUTHOR]

The North American-introduced fungal plant pathogen Heterobasidion irregulare has become invasive in pine stands of central Italy and has broadly hybridized with the native congeneric species H. annosum. In this study, by genotyping Heterobasidion fruiting bodies and mycelia in pine logs inoculated with both fungal species, we showed that H. irregulare developed fruiting bodies at a 1.9-fold higher frequency when spatially overlapping with H. annosum than when by itself. In spite of different fruiting rates, all fruiting bodies were morphologically identical, independently of where they were formed, indicating that increased fruiting rate is likely to increase production of spores. Although all possible nuclear–mitochondrial combinations were identified in hybrids formed in inoculated pine logs, hybrids with nuclei of both species and the H. irregulare mitochondrion were favoured, while hybrids with both nuclei of one species and mitochondria of the other species were less frequent. Based on these results, predictions on the wider invasion of H. irregulare in Europe and recommendations for its containment are formulated. [ABSTRACT FROM AUTHOR]

The North American fungal pathogen Heterobasidion irregulare is currently distributed in pine and oak stands along 103 km of coastline west of Rome, Italy. This paper reviews and expands the knowledge on impacts, pathways of introduction and invasion, factors driving the invasion and on the dispersal abilities of this pathogen in Italy. Further, an integrated disease management program to minimize the spread of the fungus in Europe is suggested, based both on the published literature and on new findings reported here. Observational and genetic evidence support a single introduction through infected wood during World War II, and a subsequent invasion through spore dispersal. Experimental evidence suggests transmission potential of the pathogen rather than hyper-susceptibility of native hosts is the major determinant of invasion. The current range of H. irregulare is too vast to suggest eradication, however, we recommend minimizing the risk of spread of H. irregulare outside the zone of infestation while reducing the magnitude of infestations within its current range. We provide evidence suggesting the most cost-effective management approach hinges on preventing the saprobic establishment of the fungus in stumps in a ‘buffer’ area surrounding the current zone of infestation. [ABSTRACT FROM PUBLISHER]

The recently emerged plant pathogen Phytophthora ramorum is responsible for causing the sudden oak death epidemic. This review documents the emergence of P. ramorum based on evolutionary and population genetic analyses. Currently infection by P. ramorum occurs only in Europe and North America and three clonal lineages are distinguished: EU1, NA1 and NA2. Ancient divergence of these lineages supports a scenario in which P. ramorum originated from reproductively isolated populations and underwent at least four global migration events. This recent work sheds new light on mechanisms of emergence of exotic pathogens and provides crucial insights into migration pathways. [ABSTRACT FROM AUTHOR]

Abstract: In California and Oregon, two recently described oomycete forest pathogens, Phytophthora nemorosa and P. pseudosyringae, overlap in their host and geographic ranges with the virulent P. ramorum, causal agent of “sudden oak death.” Epidemiological observations, namely broader geographic distribution and lack of landscape-level mortality, led to the hypothesis they are native to this region, whereas multiple lines of evidence indicate P. ramorum is exotic to North America. We used AFLP analysis to measure genetic variability in the homothallic P. nemorosa and P. pseudosyringae and to evaluate the hypothesis of endemism. We analysed 39 P. nemorosa and 48 P. pseudosyringae isolates (29 American and 19 European) from throughout their geographic and host ranges. In the US, both P. nemorosa and P. pseudosyringae have a dominant AFLP clone with several closely related variants. There is no evidence that genetic diversity is partitioned by host or location in P. nemorosa, but the US P. pseudosyringae clonal lineage is largely nested within a more genetically variable European group. Though the absence of highly variable sampled source populations does not allow us to determine whether each species is native or introduced in the western US with certainty, the results are most consistent with the hypothesis that both are introduced —P. pseudosyringae perhaps from Europe. Invasive Phytophthora species are increasingly being implicated in emergent forest diseases, highlighting the need to identify and characterize both native and previously unknown introduced forest Phytophthoras. [Copyright &y& Elsevier]

The genetic structure within and between USA and European populations of the emerging phytopathogen Phytophthora ramorum was examined. Four primer combinations were used for amplified fragment length polymorphism (AFLP) fingerprinting of 67 USA isolates from California and Oregon, and 18 European isolates from Belgium, Germany, The Netherlands, Spain and the UK. In addition, three DNA regions (ITS, cox II, and nad 5) of additional Phytophthora species were amplified by polymerase chain reaction, sequenced, and analysed to provide better phylogenetic understanding of P. ramorum within the genus Phytophthora. AFLP banding patterns indicate that the 85 isolates form two distinct lineages within a monophyletic group, distinct from the closely related outgroup species P. lateralis. With the exception of two isolates from an Oregon nursery, European and USA isolates clustered separately within individual clades. The AFLP profiles also indicate that a single clonal lineage dominates the North American population, while the European population consists of an array of mainly unique, closely related AFLP types. Sequences from the three DNA regions were identical among all P. ramorum isolates, and phylogenetic analysis indicates that P. ramorum is closely related to P. lateralis and P. hibernalis.