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Bursaphelenchus xylophilus, the pinewood nematode and causal agent of pine wilt disease (PWD), is a globally destructive pine pest. PWD is favoured by high temperatures and drought, but B. xylophilus can also persist in symptomless hosts in areas not prone to PWD. We compared seedlings of different Pinus species that were inoculated with B. xylophilus when well‐watered and under moderate temperatures, conditions that favour sublethal infections. We compared isolates of B. xylophilus in south‐west Europe (Portugal) and south‐east USA (Louisiana). P. pinea, P. pinaster, and P. radiata were challenged in both areas with inoculations of local B. xylophilus. P. sylvestris in Portugal and P. palustris in Louisiana were also tested. Seedling mortality was low, as expected, but nematode establishment was common. There were higher nematode densities and more damage in seedlings of P. sylvestris and P. radiata, followed by P. pinaster, with P. pinea and P. palustris proving to be more tolerant. Life history of hosts was a better predictor of resistance to B. xylophilus than geographical origin, even under sublethal conditions. Fast‐growing species (P. radiata and P. pinaster) appeared most likely to be symptomless carriers in areas not prone to the development of PWD. Cold‐adapted species (P. sylvestris) may still be suitable hosts for the pathogen, even at the sublethal conditions of their distribution areas. There were more nematodes per gram of host using nematode isolates from Portugal. Nonetheless, our results opposed the hypothesis that North American pinewood nematodes have such low virulence that they are obligatorily saprophytic. [ABSTRACT FROM AUTHOR]

Highlights • We investigated if there is a spatial structure of pine forest decline in Portugal. • Pine forest decline was spatially aggregated, presenting spatial autocorrelation. • Forest decline patterns could not be dissociated from climate effects. • Landscape diversity and forest density also seem to have an impact on pine decline. Abstract Context The pinewood nematode, Bursaphelenchus xylophilus , has been identified as being responsible for the decline of pine forests in Portugal, since its detection in the territory about 20 years ago. Forest decline due to disease and/or pests is a complex phenomenon, and a knowledge of the spatial patterns of tree mortality is important for providing an understanding of forest susceptibility and pest dynamics. Objectives The aim of the present study was to investigate the relationship between the spatial structure of pine mortality and that of different climatic, edaphic and landscape variables in an area of Portugal where B. xylophilus is known to have spread during the last decade. Methods Pine mortality was assessed in the field, in a large, complex area of Central-North Portugal. An ArcGis database with a range of environmental variables was created for the study area. Spatial autocorrelation of the different variables was investigated by means of Moran's I and Mantel r analysis. The relationship between pine decline and the different variables was analyzed using bivariate Moran's I. Results Environmental descriptors exhibit a clear spatial pattern, influenced by an undulating landscape. Pine mortality is spatially aggregated; warm, dry locations with higher evapotranspiration present high values, and in areas of extensive pine occupancy in a diverse landscape mortality decreases. Conclusions Forest mortality attributed to infestation by a non-native forest pathogen cannot be dissociated from the effects of climate and landscape diversity associated with pine forests. The analysis of landscape patterns appears to be crucial for gaining an understanding of forest pathology and decline. [ABSTRACT FROM AUTHOR]

Bursaphelenchus xylophilus is an insect-vectored pathogen and the causal agent of the pine wilt disease (PWD). In Portugal, where B. xylophilus is a recent invasive pest, the native pine sawyer beetle, Monochamus galloprovincialis, is its only vector. Epidemics of PWD emerge from three-way interactions among B. xylophilus , M. galloprovincialis , and host pine trees. However, the basic ecology of these interactions is poorly known. We studied M. galloprovincialis with respect to its life history, host selection, and phoresy by B. xylophilus in an area severely affected by PWD in Portugal. We assessed: (1) seasonal patterns of phoresy related to the flight phenology of the vector; and (2) distribution of M. galloprovincialis within and among host pine trees. Monochamus galloprovincialis in Portugal flies from May through October, and is probably bivoltine, but phoresy by B. xylophilus only occurs within a short period in early summer, when infection of new pine hosts most likely occurs. Peaks of M. galloprovincialis emergence and B. xylophilus phoresy immediately precede onset of the Mediterranean dry season, favoring PWD because B. xylophilus succeeds best on trees suffering from severe water deficits. The abundance of phoretic nematodes was more evident when their vectors were trapped as free flying adults rather than emerging from dying trees. Bursaphelenchus xylophilus were highly aggregated on a subset of M. galloprovincialis , which were aggregated in a subset of dying trees. However none of the host parameters related to size and bark thickness, could explain host selection. Despite its importance for PWD, M. galloprovincialis is a minor component of the bark and wood-boring insects of pine trees in Portugal. Interactions within this community are likely important to the epidemiology of PWD. [ABSTRACT FROM AUTHOR]