Modeling the potential spread of Xylella fastidiosa can document contingency plans in northern Europe, so far uncolonized by the bacterium. Through mark-release-recapture (MRR) and flight-mill experiments, the flight capacity of two potential vectors for temperate Europe was studied: Philaenus spumarius, the reported southern European vector, and Aphrophora salicina, a xylem-specialist feeding on potential host plants of X. fastidiosa. Aphrophora salicina displayed significantly better flight performances than P. spumarius. In flight-mills, the average distance flown was, respectively, 623 m vs. 102 m and the maximal distance flown was 6.16 km vs. 1.54 km in 2.5 h. In MRRs, A. salicina travelled more than 30 m in a single flight, with a maximal interception distance of 80 m after two days, highlighting that dispersal is driven by connectivity and host plant quality. Philaenus spumarius mainly jumped, with 1 m movements in length and a maximal interception distance of 32 m in 27 days. Models estimated P. spumarius' daily mean dispersal at 1.5 m and A. salicina's at 3.5 m. Although only a small part of the population moves over very long distances, this pool of efficient insects could already be sufficient to effectively spread an epidemic. As Salicaceae have been reported as host plants of the bacterium, the association of Aphrophoridae and Salicaceae in riparian areas could create bacterium reservoirs in corridors allowing for transportation over medium to long distances.

The sentinel plantation concept consists of assessing the impact of exotic factors, such as pests and pathogens, on plants of interest by planting them out of their native range. This tool is a way to enhance knowledge for pest risk analysis (PRA) by guiding decisions on how quarantine organisms should be regulated and where to focus prevention and surveillance efforts for an early detection. In this study, the sentinel method was used in the case of research on Xylella fastidiosa, a plant pathogenic bacterium that has recently been found established in southern Europe, but whose potential impact and possible host range are still poorly documented in northern areas where the bacterium is not known to occur. To improve knowledge on the susceptibility of potential hosts of X. fastidiosa in northern Europe, a sentinel plantation of Prunus domestica cv. Opal, Quercus petraea and Salix alba was established in the X. fastidiosa-infected area of Majorca. In order to assess the circulation of the bacterium in the sentinel plot and around it, surveys of the local flora and insect vectors were carried out, as well as the planting of a network of rosemary "spy plants". Symptomatic monitoring and molecular analyses were performed on the sentinel plants for four years. During these years, X. fastidiosa was never detected in our sentinel plants most likely because of the low infectivity pressure recorded in the surroundings. This study underlines the complexity of conducting sentinel plantation assays combined with X. fastidiosa research, highlighting the need for long-term investigation and questioning the efficiency of the sentinel tool. However, this study is placed in perspective with other valuable sentinel plantations. It also highlights the complementarity of the tool and proposes elements to improve or reorient the implementation of future sentinel projects.

The discovery of several strains belonging to three subspecies of Xylella fastidiosa in Europe has triggered major attention to the potential spread up north of the bacteria. It is essential to assess the susceptibility of the previously unexposed European flora to this pathogen. Under biosafety facility, we evaluated the susceptibility of Salicaceae such as Populus tremula, Populus canescens, Salix alba and Salix caprea by mechanically inoculating the KLN59.3 GFP-labelled X. fastidiosa at 22 °C and at 28 °C. Bacterial movement and multiplication in plants were investigated by PCR, real-time PCR, confocal or scanning electron microscopy. Nine months post-inoculation, 100% of the plants tested positive for X. fastidiosa, with the exception of 57% for P. canescens under the 22 °C-growing conditions. Bacteria were detected up to 120 cm from the inoculation point for S. alba. They were detected in the roots of all species and were successfully isolated for S. alba and P. tremula. Estimates of average CFU/g of plant tissue per species ranged from 1.5E + 03 to 3.5E + 06, with the lowest figures for P. canescens and the highest for P. tremula together with high number of totally obstructed vessels observed by confocal microscopy. The possibility of insect transmission was also evaluated using an experimental set up based on Majorca Island. There, transmission by P. spumarius of both X. fastidiosa ST1 and ST81 was proven on S. alba. We thus demonstrated that indigenous European Salicaceae such as S. alba or P. tremula are new potential hosts for X. fastidiosa.

The sentinel plantation concept consists of assessing the impact of exotic factors, such as pests and pathogens, on plants of interest by planting them out of their native range. This tool is a way to enhance knowledge for pest risk analysis (PRA) by guiding decisions on how quarantine organisms should be regulated and where to focus prevention and surveillance efforts for an early detection. In this study, the sentinel method was used in the case of research on Xylella fastidiosa, a plant pathogenic bacterium that has recently been found established in southern Europe, but whose potential impact and possible host range are still poorly documented in northern areas where the bacterium is not known to occur. To improve knowledge on the susceptibility of potential hosts of X. fastidiosa in northern Europe, a sentinel plantation of Prunus domestica cv. Opal, Quercus petraea and Salix alba was established in the X. fastidiosa-infected area of Majorca. In order to assess the circulation of the bacterium in the sentinel plot and around it, surveys of the local flora and insect vectors were carried out, as well as the planting of a network of rosemary “spy plants”. Symptomatic monitoring and molecular analyses were performed on the sentinel plants for four years. During these years, X. fastidiosa was never detected in our sentinel plants most likely because of the low infectivity pressure recorded in the surroundings. This study underlines the complexity of conducting sentinel plantation assays combined with X. fastidiosa research, highlighting the need for long-term investigation and questioning the efficiency of the sentinel tool. However, this study is placed in perspective with other valuable sentinel plantations. It also highlights the complementarity of the tool and proposes elements to improve or re-orientate the implementation of future sentinel projects.

The discovery of three subspecies of Xylella fastidiosa in Europe has triggered major attention on the potential spread up North of the bacteria. Assessing the susceptibility of a previously unexposed European flora is a key element that remains widely unknown. Under biosafety facility, we evaluated the susceptibility of Salicaceae such as Populus tremula, Populus canescens, Salix alba and Salix caprea by mechanically inoculating the KLN59.3 GFP-labelled X. fastidiosa at 22 C and at 28 C. Bacterial movement and multiplication in plants were investigated by PCR, real-time PCR, confocal or scanning electron microscopy. Nine months post-inoculation, 100 % of the plants tested positive for X. fastidiosa, with the exception of 57% for P. canescens under the 22 C-growing conditions. Bacteria were detected up to 120 cm from the inoculation point for S. alba. They were detected in the roots of all species and were successfully isolated for S. alba and P. tremula. Estimates of average CFU/g of plant tissue per species ranged from 1.5E + 03 to 3.5E + 06, with the lowest figures for P. canescens and the highest for P. tremula together with high number of totally obstructed vessels observed by confocal microscopy. The possibility of insect transmission was also evaluated using an experimental set up based on Mallorca Island. There, transmission by P. spumarius of both X. fastidiosa ST1 and ST81 was proven on S. alba. We thus demonstrated that indigenous European Salicaceae such as S. alba or P. tremula are new potential hosts for X. fastidiosa.