In agreement with Article 6(2) of the Regulation (EU) 2016/2031 on protective measures against pests of plants, the European Commission has been tasked by the Council and European Parliament to establish a list of Union quarantine pests which qualify as priority pests. The prioritisation is based on the severity of the economic, social and environmental impact that these pests can cause in the Union territory. The Commission's Joint Research Centre (JRC) is in charge of developing a methodology based on a multi‐criteria decision analysis and composite indicators. In this context, EFSA has provided technical and scientific data related to these pests, in particular: (i) the potential host range and distribution of each of these pests in the Union territory at the level of NUTS2 regions; (ii) parameters quantifying the potential consequences of these pests, e.g. crop losses in terms of yield and quality, rate of spread and time to detection. Expert knowledge elicitation methodology has been applied by EFSA in order to provide those parameters in a consistent and transparent manner.

The Plant Health Panel reviewed the paper by Guarnaccia et al. (2017) and compared their findings with previous predictions on the establishment of Phyllosticta citricarpa. Four species of Phyllosticta were found by Guarnaccia et al. (2017) in Europe. P. citricarpa and P. capitalensis are well-defined species, with P. citricarpa recorded for the first time in Europe, confirming predictions by Magarey et al. (2015) and EFSA (2008 that P. citricarpa can establish in some European citrusgrowing regions. Two new species P. paracitricarpa and P. paracapitalensis were also described, with P. paracitricarpa (found only in Greece) shown to be pathogenic on sweet orange fruits. Genotyping of the P. citricarpa isolates suggests at least two independent introductions, with the population in Portugal being different from that present in Malta and Italy. P. citricarpa and P. paracitricarpa were isolated only from leaf litter in backyards. However, since P. citricarpa does not infect or colonise dead leaves, the pathogen must have infected the above living leaves in citrus trees nearby. Guarnaccia et al. (2017) considered introduction to be a consequence of P. citricarpa having long been present or of illegal movement of planting material. In the Panel's view, the fruit pathway would be an equally or more likely origin. The authors did not report how surveys for citrus black spot (CBS) disease were carried out, therefore their claim that there was no CBS disease even where the pathogen was present is not supported by the results presented. From previous simulations, the locations where Guarnaccia et al. (2017) found P. citricarpa or P. paracitricarpa were conducive for P. citricarpa establishment, with number of simulated infection events by pycnidiospores comparable to sites of CBS occurrence outside Europe. Preliminary surveys by National Plant Protection Organisations (NPPOs) have not confirmed so far the findings by Guarnaccia et al. (2017) but monitoring is still ongoing.