Outbreaks due to non-native invasive pests have become more frequent because of the increased movement of plants and agricultural products around the world, which takes place for commercial or research purposes. The introduction of these harmful organisms can cause serious damage to the agricultural production of a territory, representing one of the causes responsible for the genetic erosion of many plant species [1]. In reference to this, a recent example is the epidemic of olive quick disease syndrome (OQDS) caused by the bacterium Xylella fastidiosa that is native to the American continent, which was detected in 2013 in South Italy ^[2][3][2,3]. Prevention of the introduction of these harmful organisms is primarily accomplished through international quarantine regulations and adequate controls on transboundary shipments of plants/propagation material. Plant health regulations allow agents to exchange only plant material that meets specific phytosanitary requirements, particularly as regards their freedom from specific pests [4]. Verification of these requirements is achieved by inspecting batches of plants or propagation material through visual surveys for the presence of symptoms, and by means of diagnosis through laboratory analysis using serological or molecular tests. The policies of different countries regarding plant quarantines are in line with the International Convention for the Protection of Plants (IPPC) [5]. This convention is a multilateral treaty that currently includes 183 countries, which was signed on 6 December 1951 under the aegis of the Food and Agriculture Organization of the United Nations (FAO) and later revised in November 1997 [6]. It aims to coordinate international policies for the prevention of the introduction and spread of harmful organisms of plants, with the term “plants” meaning living plants and parts of them, seeds and germplasm [7]. The governing body of the IPPC is constituted by the Commission on Phytosanitary Measures (CPM) while the IPPC’s secretariat defines the recommended measures for plant protection constituted by the International Standards for Phytosanitary Measures (ISPMs) (https://www.ippc.int/core-activities/standards-setting/ispms, accessed on 19 October 2022). According to the IPPC, a “pest” is defined as “any species, strain or bio-type of plant, animal or pathogenic agent injurious to plants or plant products”, while a quarantine pest is a “pest of potential economic importance to an area and not yet present, or if present is not widely distributed and subject to official controls” [8]. The IPPC guidelines provide for a hierarchical approach that considers the prevention of the unintended entry of plant pests as a priority over the eradication and subsequent containment interventions [9]. The International Plant Protection Convention (IPPC) standards are also recognized by the World Trade Organization’s (WTO) through the Agreement on the Application of Sanitary and Phytosanitary Measures (SPS) [10]. The ISPMs of the IPPC constitute a reference for the Regional Organizations for the Protection of Plants (RPPOs), which have the role of harmonizing the phytosanitary measures between the National Plant Protection Organizations (NPPOs). NPPOs are services authorized by government authorities for the implementation of the guidelines issued by the IPPC. Regulatory measures can be established by the states based on the recommendations of these plant health organizations. The RPPO for the Euro-Mediterranean region is constituted by the European and Mediterranean Plant Protection Organization (EPPO), which was founded in 1951. The EPPO, in addition to coordinating IPPC guidelines between the member NPPOs, defines plant protection standards which can be used by the NPPOs of member states to establish laws on the prevention of the introduction of harmful organisms or to limit their spread in the case of entry. The EPPO also draws up alert lists, which are lists of pests that are potentially harmful to the Mediterranean area and that can be considered as quarantine pests. Periodically, following a process of pest risk analysis (PRAs), some of these listed organisms can be inserted on the EPPO A1 and A2 lists or removed from them if the relative risk is no longer judged to be high. These EPPO A1 and A2 lists include pests recommended to be regulated as quarantine pests. The EPPO A1 list includes exotic quarantine pests which are absent from the EPPO region; instead, the A2 list reports pests whose presence has already been reported in the territory of the EPPO region. In the latter list, the pathogen X. fastidiosa was included in 2017 following its discovery in Europe, which took place initially in southern Italy, which was followed by its detection in other areas in Europe [11]. Another function of the EPPO is to standardize the phytosanitary diagnostic procedures in the EPPO region, which is accomplished through the approval of diagnostic standards on plant pests and especially for quarantine pests. EPPO Diagnostic Standards are published in the EPPO Bulletin and uploaded to the EPPO database. They include both horizontal standards covering quality assurance on performing diagnostic tests, and specific diagnostic standards consisting of protocols for detecting specific pests. Diagnostic standards have also been established for the detection of X. fastidiosa [12]. The EPPO also manages the EPPO Global Database [13], which is a constantly updated database containing the EPPO’s standards and information on plant pest species or invasive alien plants, also providing the host plants and quarantine status for each pest.

Another European reference authority for the protection of plants from harmful organisms is the European Food Safety Authority (EFSA). The EFSA is an agency of the European Union established in 2002 with the function of supporting the decisions of the European institutions and governments concerning the protection of consumer health and food safety. One of the EFSA’s mandates is to participate in the protection of EU member states from the threat posed by plant pests. In this regard, the EFSA draws up pest survey cards containing pest-specific information and guidelines based on international standards to assist EU countries in plant pest surveys. The European Community’s phytosanitary regime complies with the principles of the IPPC and of the Food and Agriculture Organization of the United Nations (FAO). In consideration of the importance of phytosanitary policies to minimize the risk of spreading harmful organisms to plants, the phytosanitary legislation in force in the European community was reviewed in this work, with particular focus on the olive tree (Olea europaea L. subsp. europaea). Furthermore, the olive tree pests transmissible by propagation material and their relative current categorization in the European Union were also described.

Harmful agents of the olive tree include fungi, phytoplasm, viruses and virus-like agents, bacteria and nematodes, which can spread even over a long distance with the movement of the infected propagation material [14] and may be responsible for negative economic effects on the production of this crop [15]. Among the fungal pests, Verticillium dahliae Kleb. [16] is a vascular, soil-inhabiting pathogen responsible for verticillium wilt, which represents one of the main diseases of the olive tree [15]. The species of phytoplasma that affect the olive tree can cause alterations such as reduced growth, “witches’ broom” phenomena, and deformation and yellowing of the leaves. They include Candidatus Phytoplasma asteris, Candidatus Phytoplasma solani, Candidatus Phytoplasma ulmi and Candidatus Phytoplasma pruni ^[17][18][17,18]. The most important olive viruses are Olive leaf yellowing associated virus (OLYaV) [19], Olive yellow mottling and decline-associated virus (OYMDaV) [20], Olive latent virus-1 (OLV-1) [21], Olive latent virus-2 (OLV-2) [22], Olive latent virus 3 (OLV-3) [23], Tobacco mosaic virus (TMV) [24], Olive vein yellowing-associated virus (OVYaV) [25], Olive mild mosaic virus (OMMV) [26], Arabis mosaic virus (ArMV) [27], Cherry leaf roll virus (CLRV) [28], the soil-borne virus Strawberry latent ring spot virus (SLRSV) [27], Cucumber mosaic virus (CMV) [29], Tobacco necrosis virus Strain D (TNV-D) [30], Olive semilatent virus (OSLV) [31] and Olive latent ringspot virus (OLRSV) [32]. SLRSV, ArMV and TNV are soil-borne; CLRV and OLV-1 can be transmitted by the seeds; CMV is transmitted by aphids; and OLV-2 and OLRSV are transmitted both mechanically and by grafting [33]. Furthermore, two new olive-infecting viruses have recently been discovered, the Olea europaea geminivirus (OEGV) [34] and the olive virus T (OlVT) [35]. In olive trees, viral infections are often asymptomatic, and some viruses can lead to symptoms only in some cultivars while remaining latent in others, as in the case of SLRSV, which is the etiological agent of the infection that has the name of bumpy fruit ^[36][37][36,37]. The development of symptoms also occurs due to infections of the OVYaV, OYMDaV and OLYaV viruses, which are responsible for leaf yellowing complex [37], and by strains of TMV and OSLV isolated from olive trees with symptoms of vein banding and vein clearing diseases, respectively [15]. Moreover, in some cases, olive tree viruses can determine a reduction in growth and rhizogenic capacity [37]. Although these viruses can remain latent or cause negligible damage, some of them can severely affect other plant species; in particular, SLRSV can cause Black line disease in walnut [33]. The olive tree is also affected by some virus-like diseases of unknown etiology, for some of which, transmission by graft has been successfully achieved. These include partial paralysis, foliar deformation, sickle leaf, infectious yellowing, spherosis and bark cracking ^[38][39][38,39]. Among the bacteria, Pseudomonas savastanoi pv. savastanoi (E.E. Smith) is a pathogen that causes a widely diffused alteration in olive knot disease [15]. In addition, the phytopathogenic bacterium Xylella fastidiosa [40] is present in Southern Italy, where a new isolate of X. fastidiosa subsp. pauca, classified as the ST53 sequence type ^[41][42][41,42], was found to be associated with olive quick disease syndrome (OQDS) [3]. Infections caused by X. fastidiosa on olive trees have also been reported in Argentina and Brazil, which were caused by two different sequence types of X. fastidiosa subsp. pauca, consisting, respectively, of ST69 ^[43][44][43,44], and ST16 [45], and have also been reported in California due to X. fastidiosa subsp. multiplex [46]. The olive tree is also affected by several species of nematodes including the species Meloidogyne arenaria Chitwood [47], Meloidogyne incognita (Kofoid & White) Chitwood [48], Meloidogyne javanica Chitwood [49], Pratylenchus vulnus [50] and Xiphinema diversicaudatum [51]. Among these, X. diversicaudatum is also responsible for the transmission of ArMV and SLRSV [52], while M. incognita and P. vulnus favor the penetration of Verticillium dahliae, causing injuries to the plants’ roots [52].