BACTERIAL DISEASES THAT MAY OR DO EMERGE, WITH (POSSIBLE) ECONOMIC DAMAGE FOR EUROPE AND THE MEDITERRANEAN BASIN: NOTES ON EPIDEMIOLOGY, RISKS, PREVENTION AND MANAGEMENT ON FIRST OCCURRENCE
Bacterial diseases are difficult to control (both chemically and biologically), and are restrained primarily by preventive measures. Most important risk factors for the introduction or spread of bacterial diseases in Europe and the Mediterranean basin are imported infected planting material and infected insect vectors. In this review the epidemiology, management and main risks of several emerging bacterial diseases approaching or already present in Europe, their causal organisms and vectors will be highlighted, especially: (a) Citrus huanglongbing (= Citrus greening), caused by the heat-tolerant "Candidatus Liberibacter asiaticus" and heat-sensitive "Candidatus L. africanus". Both liberibacters and the respective psyllid vectors Diaphorina citri and Trioza erytreae are present in the Arabian peninsula, with recent reports of huanglongbing occurring in Iran, Mali, Ethiopia and Somalia. T erytreae is already present on some Atlantic Ocean islands; (b) leaf scorch and leaf scald diseases of grape and different fruit and ornamental trees, caused by Xylella fastidiosa. For this pathogen, although its presence has not been confirmed in Europe or the Mediterranean basin, local possible vectors such as Cicadella viridis and Philaenus spumarius occur; (c) Citrus canker caused by Xanthomonas citri pv. citri, the most severe form of which, the so-called Asiatic, is already present in Iraq, Iran, Oman, Somalia, United Arab Emirates (UAE), Saudi Arabia, Yemen and Reunion. Outbreaks and/or risk and (possible) emerging character of some other bacterial pathogens not yet present in Europe (i-iv) or already present (v-xvi) are also highlighted: (i) black spot of mango, Xanthomonas citri pv. man giferindicae, present in UAE and Reunion; (ii) bacterial blight of pomegranate, X. axonopodis pv. punicae, emerging in India; (iii) bacterial blight of guava, Erwinia psidii, emerging in Brazil; (iv) bacterial spot of passion fruit, X. campestris pv. passiflorae, emerging in Brazil; (v) stem rot and leaf spot of maize and center rot of onion, Pantoea ananatis, an emerging problem for onion in the USA, isolated from seed in South Africa; (vi) almond witches' broom, "Candidatus Phytoplasma phoenicium" killing thousands of trees in Lebanon and Iran; (vii) potato stolbur, "Candidatus Phytoplasma solani", spreading from Eastern Europe westwards; (viii) zebra chip disease of potato and yellows of carrot caused by "Candidatus Liberibacter solanacearum (synonym Ca. Liberibacter psyllourous). The potato strains occurs only in North and Central America and New Zealand, but the carrot pathogen has been reported from several Scandinavian countries and Spain with the respective psyllid vectors Bactericera cockerelli and Trioza apicalis; (ix) an apparently 'harmless' "Candidatus Liberibacter europaeus" found in the pear psyllid (Capopsylla pyri) in Italy; (x) bacterial fruit blotch of cu-curbits, Acidovorax citrulli, seed-transmitted and the cause of outbreaks in Europe, Turkey and Israel; (xi) a new strain of the potato stem rot bacterium, provisionally named Dickeya solani, emerging in several northwestern European countries and Israel; (xii) Stewart's disease or bacterial wilt of maize, P. stewartii subsp. stewartii, spread by the corn flea beetle Chaetocnema pulicaria, observed in several European countries in which it has not become established due to the absence of vector; (xiii) renewed outbreaks from 2008 of Pseudomonas syringae pv. actinidiae, the agent of bacterial blight, especially on Actinidth chinensis (yellow kiwifruit) but also on A. deliciosa in central Italy and, since 2010, in France; (xiv) bleeding canker of horse chestnut, Ps. syringae pv. aesculi, emerging in western Europe; (xv) bacterial canker of stone fruits caused by X. arboricola pv. pruni, with recent outbreaks in Switzerland, Spain (on almond) and in the Netherlands on cherry-laurel (Prunus laurocerasus); (xvi) bacterial leaf spot of poinsettia, X. axonopodis pv. poinsettiicola observed in greenhouses in several north-western European countries. Ornamental and wild hosts may play an important role in spreading diseases and maintaining the pathogens and their vectors in the environment. These plants should be included in surveys. Rapid and reliable diagnosis remains a key issue, as well as breeding for resistance. All pathogens mentioned are emerging threats, with real risks of introduction and, in some cases, closely approaching or already present in the Mediterranean basin and/or Europe. Introduction of susceptible wild hosts and susceptible cultivars of cultivated hosts must be avoided, as they often lead to introduction of pathogens or to outbreaks due to a 'jump' of local pathogen strains to very susceptible cultivars. Introduction of vectors should also be prevented as history teaches that vectors appear first and the pathogens a few years later. Examples of these events are given in the description of the various disease-pathogen combinations. An efficient prevention and control strategy of the diseases mentioned in this review should be based on the so-called pathway protection, i.e. regulatory systems ensuring importation of plant material free of all quarantine and regulated non-quarantine pests and practically free of non-regulated pests, where the following conditions should be enforced and controlled: (i) place of production should have integrated pest management practices; (ii) pre-export treatments if necessary; (iii) clean growing media associated with plants; (iv) proper waste management; (v) availability of expert diagnostic services; (vi) inspections at growing sites and (vii) clean packing practices.