XYLELLA FASTIDIOSA: ITS BIOLOGY, DIAGNOSIS, CONTROL AND RISKS
The bacterium Xylella fastidiosa, a xylem-inhabiting, vector-transmitted, Gram-negative, very slow growing bacterium, was cultured and properly described for the first time in 1987 in the USA as the cause of Pierce's disease (PD) of grapevine, Vitis vinifera (disease observed already in 1884) and as the cause of phony peach disease (PPD) in peach, Prunus persica (disease observed in 1890 in the USA) and in 1993 in Brazil as the cause of citrus variegated chlorosis (CVC) or citrus X disease. Moreover, it was found that the bacterium also causes a number of so-called leaf scorch diseases in Prunus spp. (including almond leaf scorch or ALS in Prunus amygdalus and plum leaf scald or PLS in Prunus domestica), Acer spp., Carya illinoinensis (pecan.), Coffea arabica (CLC, in Brazil isolated in 1995 and also pathogenic to Citrus), Hedera helix, Morus rubra, Nerium oleander (OLS), Platanus occidentalis, Quercus spp., and Ulmus americana. It infects also Medicago saliva (alfalfa dwarf) and Vinca major (wilting symptoms). Many wild plants may carry the pathogen with, but more often without showing symptoms, such as grasses, sedges and trees. A list of main hosts is presented. All these diseases are not seed-borne and occur mainly in tropical/subtropical areas, although leaf scorch diseases also occur in much colder climate, e.g. oak leaf scorch in eastern North America up to Canada. Several pathogenic varieties of the bacterium have been described, that are often host-specific (e.g., the PD strain will not cause disease if introduced to peach or plum). The following subspecies have been described: (i) Xylella fastidiosa subsp. fastidiosa (erroneously named X. f subsp. piercei), PD and LSA, strains from cultivated grape, alfalfa, almond, and maple; (ii) X. fastidiosa subsp. multiplex, PPD, PLS, strains from peach, elm, plum, pigeon grape, sycamore and almond; (iii) X. fastidiosa subsp. pauca, CVC, strains from citrus and probably those from coffee (CLC); (iv) X. fastidiosa subsp. sandyi, strains from Nerium oleander (OLS); (v) X. fastidiosa subsp. tashke, strains from the ornamental tree Chitalpa tashkentensis. Vectors are mainly sharpshooters and froghoppers or spittlebugs (Cicadellidae) that lack a latent period, and have no transstadial or transovarial transmission of the bacterium. The pathogen shows persistence in the vector adults, and ability to multiply in the foregut. In North America main vectors (for PD unless indicated) are Cuerna costalis (PPD), Draculacephala minerva (green sharpshooter) important also in ALS in California; Graphocephala atropunctata (blue-green sharpshooter), most important before the introduction of the glassy winged sharpshooter; G. versuta (PPD); Hordnia circel-lata, most efficient; Homalodisca vitripennis [formerly H. coagulata (glassy-winged sharpshooter or GWSS)]; H. insolita (PPD), Oncometopia nigricans, O. orbona (PPD), Xyphon fulgida [formerly Carneocephala fulgida (red-headed sharpshooter)]. CVC vectors in Brazil are Acrogonia terminalis, that lays eggs externally on leaves, Dilobopterus costalimai and Oncometopia fascialis. Local possible vectors for Europe are Cicadella viridis and Philaenus spumarius (meadow spittle bug). X. fastidiosa is an emerging threat in the south-west USA, mainly due to recent establishment of H. vitripennis, providing much more efficient transmission than local vectors, and leading to very serious outbreaks of PD in grapevine, ALS and OLS. GWSS probably first entered California as eggs in plants. The eggs are deposited into plant tissues. In Central and South America X. fastidisa has become very noxious due to the rapid expansion (most likely via distribution of infected planting material) of CVC in Citrus, leading to more than a third of all trees in the area having symptoms of CVC, and CLC in coffee. For Europe there are until now only a few unconfirmed reports of the presence of X. fastidiosa in grapevine from Kosovo [erroneously mentioned as Slovenia in Janse (2006)] and in France, based on disease symptoms observation. Since X. fastidiosa has more that 150 hosts and many of them, including Vitis planting material, were and are imported, risk of introduction (especially in latent form) must not be underestimated. Absence of the diseases caused by X. fastidiosa will mainly be due to the absence of suitable vectors. However, introduction of the pathogen and vectors with plant material can not be excluded for certain. Moreover, also local Cicadellidae (see above) could become (potential) vectors. Therefore, X. fastidiosa has the A1 quarantine status in the EPPO region and H. vitripennis, that has a very large host range and also feeds on almond, peach and plum, was recently put on the EPPO alert list. As in the more northern parts of the USA, Vitis varieties in Europe are very susceptible to X. fastidiosa and this is really a risk should a vector that could survive the winters of southern Europe become established, also in wild hosts (e.g. wild and domestic plums and wild cherry are symptomless reservoirs in the USA) and cause spring infections that would most likely to persist over the years. The same risk holds true for Citrus (sweet oranges, mandarins, and tangerines) and other hosts, such as almond, plum and peach that are widely grown in south-east and south-west Europe, especially in the warmer Mediterranean basin (where a disease-favourable combination of warm nights, regular rainfall/high humidity and long growing season, is present). Possible ways to prevent introduction and to control eventual outbreaks are indicated. The conclusion is that X. fastidiosa is a real and emerging threat for Europe, not only for Vitis and Citrus but also for stone fruits (almond, peach and plum) and oleander (e.g. GWSS likes to feed on oleander), that is difficult to prevent from entering and difficult to control once established, deserving more attention than up till now. Resistance in European grapes is scarce or even absent. Vector control proved not to be very effective in the USA. Cultural practices to keep plants in optimum condition are of importance, but not sufficient and the use of avirulent strains for cross-protection is still in its infancy.