DETECTION AND IDENTIFICATION METHODS AND NEW TESTS AS USED AND DEVELOPED IN THE FRAMEWORK OF COST 873 FOR BACTERIA PATHOGENIC TO STONE FRUITS AND NUTS

Abstract

Listen

Translate article

The quarantine pathogen Xylella fastidiosa (EPPO A1 list), a xylem-inhabiting, vector-transmitted, Gramnegative, very slow growing bacterium, was cultured and properly described in 1987 in the USA as the cause of Pierce’s disease (PD) of grapevine (Vitis vinifera) and of phony peach disease (PPD) in peach (Prunus persica) and in 1993 in Brazil as the cause of citrus variegated chlorosis (CVC) in sweet orange (Citrus sinensis). Moreover, it was found that the bacterium also causes a number of so-called leaf scorch diseases in Prunus spp. [including almond leaf scorch (ALS) in P. armeniaca and plum leaf scald (PLS) in P. domestica], ornamental trees, Coffea arabica (CLC, in Brazil, isolated in 1995), and many other plant species (see under ‘Host range’). Many wild plants, including grasses, sedges and trees may carry the pathogen without showing symptoms. All these diseases are quite destructive, giving substantial yield losses. They occur mainly in tropical/subtropical areas, although leaf scorch diseases also occur in much colder climates, e.g. oak leaf scorch in eastern North America, as far north as Canada. Several pathogenic genotypes have been described, which are often host-specific (e.g., the strain will not cause disease if introduced to peach or plum). The following subspecies are currently known: (i) Xylella fastidiosa subsp. fastidiosa (erroneously named X. f. subsp. piercei) strains from cultivated grape, alfalfa, almond (two), and maple; (ii) X. f. subsp. multiplex, strains from peach, elm, plum, pigeon grape, sycamore, and almond; (iii) X. f. subsp. pauca, strains from citrus and probably those of coffee; (iv) X. f. subsp. sandyi, strains from Nerium oleander. X. f. subsp. taske was described from the ornamental tree Chitalpa tashkentensis on the basis of one gene sequence only and its status as a separate subspecies is therefore not ultimately established. Vectors are sharpshooter leafhoppers (Hemiptera:Cicadellidae) and froghoppers or spittlebugs (Hemiptera: Cercopidae), the most important of which in the USA include Cuerna costalis and Graphocephala versuta (PPD), Draculacephala minerva (green sharpshooter); G. atropunctata (blue-green sharpshooter, formerly Hordnia circellata) and Homalodisca vitripennis (formerly H. coagulata, glassy-winged sharpshooter, GWSS) (PD). CVC vectors in Brazil include Acrogonia terminalis, Dilobopterus costalimai and Oncometopia fascialis. Local possible vectors for Europe are Cicadella viridis and Philaenus spumarius (meadow spittle bug), as well as all other members of the family Cercopidae and the leafhopper subfamily Cicadellinae. The bacterium is, as far it is known, still absent from Europe. For more general information on the bacterium, epidemiology, control, etc. see Purcell (1997), Hopkins and Purcell (2002), EPPO/OEPP (1992), Chatterjee et al. (2008), Janse and Obradovic (2010 ), and http://www.cnr. berkeley.edu/xylella/ In order to study the disease and train on diagnosis in a pro-active way, the EU COST873 project organised in October 2010 a workshop specifically on X. fastidiosa detection and identification with excellent support and participation of two outstanding experts in the field, Prof. A.H. Purcell and Prof. R.P.P. Almeida from the University of California at Berkeley (USA). The workshop was hosted by IAM, Bari, Italy (http://cost873.ch/5_activites/ meeting_detail.php?ID=28). Most of the methods presented here have been found useful by the US experts over the years and were practised during the workshop.