Journal of Plant Pathology (2012), 94 (1, Supplement), S1.91-S1.96 Edizioni ETS Pisa, 2012 S1.91 EVALUATION OF XANTHOMONAS ARBORICOLA pv. PRUNI INOCULATION TECHNIQUES TO SCREEN FOR BACTERIAL SPOT RESISTANCE IN PEACH AND APRICOT D. Socquet-Juglard 1,2 , A. Patocchi 1 , J.F. Pothier 1 , D. Christen 3 and B. Duffy 1 1 Agroscope Changins-Wädenswil (ACW) Research Station, Phytopathology, Schloss 1, 8820 Wädenswil, Switzerland 2 Swiss Federal Institute of Technology Zürich (ETH-Z), Phytopathology, Universitätstrasse 2, 8092 Zürich, Switzerland 3 Agroscope Changins-Wädenswil (ACW) Research Station, Conthey Research Centre, Fruit growing and breeding group, 1964 Conthey, Switzerland SUMMARY Screening for resistance to bacterial spot of apricot (Prunus armeniaca) and other stone fruit species such as peach (P. persica) and plum (P. domestica), caused by the quarantine bacterium Xanthomonas arboricola pv. pruni requires an inoculation method which is rapid, efficient and which permits to differentiate levels of resistance to this disease. Four inoculation techniques were com- pared under greenhouse conditions: (i) infiltration in the leaves using a needleless syringe; (ii) injuries caused by a needle; (iii) injuries caused by scissors; (iv) dipping shoots of the plant into the inoculum. All inoculation techniques induced symptoms: infiltration was very effi- cient and quickly caused severe necrosis and leaf drop; the use of needle was also very efficient but the symp- toms were very weak, while the scissors technique was not as efficient as the first two approaches and the symptoms were also very weak. Finally dipping peach shoots into the inoculum was the least efficient in symp- tom induction, but this technique permitted to obtain more quantitative data suitable for evaluating the level of resistance of a cultivar than all the other techniques (e.g. number of spots per leaf, or damaged surface, cal- culated disease incidence and resistance index). For this reason, the inoculation technique by immersion was ap- plied to six apricot cultivars to be screened for resist- ance against bacterial spot. Plants were assessed 15, 28, 38 and 64 days post inoculation for spot number, leaf surface damaged, disease incidence and severity of symptoms. Cv. Orangered was the most resistant, show- ing low levels of damage to the leaves (10% +/- 3.54) and only a few symptomatic leaves per plant (17.39 +/- 0.56). Cv. Goldbar was the most susceptible with 76.74 +/- 19.39% of symptomatic leaves and high levels of damage per leaf (85 +/- 11.34%). Four other cvs., Bergarouge, Bergeron, Goldrich and Kioto showed in- termediate levels of susceptibility to the bacterium. Re- sults of resistance testing were comparable to those ob- served under field conditions as reported in the litera- ture. This indicates that shoot immersion into the in- oculum is an efficient and reliable technique to screen for resistance against bacterial spot of stone fruits under greenhouse conditions. Key words: bacterial spot, Prunus, inoculation tech- niques, screening for resistance. Xanthomonas arboricola pv. pruni (Xap), the causal agent of bacterial spot of stone fruits, is a quarantine pathogen for the European Union (EU) and the Euro- pean Plant Protection Organisation (EPPO). It mainly causes necrotic lesions on leaves and fruits of peach, plum, apricot and nectarine, but in more severe cases, cankers, defoliations and death of the trees can be ob- served (du Plessis, 1988). This bacterium has been re- ported from all continents, and is still expanding its range (EPPO, 2006). In Europe, bacterial spot has been identified first in northern Italy in 1934 (Petri, 1934) where it is now considered as endemic (Battilani, 1999). The disease continues to spread in Europe and out- breaks have occurred more recently in France, Bulgaria, Romania and Ukraine (EPPO, 2006). In Switzerland the disease was detected in 2005 on apricot trees (Poth- ier et al., 2010), and it is now considered as locally es- tablished. Due to lack of efficient chemical or biological control methods, host resistance might be the most promising solution to combat the disease. Different levels of toler- ance against Xap have been detected in peach, nec- tarine, apricot and plum based on field observations (Simeone, 1985; Topp et al., 1989; Layne and Hunter, 2003; Garcin et al., 2005; Garcin and Bresson, 2011). The availability of an artificial inoculation technique al- lowing evaluations in a standardized quantitative man- ner of the degree of susceptibility or resistance (toler- ance) of different cultivars under greenhouse conditions may foster the selection or identification of resistant or tolerant cultivars. Few techniques for leaf inoculation have been described, mostly on peach (Prunus persica) and to a less extent on plum (P. domestica) and apricot (P. armeniaca). For practical aspects, detached leaf as- says have often been used for pathogenicity assays. For example, Boudon et al. (2005) immersed detached Corresponding author: A. Patocchi Fax: +41.44.7836 05 E-mail: andrea.patocchi@acw.admin.ch 015_COST(Patocchi)_S91 15-06-2012 11:58 Pagina 91