EVALUATION OF XANTHOMONAS ARBORICOLA pv PRUNI INOCULATION TECHNIQUES TO SCREEN FOR BACTERIAL SPOT RESISTANCE IN PEACH AND APRICOT
D. Socquet-Juglard, A. Patocchi, J.F. Pothier, D. Christen, B. Duffy
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 compared 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 efficient and quickly caused severe necrosis and leaf drop; the use of needle was also very efficient but the symptoms 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 symptom 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, calculated disease incidence and resistance index). For this reason, the inoculation technique by immersion was applied to six apricot cultivars to be screened for resistance 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, showing 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 intermediate levels of susceptibility to the bacterium. Results of resistance testing were comparable to those observed under field conditions as reported in the literature. This indicates that shoot immersion into the inoculum is an efficient and reliable technique to screen for resistance against bacterial spot of stone fruits under greenhouse conditions.