Bacterial spot, caused by Xanthomonas arboricola pv. pruni (Xap), affects almond (Prunus dulcis) production in Australia and worldwide. While almond varieties may differ in susceptibility to bacterial spot, limited information is available regarding their level of resistance to this disease. However, screening for resistance to bacterial spot is important for effective disease management and the development of resistant almond varieties. Therefore, the primary objective of this study was to assess the efficacy of five artificial inoculation methods for screening for bacterial spot resistance in almonds in South Australian conditions. Experiments were conducted from October to December 2024 using four varieties (“Nonpareil”, “Fritz”, “Carmel” and “Marcona”) located at the Waite Campus of the University of Adelaide. Suspensions of Xap strains DAR 72009 and DAR 82623, at 107 CFU/ml, were applied separately to: (1) detached leaves in vitro using a needleless syringe; (2) detached leaves in vitro via a needle prick; (3) leaves on detached shoots in vitro using a needleless syringe; (4) leaves on small trees in a shade-house using a needleless syringe; (5) leaves on mature trees in an orchard. Controls were sham-inoculated with physiological saline. Methods 1 and 2 were applied to all four varieties, method 3 to Nonpareil and Marcona, and methods 4 and 5 to Nonpareil. Disease severity was assessed based on the extent of the leaf area affected and characteristics of the lesions, and the presence of Xap was confirmed by isolation and PCR using Xap-specific primers. All inoculation methods, using either Xap strain, resulted in bacterial spot symptoms. Further, none of the almond varieties tested was immune. Method 3 resulted in the fastest symptom development (6 days), followed by method 1 (6-14 days). Methods 2 and 4 required approximately 21 days for symptoms to develop. Leaves on mature trees in the orchard (method 5) took nearly 4 weeks to express symptoms of bacterial spot. Inoculation using methods 1, 2 or 3 required specific conditions and inputs. While the method 5 required the least labour and inputs, disease development was comparatively slow and was affected by environmental factors. Overall, method 4, in which leaves on trees in a shade-house were inoculated using a needleless syringe, was easy to perform, required minimal resources, and could be used for the simultaneous assessment of multiple trees. As such, it has potential for screening for resistance to Xap. However, further studies using multiple almond varieties and additional Xap strains are needed to verify suitability for screening for bacterial spot resistance in almonds.
Key words: Bacterial spot, Almond (Prunus dulcis), Australia, Artificial inoculation, Xap