Utilization of phytogenic silver nanoparticles and bacterial agents for the management of citrus canker disease

Citrus is one of the most important and widely cultivated fruit crops globally. However, its productivity is increasingly threatened by various diseases, with bacterial infections being particularly the most destructive. Among these, citrus canker, caused by Xanthomonas axonopodis pv. citri (Xac), poses a significant challenge, requiring effective management strategies. Traditional approaches have shown limited success, but nanotechnology has emerged as a promising alternative for disease control. In this study, Xanthomonas spp. was isolated from two locations: Koont Farm and the National Agricultural Research Centre (NARC) citrus orchards in Islamabad, Pakistan. The isolates were identified through morpho-biochemical methods. Moreover, three Bacillus isolates (AAUR-1, Btk-2, and NARC-3) were evaluated in combination with silver nanoparticles (AgNPs). Initially, the three Bacillus isolates were tested against Xanthomonas spp. using a dual culture assay. The results demonstrated that Btk-2 showed significant inhibitory activity against Xanthomonas spp. AgNPs were synthesized using the green method with Azadirachta indica leaf extract. Characterization techniques, including ultraviolet-visible spectroscopy (200–400 nm), scanning electron microscopy (15–20 nm in size), and energy-dispersive X-ray spectroscopy, confirmed formation of the AgNPs. AgNPs at concentrations of 1000, 1300, and 1500 ppm were evaluated for antibacterial activity against Xac using the disc diffusion method. The 1300 ppm concentration inhibited Xac by 0.1 mm, compared to 0.18 mm inhibition by streptomycin. The antibacterial efficacy of Btk-2 and AgNPs was further tested using the leaf-detach method. While untreated leaves developed symptoms, the AgNP-treated leaves showed resistance. Hence, it was concluded that the combination of Btk-2, AgNPs, and antibiotics like streptomycin can be an effective strategy to manage citrus canker caused by Xac.