Biological control of citrus canker by endophytic bacteria

Citrus is an important fruit crop in India, playing a significant role in the agricultural economy and experiencing high demand due to its rich nutritional content. Despite its economic importance, citrus canker, caused by the Gram-negative bacterium Xanthomonas citri subsp. citri, poses a significant threat to citrus production globally, including in India. This study focuses on using endophytic bacteria, specifically Bacillus amyloliquefaciens ESK-8 and Bacillus subtilis EPM-3, for the biological management of this pathogen. A roving survey in major citrus cultivating areas of Tamil Nadu revealed varying disease prevalence rates (21% - 65%). The citrus canker pathogen and endophytic Bacillus isolates were isolated from the surveyed locations and identified based on biochemical and molecular analyses using 16S rDNA sequences. In vitro assays through agar well diffusion method using culture filtrates of various Bacillus isolates revealed significant inhibition rates, with Bacillus amyloliquefaciens ESK-8 and Bacillus subtilis EPM-3 exhibiting inhibition diameters of 14.1 mm and 11.6 mm, respectively, compared to the control. GC-MS analysis of Bacillus amyloliquefaciens ESK-8 and Bacillus subtilis EPM-3 unraveled important antibacterial compounds such as bis(2-ethylhexyl) phthalate, n-hexadecanoic acid, D-erythro-pentose, 2-deoxy, hexadecanoic acid, octadecanoic acid, and tridecane. Furthermore, the metabolic pathways related to these compounds include glycerolipid metabolism, glutamate metabolism, and tryptophan metabolism, all of which play significant roles in plant growth and antagonism-promoting activities. Additionally, pot culture studies confirmed the antagonistic potential of Bacillus amyloliquefaciens ESK-8 and Bacillus subtilis EPM-3 against the citrus canker pathogen. This research highlights the potential of endophytic Bacillus isolates for the sustainable management of citrus canker disease.