Biocontrol of Citrus Canker Disease Caused by Xanthomonas citri subsp. citri Using an Endophytic Bacillus thuringiensis

Citrus canker is a devastating disease of citrus caused by Xanthomonas citri subsp. citri (Xcc). A total of 134 endophytic bacteria were isolated from various gymnospermic and angiospermic plants. They were screened for their antagonistic activities against three wild-type and six streptomycin-resistant Xcc strains. TbL-22 and TbL-26, both later identified as Bacillus thuringiensis, inhibited all the wild and resistant Xcc strains. TbL-22 exerted the highest antagonistic activity against XccW3 and XccM6 with inhibition zones of 20.64 +/- 0.69 and 19.91 +/- 0.87 mm, respectively. Similarly ethyl acetate extract of TbL-22 showed highest inhibition zones 15.31 +/- 2.08 and 19.37 +/- 3.17 mm against XccW3 and XccM6, respectively. TbL-22 reduced canker incidence on infected leaves by 64.05% relative to positive controls. Scanning electron microscopy revealed that the cell membranes of Xcc treated with ethyl acetate extract of TbL-22 were ruptured, lysed, and swollen. B. thuringiensis TbL-22 can effectively and sustainably controls streptomycin-resistant citrus canker.