Botanical-Based Strategies for Controlling <i>Xanthomonas</i> spp. in Cotton and Citrus: In Vitro and In Vivo Evaluation

Citrus canker, caused by <i>Xanthomonas citri</i> subsp. <i>citri</i>, and bacterial blight, caused by <i>Xanthomonas citri</i> subsp. <i>malvacearum</i>, results in substantial economic losses worldwide, and searching for new antibacterial agents is a critical challenge. In this study, regional isolates AE28 and RQ3 were obtained from characteristic lesions on <i>Citrus limon</i> and <i>Gossypium hirsutum</i>, respectively. Essential oils extracted by steam distillation from the fresh aerial parts of <i>Pelargonium graveolens</i> and <i>Schinus molle</i> exhibited complete (100%) inhibition of bacterial growth in vitro at a concentration of 1000 ppm, as determined by diffusion tests. To evaluate the potential of these essential oils for controlling <i>Xanthomonas</i>-induced diseases, in vivo assays were conducted on lemon leaves and cotton cotyledons inoculated with the regional AE28 and RQ3 strains. Two treatment approaches were tested: preventive application (24 h before inoculation) and curative application (24 h after inoculation). Preventive and curative treatments with <i>P. graveolens</i> essential oil significantly reduced citrus canker severity, whereas <i>S. molle</i> essential oil did not show a significant reduction compared to the control. In contrast, regardless of the treatment’s timing, both essential oils effectively reduced bacterial blight severity in cotton cotyledons by approximately 1.5-fold. Gas chromatography–mass spectrometry (GC-MS) analysis identified geraniol and citronellol as the major components of <i>P. graveolens</i> essential oil, while limonene and t-cadinol were predominant in <i>S. molle</i>. These findings highlight the promising potential of botanical products as bactericidal agents, warranting further research to optimize their application and efficacy.