The effect of aqueous extract of some medicinal plants against Xanthomonas translucens and X. citri subsp. citri in vitro and their effect on the control of bacterial leaf streak of wheat under greenhouse conditions

Bacterial leaf streak of wheat caused by Xanthomonas translucens and Asiatic citrus cankercaused by Xanthomonas citri subsp. citri are two important bacterial diseases that cause considerable damage in Iran. Copper compounds are usually applied to control these diseases. However, their phytotoxicity and development of copper-resistant bacterial strains have made to develop new antimicrobial compounds indispensable. On the other hand, because of awareness of the chemical pesticides' threats to the environment and human health, biopesticides were considered. Hence, this study aimed to identify plant extracts that effectively control these two bacteria.

Identifying bacterial isolates was accomplished based on phenotypic, molecular and pathogenicity tests. In the first step, the effect of aqueous extract of five medicinal plants was examined against X. translucens and X. citri subsp < em>. citri in vitro using the agar well diffusion method and minimum inhibitory concentration (MIC), and minimum bactericidal concentration (MBC) were determined by serial dilution method. Then, according to the results of laboratory experiments, two extracts were chosen, and their effect was assessed on the control of wheat bacterial leaf streak under greenhouse conditions.

In the laboratory tests, measuring growth inhibitory zone diameter showed that the extracts of gol-e-arvaneh (Hymenocrater longiflorus Benth), yarrow (Achillea millefolium L.), garlic (Allium sativum L.) and Teucrium polium L. have different degrees of significant antibacterial effect on both examined bacteria. The extract of sage (Salvia officinalis L.) had no inhibitory effect on both bacteria. The extracts of gol-e-arvaneh and yarrow had the highest antibacterial activity against X. translucens with the inhibitory zone of 31 and 24 mm, respectively. In the case of X. citri, the extracts of T. polium and gol-e-arvaneh had the highest antibacterial activity with the inhibitory zone of 25 and 22 mm, respectively. In the greenhouse experiments, applying the extracts of yarrow and gol-e-arvaneh to control wheat leaf streak led to remarkable reduction of disease incidence and disease severity percent.

According to the laboratory experiments, the intensity of the inhibitory effect of extracts varied based on the bacterial species. Therefore, to produce a plant extract as an antimicrobial agent against a bacterial pathogen, the performance of screening tests is unavoidable. The affectivity of the extracts of yarrow and gol-e-arvaneh for controlling X. translucens in laboratory and greenhouse conditions showed that these two extracts are suitable candidates for further investigation, to synthesis safe antibacterial compounds for the management of wheat bacterial leaf streak.