Investigation of efficiencies of endophytic bacteria in biological control of halo blight disease of bean (Pseudomonas syringae pv. phaseolicola)

The common bean (Phaseolus vulgaris L.) is one of the most important legumes in the human diet and serves as a significant source of proteins. In recent years, bacterial disease incidences were frequently reported in major bean production areas due to abnormal rainfall and high fluctuating temperature and constitute serious constraints in bean crops. Bacterial halo blight of bean, caused by seed-borne Pseudomonas syringae pv. phaseolicola (Psp), is a serious threat for bean production not only in Turkey but also worldwide. The present study was conducted to (1) isolate and identify diversity of endophytic plant growth promoting bacterial isolates (EB) from healthy bean plants grown in different major bean producing provinces; (2) to screen antagonistic potentials of EB on bacterial growth and disease incidence in vitro and in vivo conditions; (3) to determine antagonistic and plant growth promoting (PGP) traits such as siderophore, protease, ammonium, indole-3-acetic acid (IAA) production and phosphate solubilisation by EB qualitatively and quantitatively. A total of 343 putative EB were obtained from different parts of healthy bean plant growing in Niğde, Konya, Eskişehir, Burdur and Ankara provinces. Fifty-six representatives putative EB were selected based on regions, biochemical and pathogenicity tests and identified in species level by morphological characterization and MALDI-TOF analyses. By using dual culture test, EB isolates were screened to inhibit Psp in vitro conditions. The results from the dual culture test, inhibition of Psp growth by EB, varied widely and significantly among isolates, with antagonistic index values ranging from 1.22 to 4.85. Among the 56 antagonist EB isolates, 14 isolates showed strong inhibition (inhibition indices >3.0), 20 isolates moderate inhibition (inhibition indices between 2.0-3.0), 22 isolates weak inhibition (inhibition indices >2). The highest antagonistic activities were displayed by EB isolates Pseudomonas gessardii N35.1 (4.85 inhibition index) followed by Arthrobacter ilicis N11.3, Stenotrophomonas sp B6.6, Achromobacter spanius B1.1, Pseudomonas putida K15.1 and Arthrobacter gandavensis N19.1, respectively. Among the screened EB, 54 isolates were positive for the production of siderophore, 23 isolates positive for protease production and 12 isolates solubilised phosphate in varying degree. All selected EB isolates produced IAA and ammonia in relatively varying concentrations. The 32 antagonist EB isolates were further evaluated for their biocontrol efficacy on bean pods and leaves in vivo experiment. Although all isolates suppressed bacterial disease incidence varying ratios, isolates of Pseudomonas gessardii N35.1, Arthrobacter nicotinovorans B11.2, Stenotrophomonas sp B6.6, Arthrobacter ilicis N11.3, Achromobacter spanius B1.1 ve Pseudomonas putida K15.1 and Arthrobacter gandavensis N19.1 showed significant biocontrol activities against disease caused by Psp on bean pods and leaves. Seed coating by EB isolates Stenotrophomonas sp E13.1, Pseudomonas gessardii N35.1, Arthrobacter nicotinovorans B11.2 promoted seedling growth and development significantly. The biocontrol and plant growth promoting and activities displayed by Pseudomonas gessardii N35.1, Arthrobacter nicotinovorans B11.2, Stenotrophomonas sp B6.6, Arthrobacter ilicis N11.3, Achromobacter spanius B1.1 ve Pseudomonas putida K15.1 and Arthrobacter gandavensis N19.1 isolates identified in this study highlight their potential suitability as a biocontrol bacterial agents against halo blight disease of bean.