Determination of the effects of some rhizobium and bacillus species in controlling bean bacterial common blight (Xanthomonas axonopodis pv. phaseoli) disease

The common bean (Phaseolus vulgaris L.) is a key legume crop cultivated worldwide and plays a vital role in human nutrition, serving as a rich source of protein, carbohydrates, vitamins, and minerals. This crop is affected by common bacterial blight (CBB), caused by Xanthomonas axonopodis pv.phaseoli (Xap), which is one of the most destructive diseases, leads to significant economic and yield losses and poses a serious threat to food security in both developed and developing countries. Traditional disease control methods, which rely heavily on copper-based compounds and antibiotics, have shown limited effectiveness and raise concerns regarding environmental sustainability and public health. This study evaluated the biocontrol potential of selected strains of Rhizobium and Bacillus species against CBB in two bean varieties, PV12 × Alberto (dry bean) and Riberia (green bean), under both in vitro and in vivo conditions. In vitro treatments included Rhizobium gallicum (Rg), Rhizobium leguminosarum (Rl), their mixture (Rl + Rg), and Bacillus subtilis, compared with streptomycin and sterilized distilled water (SDW) as controls. Colony zone, inhibition diameter, and inhibition index parameters were used to assess efficacy. In vivo experiments involved the same bioagents compared to copper treatment and a pathogenonly control. Disease parameters, including disease incidence (DI), disease severity index (DSI), severity score (SC), and disease control (DC), were evaluated at 7, 14, and 21 days after inoculation (DAI), along with the area under the disease progress curve (AUDPC). Growth traits measured included plant height, number of leaves, and number of primary branches per plant, while root characteristics assessed at final sampling included root length, fresh root weight (FRW), dry root weight (DRW), and root water content. Data were analyzed using Origin Pro 2025 software, applying LSD, coefficient of variation (CV), and Tukey's test (p ≤ 0.01), alongside correlation analysis between disease and growth parameters. According to the in vitro results, Bacillus subtilis demonstrated the highest efficacy after streptomycin, with a 17.00 mm inhibition zone classified as very effective. This was followed by Rl (12.33 mm, moderately effective) and the Rhizobium mixture (10.66 mm). Under in vivo conditions, the mixed treatment (Rl + Rg) showed the highest disease control, with 68.49% in PV12×Alberto and 54.44% in Riberia, at 21 DAI. It also resulted in the lowest disease incidence, at 22.24% and 29.37%, respectively. This was followed by Rg in PV12×Alberto (dry bean), which controlled the disease by 59.12% with a reduction in incidence of 28.86%, and B. subtilis QST713 in Riberia, which controlled the disease by 53.18% and reduced incidence to 30.18%, both measured at 21 DAI. All treatments promoted growth traits and enhanced root development. Correlation analysis exhibited strong and significant positive relationships among disease parameters, while significant negative correlations were observed between disease parameters and growth traits. In conclusion, the use of Rhizobium and Bacillusspecies can effectively reduce and control the Xap pathogen, as they are eco-friendly bacteria that support long-term sustainability. To fully realize the potential of these bioagents, namely, eradicating plant pathogens, reducing reliance on chemical pesticides, and promoting environmentally friendly farming systems, future studies should focus on identifying the virulence mechanisms of the Xap pathogen and the regulatory pathways of host plant genes related to resistance. Such insights are expected to significantly improve the effectiveness of breeding programs and aid in the development of innovative strategies to combat this pathogen.