Unveiling the potential of endophytic bacteria in combatting walnut bacterial canker disease

Bacterial endophytes within plant tissues can be used to develop sustainable disease protection strategies. This study aimed to identify endophytic bacteria with biocontrol ability of walnut bacterial canker disease. For this purpose, leaf, stem, and root samples from healthy walnut trees (cv. Chandler) were collected, resulting in the selection of 340 bacterial endophytes on nutrient agar and King’s medium B. In vitro, 16%, 32%, and 12% of the strains produced the inhibition zone diameters of more than 5 mm against Brenneria rosae subsp. rosae strain 181 (Br181), Gibbsiella quercinecans strain 128 (Gq128) and Brenneria nigrifluens strain 200 (Bn200) respectively, displaying a significant difference at the 1% level. Among them, eight antagonist strains showed antibacterial activity against all three pathogenic species. The partial 16S rRNA gene sequences and multilocus sequence analysis, followed by a phylogenetic analysis revealed that the strains JD62, JD81 and JD87, JD82’, and JD46 belonged to Pseudomonas protegens (rpoB, rpoD and gyrB), Bacillus velezensis (gyrA and rpoB), Serratia liquefaciens (rpoB and infB), and Chryseobacterium lathyri (rpoB), respectively. Strains JD61, JD105, and JD42 were identified as Delftia tsuruhatensis and Delftia sp., respectively. In greenhouse conditions, strain JD81 demonstrated an 84.74% and 87.93% reduction in symptom severity caused by Br181 and Bn200, respectively, on the trunk. Moreover, strain JD62 significantly reduced symptom severity caused by Br181 and Gq128 by 71.18% and 81.01%, respectively, on the trunk. Thus, B. velezensis strain JD81 and P. protegens strain JD62 can be used to develop effective agents to manage bacterial cankers on walnut trees.