Investigation of efficacies of endophytic and epiphytic bacteria in the biological control of watermelon bacterial seedling blight disease (Acidovorax citrulli)

Watermelon is one of the important crops in Turkey, which is the 3rd largest watermelon producing country in the world. In recent years, it was found that high fungal and bacterial disease incidences were noted due to abnormal rainfall and high fluctuating temperature and constitute serious constraints in watermelon crops. Bacterial seedling blight (also known as fruit blotch) of watermelon, which is caused by seed-borne Acidovorax citrulli, is a serious threat to watermelon growers around the world. The present study was conducted to isolate, identify and screen antagonistic potentials of epifitic and endophytic bacterial isolates against A. citrulli and determine their efficacy on disease agent and plant growth parameters such as siderophore, protease, ammonium, indole-3-acetic acid (IAA) production and phosphate solubilization by bacterial isolates were also determined qualitatively and quantitatively. A total of 432 putative endophytic and epiphytic bacterial strains were isolated from different parts of healthy watermelon and relative plant species from 8 locations of Antalya province. Among them, 37 representative putative bacterial isolates were selected based on several tests. These strains were further identified in species level based on cultural and morphological characterization, MALDI-TOF MS (also some strains identified in species level based on 16S rRNA gene sequencing analyses). These 37 isolates were rated for their potential as biocontrol agents against Aac by using antagonistic and plant growth promoting examination assessment system. Among the 37 isolates, 13 isolates showed potential antagonistic activities in vitro dual culture tests. The results showed that the test strains displayed antagonistic efficiency ranging from antagonistic index (AI) values of 1.24 to 8.16. The most efficient bacterial isolates were found Paenibacillus polymyxa KMEn1 and BTCPI followed by Brevibacillus brevis (7.04 AI value) and Bacillus velezensis (5.28 AI value). Isolates of Ochrobactrum intermedium, Brevibacillus formosus, Stenotrophomonas maltophilia, Bacillus pumilus and Bacillus amyloliquefaciens were also displayed in vitro antagonistic activities in varying degree. All isolates produced at least one of the mechanisms tested such as production of siderophores, protease, ammonia, indole-3-acetic acid and solubilize phosphatase. The 8 antagonists that achieved high inhibition to Acidovorax citrulli in vitro were evaluated for their biocontrol efficacy and plant growth-promoting efficacy on watermelon in vivo experiment. Notably, treatment of seeds of watermelon with most potent isolates, resulted in significant suppression seedling blight disease incidence ranging from 10.13% to 79.75%. The most potent antagonist isolates P. polymyxa KMEn1, Brevibacillus brevis GSEp3, Bacillus amyloliquefaciens BT113 and P. polymyxa BTCPI showed the highest biocontrol efficacies, resulted in 79.75%, 59.49%, 44.30% and 40.51% suppression of disease incidence in vivo conditions, respectively. All isolates also enhanced growth of watermelon plants, in comparison with pathogen inoculated control plants. The biocontrol and plant growth promoting and activities displayed by P. polymyxa KMEn1, Brevibacillus brevis GSEp3, Bacillus amyloliquefaciens BT113 and P. polymyxa BTCPI isolates identified in this study highlight their potential suitability as a biocontrol bacterial agents.