Phyllosphere antagonistic bacteria induces growth promotion and effective anthracnose control in cucumber

The phyllosphere, aerial parts of plants, serves as a crucial habitat for diverse microorganisms. Phyllosphere bacteria can activate protective mechanisms that help plants resist diseases. This study focuses on isolating and characterizing phyllosphere bacteria from cucurbits to evaluate their potential in controlling Colletotrichum orbiculare, a pathogen causing anthracnose in cucumbers. Among 76 bacterial isolates collected, 11 exhibited strong antagonistic effects against C. orbiculare in vitro. Morphological and 16S rRNA analyses identified these isolates as different Bacillus species, including B. vallismortis, B. velezensis, B. amyloliquefaciens, and B. subtilis. These bacteria demonstrated essential plant-growth-promoting and biocontrol traits, such as motility, biofilm formation, phosphate solubilization, nitrogen fixation, and production of indole acetic acid. Most of the bacterial strains also produced biocontrol compounds like ammonia, acetoin, siderophores, hydrogen cyanide, chitinase, protease, lipase, and cellulase. Application of these bacteria significantly enhanced cucumber growth in both non-manured and organic-manured soils, showing improvements in root and shoot length, chlorophyll content, and biomass accumulation. Additionally, bacterial treatments effectively reduced anthracnose severity, with isolates GL-10 and L-1 showing the highest disease suppression in both soil types. Colonization studies showed that phyllobacteria preferentially colonized healthy leaves over roots and diseased tissues, and were more effective in manure-amended soils. These results suggest that Bacillus phyllobacteria have strong potential as sustainable bio-stimulants and biocontrol agents, offering an effective approach to enhance cucumber growth and disease control under both fertilized and unfertilized soil conditions.