Phyllosphere Antagonistic Bacteria Induce Growth Promotion and Effective Anthracnose Control in Cucumber

The phyllosphere, the aerial part of plants, serves as a crucial habitat for diverse microorganisms. Phyllosphere bacteria can activate protective mechanisms that help plants resist disease. 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 the 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 the production of indole acetic acid. Most of the bacterial strains also produced biocontrol compounds such as ammonia, acetoin, siderophores, hydrogen cyanide, chitinase, protease, lipase, and cellulase. The application of these bacteria significantly enhanced cucumber growth in both non-manured and organically 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 they 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 for enhancing cucumber growth and disease control under both fertilized and unfertilized soil conditions.