Bacillus atrophaeus Strain BaAZ2 Shows Antagonism Against Phytophthora infestans In Vitro and Induces Defense Reaction to Late Blight in Potato

Potato late blight, caused by Phytophthora infestans, is the most devastating disease affecting potatoes, leading to substantial annual yield losses. This study investigated the potential of three bacterial strains for the biological control of this disease under both in vitro and greenhouse conditions. In vitro, in a dual-culture test, Bacillus atrophaeus strain BaAZ2 demonstrated an antagonistic effect against P. infestans stronger than the Stenotrophomonas rhizophila strain SrAZ1 and Bacillus halotolerans strain BhAZ6. In planta, treatment with strain BaAZ2 led to a significant reduction in hydrogen peroxide accumulation in potato leaf tissue. Total phenolic content, and the activity of defense-related enzymes (polyphenol oxidase, and phenylalanine ammonia-lyase) as well as antioxidant enzymes (catalase, ascorbate peroxidase, and peroxidase) were significantly elevated in response to BaAZ2 treatment. Furthermore, the expression levels of stress and defense-related genes StuPR, StuMAPK4, StuWRKY1, StuPPO9, and StuPAL increased in strain BaAZ2-treated plants, while SrAZ1 showed moderate activity and BhAZ6 displayed comparatively limited responses. These findings highlight the strain-specific nature of bacterial biocontrol efficacy and emphasize the importance of evaluating individual isolates before their potential application in sustainable late blight management.