Bacillus amyloliquefaciens Activates the Basal Defense of Chinese Cabbage Against Alternaria brassicicola

Alternaria brassicicola is a necrotrophic plant pathogen that causes substantial damage to a broad range of host plants, especially Brassica species. Various strategies were employed to control this disease, including the application of antagonistic microorganisms. In this study, we evaluate two strains of Bacillus amyloliquefaciens (PMB04 and PMB05) as biological control agents for black leaf spot disease caused by A. brassicicola on Chinese cabbage (Brassica rapa var. chinensis). We found that these two strains inhibited mycelial growth and spore germination of A. brassicicola and reduced disease severity on Chinese cabbage. However, PMB04 showed a better in vitro inhibitory effect than PMB05, but PMB05 has a better biocontrol efficacy than PMB04. To gain more insight into the relationship between the inhibitory effect of these two strains and their biocontrol efficacy, the plant immune response was enhanced with the B. amyloliquefaciens bacterial cell and its cultural filtrate. The results exhibited that both forms of PMB05 significantly intensify the plant immunity in leaf tissue upon the inoculation of A. brassisicola spores, rather than those of PMB04. This suggests that B. amyloliquefaciens strain PMB04 and PMB05 have different biocontrol mechanisms.