Management of Nematode-Bacterium Leaf Spot Disease Complex of Beetroot by Purpureocillium lilacinum, Pseudomonas fluorescens and Bacillus subtilis

The efficacy of Purpureocillium lilacinum, Pseudomonas fluorescens, and Bacillus subtilis, applied individually and in combination, was evaluated against the disease complex of beetroot (Beta vulgaris L.) caused by Meloidogyne incognita (2000 J2/plant) and Pseudomonas syringae pv. aptata. Inoculation with the pathogens significantly reduced plant growth, chlorophyll, and carotenoid contents, with combined inoculation causing the most severe adverse effects. The soil application of the biocontrol agents (10 ml suspension/plant) significantly enhanced the plant growth and physiological attributes, including chlorophyll, carotenoid, and proline contents, in both pathogen-inoculated and non-inoculated plants. The combined application of all three biocontrol agents, as well as the dual combination of P. fluorescens and B. subtilis, produced the greatest improvements in plant growth and photosynthetic pigment contents. Proline accumulation increased in response to both pathogen infection and biocontrol treatments. Among the individual biocontrol agents, P. fluorescens was found to be most suppressive to the nematode. The root colonization was also highest for P. fluorescens. The combined application of all three biocontrol agents resulted in the greatest reduction in the nematode population density, root gall formation, and bacterial leaf spot severity. Overall, the combined use of P. fluorescens and B. subtilis, either alone or integrated with P. lilacinum, proved highly effective in suppressing the disease complex and improving plant health. The integration of compatible biocontrol agents offers an effective strategy for managing the bacterial leaf spot–root-knot nematode disease complex in beetroot.