A mixture of Azotobacter, Azospirillum, and Klebsiella strains improves root-rot disease complex management and promotes growth in sunflowers in calcareous soil

Abstract

Biological farming using a mixture of bio-agents that are compatible with each other and adapted to the plant rhizosphere is a strategic approach to manage crop disease. This study is a unique approach to manage the root-rot disease complex caused by Macrophomina phaseolina, Rhizoctonia solani, and Fusarium solani in sunflower plants grown in calcareous soil using a mixture of rhizobacteria strains, namely Azotobacter chroococcum ZCR, Azospirillum brasilense SBR, and Klebsiella pneumoniae KPR. These strains were screened in vitro for N2 fixation and phosphate solubilization, as well as the production of indoleacetic acid, siderophore, and hydrogen cyanide. Interestingly, the rhizobacteria strains exhibited promising biofertilizer and biocontrol properties. Furthermore, they significantly inhibited the growth of root-rot pathogens in a dual culture assay. Notably, the rhizospheric bacteria exhibited successful colonization of the rhizoplane of sunflower plants and persisted at high levels for up to 60 days (8 × 104–12 × 104 cfu g−1 fresh root). In field experiments under naturally infested soil conditions, application of a bio-formulation containing a plant growth-promoting rhizobacteria mixture (HALEX bio-formulation) as a seed treatment as well as by soil drenching was found to be effective in controlling sunflower root-rot disease and significantly decreased the disease severity rating by 44–55% of the non-treated control in 2 years. In addition, treating with rhizobacteria significantly improved plant growth, yield, and oil content and decreased weight loss due to pathogen-induced stress. Consequently, it is suggested that farmers use this rhizobacteria mixture as a promising eco-friendly approach for effective management of root-rot disease complex in sunflowers in semi-arid regions.