Characterization of maize rhizospheric microflora and evaluation of their antagonistic potential against Ustilago maydis

Abstract

Maize common smut, attributed to Ustilago maydis, poses a significant risk to maize farming across Kenya. Despite the known potential of rhizospheric microorganisms in biological disease control, limited information exists on their population, characterization, and efficacy against U. maydis in local maize cultivars. Additionally, data on their in vivo effect on disease incidence and severity remain largely unexplored. This study was designed to determine the bacterial and fungal populations within the maize rhizosphere, characterize them using morphological and biochemical characters, and evaluate the impact of selected microbial isolates on disease incidence and severity of common smut in maize. The study was conducted under field, laboratory, and greenhouse conditions at Maseno University. Two maize varieties, DK 8033 and Duma 43 were sown at the university farm in a completely randomized block design. Rhizospheric soil samples were collected at 30 and 60 days after sowing with a soil auger and trowel, into aseptic containers, then conveyed to the botany lab for analysis. Triplicate soil samples were processed via serial dilution followed by plating to recover colonies, which were counted and characterized morphologically and biochemically. Dual culture assays were conducted in vitro to evaluate antagonism against U. maydis. Four promising antagonists: Serratia spp., Bacillus spp., Aspergillus spp., and unidentified fungal isolate MF14, were selected for in vivo greenhouse trials in three replicates, with ten treatments arranged in a completely randomized layout. Treatments included maize cultivars with microbial isolates alongside pathogen inoculation. Controls comprised maize inoculated with the pathogen and distilled water. Comparisons among treatment means used Fisher’s Least Significant Difference, at P ≤ 0.05. There were 25 bacterial and 26 fungal isolates obtained and characterized. Results revealed significantly higher bacterial counts (134.69 × 10⁸ cfu /g) than fungal counts (34.58 × 10⁸ cfu /g), both peaking at 60 days. There were 15 bacterial genera, including Bacillus, Pseudomonas, and Serratia, and six fungal genera (Penicillium, Trichoderma, Fusarium, Alternaria, Aspergillus, Curvularia). In vitro, MF14 isolate showed the highest inhibition zone (22.00 mm), followed by Serratia spp. (19.00 mm), Bacillus spp. (16.00 mm), and Aspergillus spp. (15.00 mm). Greenhouse trials showed that the unidentified fungal isolate MF14 reduced disease incidence and severity to 49.3%, outperforming other treatments. Serratia spp and Aspergillus spp. showed moderate suppression, while Bacillus spp. showed minimal but superior protection to control. This study highlights indigenous rhizospheric microorganisms with potent antagonistic activity, particularly MF14 isolate. Findings provide foundational data for developing microbial-based biocontrol strategies, offering eco-friendly biocontrol options for maize farmers in Kenya upon further research.