Biocontrol potential of bacteria isolated from Carpobrotus spp. as seed treatments against Fusarium pathogens of maize

Abstract

Fusarium spp. cause significant losses in yield and quality in the maize (Zea mays L.) industry. Fusarium pathogens affect the development of the plant tissue and the quality of the grain. The mycotoxins produced by Fusarium spp. are also a threat to the health of animals and humans. This, therefore, has a negative impact on food safety and security of the countries which depend on maize as a staple food crop, such as the Republic of South Africa (RSA). Large yields of maize are necessary in RSA since maize is also an export crop, boosting the gross economic value total of field crops. The demand for sustainable maize production strategies necessitates intervention against Fusarium pathogens. For years, Fusarium pathogens have been controlled through the use of chemical fungicides whose progressive use is hindered by public concerns about the environmental and health safety of accumulated fungicide residues, and development of resistance against fungicides. Possibly safer, more effective and environmentally friendlier alternative control methods like biological control (biocontrol) continue to be evaluated. The application of naturally occuring organisms and their secreted metabolites to reduce disease incidence and severity may be a long-term means of control. The ability of these biocontrol agents (BCAs) to withstand harsh conditions while protecting the maize seed or plant is what is desired for the development of a potential biocontrol product for seed treatment. Bacillus spp. show potential as BCAs as they employ different modes of pathogen antagonism, stimulate plant growth and form stable endospores to help them survive harsh conditions. Bacteria (Bac A and Bac B) isolated from seeds of a mesemb, Carpobrotus acinaciformis (L.) L. Bolus were investigated for their antagonistic potential against Fusarium graminearum Schwabe, Fusarium verticillioides (Saccardo) Nirenberg, and Fusarium solani (Martius) Appel & Wollenweber emend. Snyder & Hansen. In vitro dual culture and two-plate inverted culture assays showed that Bac A and Bac B were able to inhibit all three selected Fusarium spp. by the production of antifungal diffusible and volatile metabolites. Further metabolite characterization was done for Bac A as Bac A showed better inhibition of the Fusarium spp. in vivo, and the antifungal diffusible metabolites produced by Bac A were characterized as terpenoids. The efficacy of Bac A in vivo seed treatment against F. graminearum and F. verticillioides on maize was evaluated on Bac A-treated seeds, which were placed on potato dextrose agar (PDA). The results thereof revealed that 1.5 × 109 CFU was a more efficient inhibitory concentration gainst the Fusarium spp. As Bac A was not effective for control of F. solani in vivo, phytotron investigations of the in vivo potential of Bac A as a BCA for seed treatment was evaluated against F. graminearum and F. verticillioides. Results showed that Bac A enhanced plant shoot height in Trial 1, and dry shoot mass and dry root mass of maize seedlings in the presence F. verticillioides in Trial 2. Bac A also increased shoot height of maize seedlings in the presence of F. graminearum in Trial 1. In addition, Bac A showed no adverse effects on the germination and emergence of maize. Bac A reduced shoot and root rot symptoms caused by F. verticillioides in both Trials 1 & 2. Only in Trial 1 were shoot and root rot severity caused by F. graminearum reduced by Bac A. The shelf-life of Bac A formulation for seed treatment over a storage period of three months was evaluated. Bac A formulation for seed treatment was consequently determined to retain viability and bioactivity of Bac A for the entire three months’ period of storage. Bac A proved to survive with its antifungal activity viable at 4 °C and at 25 °C. However, the antagonistic potential of Bac A after three months of storage was found to better at 4 °C storage conditions. Bac A had no negative impact of the germination of maize seeds after one month, two months, and three months of storage and at both temperatures. Bac A and Bac B showed significant biocontrol potential in that they were antagonistic against the selected Fusarium pathogens associated with maize in vitro. In addition, Bac A showed further protective ability in vivo against root and stem diseases caused by F. graminearum and F. verticillioides, as well as enhanced growth of maize shoot height and dry mass of seedlings. With simple Bac A suspension formulation, Bac A retained a better shelf-life at 4 °C for three months. Bac A has shown potential as a biofungicide and as a potential biofertilizer, which are both beneficial and cost effective in crop protection.