Efficacy of rhizobacteria for growth promotion and biocontrol of Fusarium oxysporum and Rhizoctonia solani on wheat in South Africa

Abstract

Plant growth promoting rhizobacteria (PGPR) are found in the rhizosphere and directly contribute to nutrient availability and plant growth. Due to the global move away form reliance on chemical fertilisers and crop protection products, plant growth promoting rhizobacteria (PGPR) and their application for improving crop production is receiving much attention. In the current work the selected PGPR isolates from the University of Pretoria PGPR Culture Collection were evaluated on wheat seedlings for their ability to promote seedling growth and control of selected root diseases. The ability of the PGPR to control root and crown rot of wheat caused by Fusarium oxysporum, F. graminearum and Rhizoctonia solani was evaluated in greenhouse pot trials. In addition the seedling tray assay was also evaluated as a means to screen PGPR for biocontrol. Under the test conditions, the seedling tray assay was unsuccessful as a method to screen the PGPR for biocontrol of wheat root pathogens. In the greenhouse pot trials several isolates were able to improve the growth of wheat seedlings cultivated in pathogen inoculated soil. For growth promotion the bacterial isolates were assessed in greenhouse trials. Following initial selection trials further analyses were conducted with selected isolates to determine what effect the PGPR inoculum dose and fertiliser application has on growth promotion efficacy. The isolates T06, T07, T11, T13, T19, T21, T23 and T24 were most effective in promoting the growth of wheat seedlings in the greenhouse. The dose response and fertiliser trials indicated that these factors do affect isolate activity. To facilitate understanding of the isolates and their activities, the isolates that performed well in the greenhouse were identified through 16S rRNA sequence data and selected modes of action determined. The isolates were predominantly Bacilli, from the genera Bacillus, Lysinibacillus and Paenibacillus. In vitro assessments revealed that the isolates have a wide spectrum of activity, including phosphate solubilisation, indole acetic acid production, growth in nitrogen free media, inhibition of pathogens in dual culture, chitinase production and siderophore production. A selection protocol was developed to assist with the selection of the best isolates. Based on ratings given for the greenhouse and in vitro assay the top performing isolate were selected. Spider diagrams were then constructed to visually represent each isolate’s performance. The following best performing isolates were selected as a) T10 (Bacillus cereus) for control of R. solani, b) T28 (Bacillus cereus) for control of F. oxysporum, c) T31 (Bacillus cereus) for control of F. graminearum, d) T13 (Bacillus sp. / Bacillus pumilus) for growth promotion of wheat and e) T29 (Paenibacillus alvei) as the most versatile isolate.