Utilization of Bacillus spp. as plant probiotics.

Abstract

Numerous microorganisms produce beneficial effects on plant developmentwhen applied to crop seeds or incorporated into soil. Research efforts worldwide over the past two decades have renewed commercial interest in plant growth promoting rhizobacteria (PGPR).With successes being recorded in PGPR research, it is expected that within the next few years, more commercial PGPR products will be available on the market. In particular, commercial PGPR could be advantageous to plant nurseries if they enabled earlier sale of plants, more rapid turnover of seedlings and further crop production cycles. Trials were carried out to evaluate the growth stimulation and biological control abilities of Biostart™, a Bacillus-based plant probiotic comprising seven Bacillus spp. and Biostart® 2000 (a combination of three of the Bacillus spp.). The first priority was to determine the survival pattern of six Biostart™ Bacillus spp., namely B. chitinosporus, B. uniflagellatus, B. laterosporus, B. pumilus, B. subtilis and B. licheniformis in potting soil in the presence or absence of a crop plant, i.e., cucumber, with or without NutriStart-AC. Bacterial numbers in pots in the absence of cucumber seedlings, with or without NutriStart-AC, declined slowly but steadily. Population sizes in pots without NutriStart-AC decreased steadily from Day 1 to Day 14 for all six Bacillus spp. and thereafter remained constant between 6.19 and 6.15 log cfu g -1 of wet soil for all six Bacillus spp. up to termination of the experiment on Day 35. A similar effect was observed in pots supplemented with one gram of NutriStart-AC. In the presence of cucumber seedlings, population sizes in pots without NutriStart-AC supplement declined faster until Day 14 than those in the NutriStart-AC supplemented pots. Populations remained stable after Day 14 for all six Bacillus spp. in the NutriStart-AC unsupplemented pots, while there was a variation in population sizes among Bacillus spp. in pots supplemented with NutriStart-AC. Growth stimulation trials in tunnels were carried out using four crops, i.e., lettuce, tomato, sorghum and beans. Seed treatment and seed treatment plus drenching with or without NutriStart-AC were evaluated. All Bacillus spp. used stimulated plant growth. Growth stimulation was more pronounced with a 4% NutriStart-AC supplement. Growth stimulation was best in lettuce, with Biostart® 2000. There was an increase of 466% compared to the dry biomass of the water control lettuce seedlings. The lowest responses were recorded in sorghum and beans. Three tomato cultivars, i.e., Roma, Floradade and Rodade and a pepper cultivar Thai were evaluated for growth stimulation by applying Biostart™ as seed treatment and seedling drench. The highest growth stimulation, 96%, was obtained using B. licheniformis on Roma as a seedling drench. Growth response was better in Roma and Floradade cultivars than in the Rodade cultivar. Pepper plants drenched with Biostart™ Bacillus spp., and supplemented weekly with a 4% NutriStart-AC suspension, showed increased fruit yield. Using B. subtilis, a 533% increase in fruit yield was recorded when seedlings were supplemented weekly with a 4% NutriStart-AC suspension. Similar results were recorded using an unidentified Bacillus strain CM-33 (433%) and B. licheniformis (333%). In a nematode control trial, no galls were found on the roots of treated and untreated control seedlings inoculated with Meloidogyne spp. Early inoculation of seedlings might have failed because there were no roots for the nematodes to attack at the time of inoculation. In a biological control trial, Biostart™ Bacillus spp. were applied by seed treatment and seedling drench to control Rhizoctonia causing damping-off of marigold, cabbage and eucalyptus seedlings. Biostart™ was ineffective under the conditions of this trial. Most seedlings died seven days after pathogen inoculation and by Day 21 about 90% of the seedlings were dead. The results presented in this thesis have some practical applications to seedling growers in South Africa, especially in growth promotion. Applying Biostart™ probiotic Bacillus spp. may increase the turnover of seedlings in nurseries. More trials are needed if the growth promotion and biological control potentials of Biostart™ probiotic Bacillus spp. are to be fully exploited.