Fermentation, formulation and evaluation of PGPR Bacillus subtilis isolate as a bioagent for reducing occurrence of peanut soil-borne diseases

Four isolates of Bacillus subtilis coded, B4, B7, B8 and B10 were examined as biocontrol agents for their abilities and antagonistic effect on the in vitro growth of certain phytopathogenic fungi of peanut, Rhizoctonia solani and Sclerotium rolfsii. Bacillus subtilis isolate B4 (GenBank accession no. EF150884) was the highly effective one for inhibiting the fungal mycelial growth. Batch fermentation of B. subtilis isolate B4 was carried out and the maximum biomass achieved was 4.53 g L-1 at 11 h. Bacillus subtilis isolate B4 was formulated and evaluated as a biofungicide to reduce peanut soil-borne diseases under greenhouse and field conditions at the side of Rizolex-T (fungicide) as standard. Treatments by formulated plant growth-promoting rhizobacteria (PGPR) B. subtilis B4 and Rizolex-T in a soil infested with R. solani, S. rolfsii and mixture of them were more effective in decreasing percentage of damping-off, root and pod rot disease incidence (%) in greenhouse and open field environment during the two seasons 2015 and 2016. Treatments by PGPR gave highly dry weight and number of healthy pods compared to control of fungi treatment which was nearby to dry weights of healthy pods achieved by treatments by Rizolex-T in a soil infested with S. rolfsii, R. solani and mixture of them. Formulated PGPR B. subtilis B4 gave higher increasing of yield percentage than treatment by Rizolex-T in the two evaluated seasons 2015 and 2016. It can conclude that the produced bioforumlated agent was more efficient as fungicide when compared with the other chemical synthesized fungicides, safe for human and the environment and economy.