Characterization of plant growth promoting Bacillus strains and their potential as crop protectants against Phytophthora capsici in tomato

Bacillus spp. are well known rhizosphere residents of many crops and usually show plant growth promoting (PGP) activities that include biocontrol capacity against some phytopathogenic fungi. A total of 20 PGP rhizobacterial strains belonging to the genus Bacillus, previously isolated from the rhizosphere of several crops, were evaluated for their efficacy against Phytophthora capsici causing root and crown rot of tomato. These isolates were characterized for their PGP traits and in vitro antibiosis. Based on their in vitro antibiosis, liquid based formulations of four isolates - S-21 and S-25 from tomato, B-6 from apple and A(10) from turmeric rhizosphere were used for in planta evaluation against P. capsici under net house conditions singly and in conjoint applications. Liquid based culture of strain S-25 was found to be the most promising biocontrol treatment and reduced the disease severity by 83.3%. Conjoint application of four strains also had significant effect in achieving biocontrol efficacy of 66.7%. Inoculation with strain S-25 and consortium of the four strains resulted in significant increase in most plant parameters including root length, shoot length, plant fresh weight and plant dry weight over uninoculated control. 16S rDNA and phylogenetic analysis showed that strain S-25 exhibits 99% level of similarity with Bacillus subtilis. Therefore it was designated as B. subtilis strain CKTR. Hence liquid based culture of strain S-25 can be used as a promising antagonistic agent for management of Phytophthora blight. The effectiveness of the strain under field conditions and modes of action at the molecular level needs to be explored.