Screening of Rhizo-Bacterial Isolates for Bio-control Potential against Soil-Borne Diseases in Brinjal

Studies were carried out from 2013 to 2015 with the objectives to isolate and identify some indigenous rhizobacterial strains against major soil-borne pathogens of brinjal. Seventy thizobacterial isolates were characterized based on colony characters, morphological and biochemical test and identified as Pseudomonas flourescens (30), P. aeruginosa (13), P. aureofaciens (2) and Bacillus subtilis (25). In vitro bioassay of the seventy rhizobacterial isolates revealed that the isolate I-58 was the most effective rhizobacterial isolate, followed by I-30 and I-55 isolates in inhibiting the radial growth of test pathogens.

Brinjal or eggplant (Solanum melongena L.), of the family Solanaceae is quite popular as the poor man's crop suffer from various soil-borne disease which emerged as major biotic stress in successful cultivation of this crop in India (Gupta et al., 2013).Soil borne plant pathogens responsible for the diseases due to their persistence nature in soil become more challenging and simultaneous infection by multiple soil-borne plant pathogens results in disease complex that further reduced the yield and quality of the brinjal crop (Koike et al., 2003).Currently, the management of soil-borne diseases has been done with the application of chemical fungicides.However, it may not be sustainable in the longer run as chemical fungicides are known to cause residual toxicity, toxicity to non-target organisms and other environmental hazards.In recent times much emphasis is being given to manage the soil borne diseases of the crop by employing the plant growth promoting rhizobacteria (Compant et al., 2005).
The mechanisms of PGPR included regulating hormonal and nutritional balance, inducing resistance against plant pathogens and solubilizing nutrients for easy uptake by plants (Vejan et al., 2016).The use of PGPR inoculants as biofertilizers is due to the production of some plant growth promoting substances, production of enzymes and production of some antifungal and antibacterial secondary metabolites (Weller, 1988) and as antagonists of phytopathogens due to secretion of antibiotics provide a promising alternative to chemical fertilizers and pesticides (Apastambh et al. 2016;Glick, 2012).The objectives of this study is to isolates the plant growth promoting bacteria from the rhizosphere of brinjal crop, their biochemical characterization and bio-control potential against major soil -borne plant pathogens of brinjal crop.

Isolation of rhizobacterial isolates
Soil samples were collected from the rhizosphere of healthy brinjal plants was processed for isolation of rhizobacterial isolates by serial dilution method (Dhingra and Sinclair, 1995).Single colonies formed were picked and streaked separately on petriplates containing nutrient agar and King's B media and purified by streak plate method (Koch, 1881).

In vitro evaluation of rhizobacterial isolates against the soil borne pathogens
The isolated rhizobacterial isolates were tested under in vitro conditions, against the major soil borne fungal pathogens viz., Fusarium oxysporum f. sp.melongenae, Rhizoctonia solani and Sclerotium rolfsii responsible for causing wilt, collar rot and root rot respectively in brinjal crop by using dual culture technique (Morton and Stroube, 1955).Observation regarding per cent inhibition of mycelial growth of pathogens was calculated (Wong et al., 2003)

Isolation and identification rhizobacterial isolates
Seventy rhizobacterial isolates were isolated from the brinjal fields across the locations surveyed.The selected rhizobacterial strains were purified by streak plate method and characterized based on colony characters, morphological and biochemical characteristics.The isolated rhizobacterial isolates exhibited variations in their colony characteristics (Table 1).Out of 70 isolates, 23 produced light green, 24 green, 17 white and 6 cream coloured colonies.Of these, 25 colonies were round and 45 were irregular in shape.Forty five isolates produced non-spreading type of colonies, whereas, 25 produced colonies that were spreading in nature.Further, microscopic examinations of isolated rhizobacteria revealed that out of the 70 isolates, 41 isolates had long rod shaped cells and the remaining 29 had short rod shaped cells, of which 45 strains showed Gram negative reaction and remaining 25 showed Gram positive reactions.

Biochemical characterization
Data in the table 2 indicated that the isolated rhizobacterial strains were tested for their     oxysporum f. sp.melongenae, R. solani and S. rofsii.In case of F. oxysporum f. sp.melongenae the minimum radial growth of 13.06 mm was recorded thereby effecting 85.48 per cent reduction in radial growth over control.In case of R. solani the isolate I-58 effected 82.90 per cent reduction over control (15.33 mm radial growth).With the isolate I-58 minimum radial growth of 14.00 mm with 84.44 per cent inhibition over control was recorded against S. rofsii.The rhizobacterial isolate I-58 was statistically at par with P. flourescens isolates I-55, I-43, I-50, I-67 and I-70 and Bacillus subtilis isolates I-30, I-33 and I-51.As per the effectivity of the rhizobacterial isolates, I-58 was followed by isolate I-30.influenced radial growth of 14.50 mm resulting in growth inhibition of 83.88 per cent over control in case of F. oxysporum f. sp.melongenae, 16.66 mm growth with 81.48 per cent growth inhibition over control in case of R. solani and 14.66 mm growth with 83.71 per cent reduction in growth over control in case of S. rofsii.The isolate I-30 was found to be at par with P. flourescens isolates I-55 and I-67 and Pseudomonas aeruginosa isolate I-41.T h e d a t a f u r t h e r exhibited that isolate I-55 resulted in the radial growth of 15.00 mm with 83.33 per cent reduction in growth over control in case of F. oxysporum f. sp.melongenae.It also resulted in radial growth of 15.66 mm with 82.60 per cent reduction in growth over control in case of R. solani.Isolate I-55 caused 81.11 per cent inhibition of mycelial growth of S. rofsii (17.00 mm radial growth).However, I-55 was at par with the P. flourescens isolates I-32 and I-70.Tennakoon (2007) reported the inhibition of the mycelial growth of one or the other pathogens (Pestolotia thea, Fusarium oxysporum f. sp.carthami and Rhizoctonia bataticola) by volatile metabolites produced by 7 out of 11 isolates of fluorescent pseudomonads and two out of three Bacillus isolates.Biswas and Singh (2008) used Bacillus subtilis, P. fluorescens and T. viridae against wilt of tomato and observed that P. fluorescens was effective in minimizing the disease.B. subtilis inhibited the growth of F. oxysporum and and recommended to purify antifungal compounds (Khan et al., 2011).As reported by Nandakumar et al. (2001) and Asha et al. (2011) inhibition of mycelial growth of R. solani was due to nutritional competition, colonization of fungal hyphae production of inhibitory compounds by the rhizobacteria.

Table 1 .
Cultural and morphological characteristics of rhizobacterial isolates

Table 2 .
Biochemical characteristics of rhizobacterial strains

Table 4 .
In vitro evaluation of rhizobacterial isolates against the soil borne pathogens obtained from brinjal plants