Biomanagement of root rot and leaf spot disease of stevia rebaudiana using plant growth promoting rhizobacteria

Abstract

Stevia rebaudiana Bertoni a natural sweetener contains two main sweetest compounds newlinestevioside ST and rebaudioside A tasting about 300 and 450 times sweeter than sucrose newlinerespectively This commercially important plant also suffers by a root rot disease caused by the fungus Sclerotium rolfsii and leaf spot disease caused by Alternaria alternata in various districts of Tamil Nadu India In the present study ten isolates of fluorescent pseudomonads and ten isolates of Bacillus subtilis were evaluated for their ability to control root rot and leaf spot in Stevia These isolates were characterized as Pseudomonas fluorescens and Bacillus subtilis based on newlinebiochemical tests and molecular characterisation Among these isolates P fluorescens isolate AUPF6 newlineand AUPF5 and Bacillus subtilis isolates AUBS2 and AUBS5 showed the maximum inhibition of newlinemycelial growth of Sclerotium rolfsii and Alternaria alternata They also increased plant growth in newlineStevia apart from reducing the root rot and leaf spot incidence under greenhouse condition The isolates AUPF6 and AUPF5 AUBS2 and AUBS5 were further tested for their ability to induce production of defense related enzymes and chemicals in plants Significantly increased activities of phenylalanine ammonia lyase PAL peroxidase PO and polyphenol oxidase PPO were observed in P fluorescens isolates AUPF6 and AUPF5 and Bacillus subtilis isolates AUBS2 and AUBS5 pretreated stevia plants challenged with Sclerotium rolfsii and Alternaria alternata Moreover higher accumulation of phenolics was noticed in plants pretreated with P fluorescens isolates AUPF6 and AUPF5 Bacillus subtilis isolates AUBS2 and AUBS5 challenged with Sclerotium rolfsii and Alternaria alternata Thus the present study shows that in addition to direct antagonism and plant growth promotion induction of defense related enzymes involved in the phenyl propanoid pathway collectively contributed to enhance resistance against the invasion of Sclerotium rolfsii and Alternaria alternata in Stevia plants