MANAGEMENT OF FUSARIUM WILT IN TOMATO USING NON-PATHOGENIC INDIGENOUS PSEUDOMONAS STRAINS

Abstract

Tomato (Solanum lycopersicum L.) being an important vegetable crop is cultivated worldwide. Several diseases affect its production among which pathogenic fungi especially Fusarium oxysporum f. sp. lycopersici inciting wilt disease in tomatoes is the most challenging one. Management strategies to deal this disease mostly rely on expensive inputs which not only raise economic concerns but also environmental safety issues due to toxicity caused by agrochemicals. So there is dire need of some control measures that show tendency to suppress this disease with less reliance on chemicals and are ecofriendly. Biocontrol of plant diseases through induction of systemic resistance is the best substitute to chemicals in crop protection measures. Different biotic and abiotic elicitors can trigger the plant for induced resistance. Present study was designed to explore the potential of indigenous Pseudomonas strains in inducing systemic resistance in tomato against Fusarium wilt. Concentration was confined on searching of potential ISR determinant/s from the competent bacterial strain/s involved in activating defence system of the host plant.  First, seven strains of Fusarium oxysporum f. sp. lycopersici (FOL) were isolated and were tested for their virulence against fifteen varieties of tomato. On the basis of virulence Fusarium strain “FOLc” was selected whereas on the basis of susceptibility two tomato varieties Rio-Grande (71.59%) and Nagina (70.84%) were selected for further experimentation.  Thirty one nonpathogenic indigenous strains of Pseudomonas were isolated from healthy tomato plants growing in different areas of Lahore, Pakistan. These bacterial strains were subjected to split root experiment under greenhouse conditions for screening. Eight Pseudomonas strains were selected that reduced disease index significantly as compared to the pathogen control. Among these eight the two most efficient strains were Pseudomonas aeruginosa (PM12) and P. putida (PM29) that provided ≥ 75% reduction in disease index. Selected eight bacterial strains were again subjected to split root experiment to document the changes in defense related biochemicals like phenolics and defense related enzymes. It was found that Pseudomonas VI aeruginosa (PM12) and P. putida (PM29) significantly increased these biochemicals as compared to the control plants.  Next study was conducted to check the induced systemic resistance on molecular and histological grounds. Pseudomonas aeruginosa (PM12) and P. putida (PM29) showed high expression profiles of Pathogenesis related (PR) genes and histological defence barriers like phenolics, peroxidases and lignin in stem and root tissues.  Field trials of the top most bacterial strains viz; Pseudomonas aeruginosa (PM12) and P. putida (PM29) were conducted using sugarcane press mud based formulation. Both the strains applied as consortia reduced disease index to 69.89% and 75.40% in variety Rio-Grande during the years 2013 and 2014 respectively along with increase in yield.  Study regarding searching of potential ISR determinant/s from the most competent strain Pseudomonas aeruginosa (PM12) showed that extracellular metabolites were active in reducing the disease then GC/MS analysis confirmed the involvement of 3-hydroxy, 5-methoxy benezene methanol (HMB) in triggering ISR against the pathogen.  Total metabolome analysis of tomato plants through GC/MS revealed induced metabolic transitions in plants under the influence of ISR elicitor, HMB. Metabolites involved in primary metabolism, phenylpropenoid pathway, signaling pathway and tricarboxylic acid cycle were found significantly increased by the elicitor compound in order to activate the defence machinery of the plant.  To check the results statistically all the data was subjected to analysis of variance (ANOVA) followed by Duncan’s Multiple Range Test (DMRT). This study concludes that Pseudomonas aeruginosa PM12 has the ability to produce elicitors that can induce systemic resistance in crop plants like tomato against Fusarium wilt. Application of this bacterium or its elicitor compounds to mitigate Fusarium wilt can provide a better solution to the agro-environmental challenges.