Biological control of phytopathogenic fungi of vanilla through lytic action of Trichoderma species and Pseudomonas fluorescens

Abstract

Trichoderma, soil-borne filamentous fungi, are capable of parasitising several plant pathogenic fungi. Secretion of lytic enzymes, mainly glucanases, proteases and chitinases, is considered the most crucial step of the mycoparasitic process. The lytic enzymes degrade the cell walls of the pathogenic fungi, enabling Trichoderma to utilise both their cell walls and cellular contents for nutrition. Twenty-one strains of Trichoderma spp. were isolated from vanilla rhizosphere and suppressive soils from different regions of Tamil Nadu. The antagonistic effect of Trichoderma was studied towards a range of phytopathogenic fungi; Fusarium oxysporum f.sp. vanillae, P. meadii and Colletotrichum vanillae in vanilla. Coculture of the phytopathogens and Trichodema under laboratory conditions clearly showed dominance of the Trichoderma species. In all cases, Trichoderma overgrew the phytopathogens and subsequently developed a conidial lawn over the surface. The percent inhibition of pathogen growth by isolates T1 and T17 were significantly high by 49.5 and 48.2% respectively. In these studies the lytic action of the pathogen was clearly apparent and the inhibition of growth appears directly related to its ability to hydrolyse the cell walls of the tested microorganisms. Protease, chitinase and β-1,3-glucanase activities were determined in the extracellular fluid of Trichoderma. The data imply that the nature of Trichoderma antagonism is based on mycoparasitism (lysis) and appears to optimalize with contact between the mycelia. Six of them (Tricho1, Tricho6, Tricho10, Tricho12, Tricho15 and Tricho17) were prioritized for further evaluation to utilize different substrates by chitinase production. T. viride isolates T1 and T17 produced more extracellular protease, chitinase and β-1,3 glucanase, which cause disruption of the cell wall and membrane structure of pathogen. The increase in mycelial weight, inhibition behaviour and production of extracellular enzymes by these isolates suggest that the isolates can be used as biological control agents in vanilla plantations. In vanilla field evaluation with microbial consortia involving P. fluorescens strains and Trichoderma isolates, the combinations viz., Pf1 + FP7 + chitin (soil and foliar), Pf1 + Tricho1 + chitin (soil) + Pf1 + chitin (foliar) and Pf1 + Tricho17 + chitin (soil) + Pf1 + chitin (foliar) performed well in reducing the disease incidence.