PhD

The study on “Enhancement of resistance to bacterial wilt in tomato by
endophytic microbial communities” was conducted in the Department of Plant
Pathology, Department of Agricultural Microbiology, and Centre for Plant
Biotechnology and Molecular Biology, College of Horticulture, Vellanikkara during
the period 2011-2014.
The endophytes were isolated from root and stem of healthy tomato plants
from 16 locations of north, central and south Kerala. Endophytic microbial
population varied with the plant samples and the population was more in root than
stem. Bacterial population was higher than fungi and actinomycetes in root and stem.
Among 154 endophytes isolated, 12 out of 79 bacteria, 16 out of 68 fungi, and four
out of seven actinomycetes were antagonistic to R. solanacearum in in vitro. Among
them, five bacteria, eight fungi, and two actinomycetes were promising in planta.
Mutually compatible endophytic isolates were selected for the development of
consortia and these were identified based on cultural, morphological and molecular
characters. Of the five consortia tested, the one consisted of Trichoderma viride-1,
T. viride-2, T. harzianum-1, Bacillus subtilis, and Streptomyces thermodiastaticus
showed effect in reducing wilt incidence.
Comparative study of the microbial consortium with individual endophytes
showed the higher efficacy of consortium in reducing the wilt incidence. The
endophytes were reisolated from soil, root, and stem of tomato plants. In pot culture
experiment, the consortium applied as seed treatment + seedling dip + soil application
at 45 DAP showed the minimum wilt incidence. Studies on the mechanism of
antagonism of endophytic isolates showed, positive reaction towards ammonia and
negative for HCN and siderophore production. The volatile and nonvolatile
metabolites of the endophytes inhibited the pathogen. The endophytes showed
varying levels of IAA and salicylic acid production with the maximum in
T. harzianum-1 and B. subtilis respectively.
Study on the effect of secondary metabolites of endophytes on the disease
indicated that, seedlings dipped first in culture filtrate for 2 h and later dipped in
bacterial inoculum for 30 min before planting showed the lowest per cent wilt
incidence with the minimum for T. viride-2. Induction of systemic resistance was
studied by assay of defense related compounds such as phenols, oxidative enzymes
and PR proteins. The plants treated with microbial consortium showed higher activity
of the defense related compounds with the maximum in plants inoculated with both
consortium and pathogen.
Field evaluation of endophytic consortium against bacterial wilt showed
reduction in wilt incidence of 40.85 per cent in highly susceptible variety, PKM-1,
46.94 per cent in susceptible F1 hybrid, COTH-3, and 52.81 per cent in moderately
resistant variety, Mukthi.
Thus, the study revealed that, the application of endophytic consortium can
enhance the resistance mechanism in tomato against bacterial wilt pathogen,
R. solanacearum.