Abstract:
Seven selected bacterial antagonists isolated from healthy tomato rhizosphere
and phyllosphere were evaluated in vitro for their biocontrol efficacy against
Alternaria alternata, the causal agent of tomato early blight. A. alternata was
isolated from a tomato leaf sample which showed early blight symptoms and
verified by their spore morphology according to literature. Disease-free
tomato rhizosphere soil and phyllosphere extracts were used as the potential
sources of antagonists from which 30 bacterial isolates and 22 bacterial
isolates could be isolated using the serial dilution technique in order. Out of
those, 24 different isolates were subjected to screening procedure of
antagonists against A. alternata. It was observed that, 20 bacterial isolates out
of 24, inhibited the growth of A. alternata significantly (p<0.05) in cocultivation
and dual culture plate assays showing an average radial growth
inhibition of 69%. Seven different bacterial isolates coded as RA 8, RA 12,
RA 17, RA 18, RA 29, PA 4 PA 12 were selected for further studies.
Microscopic observation of A. alternata hyphae dual cultured with bacterial
antagonists showed alterations in hyphal characteristics such as thickening,
swelling and vacuolation. Inhibition of A. alternata spore germination by
selected antagonists was significant (p<0.05) in which the average percent
inhibition was 64%. All the selected antagonists produced diffusible
antifungal substances and showed hyperparasitism and competition as their
potential mechanisms in antagonizing the fungal pathogen A. alternata. These
results suggested that above mentioned seven bacterial isolates can be further
studied as potential biocontrol agents in controlling early blight disease of
tomato.
