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Extracts from 20 non-host plant species were tested for their ability
to inhibit the mycelial growth of Alternaria solani, the causal agent of
early blight disease of tomato. In vitro studies indicated that leaf
extract of Zimmu (Allium cepa L. xAllium sativum L.) demonstrated the
highest inhibition of mycelial growth (87%) of A. solani. Known biocontrol
agents Pseudomonas fluorescens (Pf1 and Py15) and Bacillus subtilis (Bs16)
were also tested alone, together, and in conjunction with the most
effective plant extract, Zimmu, in both in vitro and in vivo experiments
for control of A. solani. All isolates were compatible with each other and
with Zimmu leaf extract. Among the various bioformulations tested as seed
treatment and foliar application, the talc-based formulation of
Pf1+Py15+Bs16+Zimmu was superior in reducing the early blight disease
incidence when compared to other treatments. Further, induction of defense
enzymes, such as peroxidase (PO) and polyphenol oxidase (PPO) phenylalanine
ammonia-lyase (PAL), chitinase and @b-1,3-glucanase and accumulation of
phenolics were studied. The enzyme accumulation was greater in
Pf1+Py15+Bs16+Zimmu treated plants compared to control. The study revealed
the probable influence of plant growth promotion and induced systemic
resistance (ISR) in enhancing the disease resistance in tomato plants
against early blight disease by Zimmu based PGPR mixture.
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