Management of early blight disease of tomato (Solanum lycopersicum L.) under protected cultivation
Loading...
Files
Date
2020
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Department of Plant Pathology, College of Horticulture, Vellanikkara
Abstract
Tomato (Solanum lycopersicum L.) is one of the most remunerative and widely
grown vegetables all over the world. With the coordinated efforts of central and state
governments, protected cultivation of tomato is now gaining popularity in Kerala. Despite
being a versatile crop adapted to various agroclimatic regions and seasons, cultivation of
tomato is constrained by various fungal, bacterial and viral diseases. Among the fungal
diseases, early blight caused by Alternaria solani is the most common, destructive and
widespread in all the tomato growing tracts. Fungicides and bioagents are commonly used
to manage plant pathogens. But little is known about their effects on the non-target
microbial communities that inhabit inside and outside the plant. Hence, it has become
necessary to consider the effect of different fungicidal and bioagent treatments on target
and non-target microbial communities while formulating disease management strategies.
So, the present investigation was carried out with the objectives to formulate suitable
management strategies against early blight disease of tomato under protected cultivation
and to assess their impact on culturable and non-culturable microflora associated with the
plant.
Isolation of the pathogen from infected tomato leaf samples revealed the association
of the fungus, Alternaria sp. and its pathogenicity was established by inoculating on threemonth-
old tomato seedlings. Symptoms observed on leaves, shoot and fruits were almost
same under both natural and artificial conditions. Cultural and morphological characters of
pathogen was studied on potato dextrose agar (PDA). Initially, pathogen produced greenish
brown mycelium and later turned to grey colour. Hyphae are septate and the colony has
aerial topography and irregular rough growth patterns with concentric zonation.
Sporulation was observed after six days of incubation and conidiophores were straight or
flexuous brown to olivaceous brown in colour. The conidia are solitary straight or muriform
or oblong, pale or olivaceous brown, length 40-110 μm and 7-15 μm thick with 2-8
transverse and 0-3 longitudinal septa. The cultural and morphological characters of the
pathogen completely fit into the description of Alternaria solani by Alexopoulos et al.
(1996). Hence, it is confirmed that the symptom observed on tomato leaves are those of
early blight disease caused by A. solani.
In vitro evaluation of fungicides and bioagents showed complete inhibition of the
pathogen with propineb (0.1%, 0.2% & 0.3%), hexaconazole (0.05%, 0.1% & 0.15%),
iprodione + carbendazim (0.1%, 0.2% & 0.3%), difenoconazole (0.075%), Trichoderma
viride (KAU), T. viride (PGPM mix), T. harzianum (PGPM mix) and plant growth
promoting microbial consortium (PGPM mix of KAU). Among the bacterial antagonists,
Bacillus subtilis (endophyte from cocoa) showed maximum growth inhibition of the
pathogen. All the three bioagents recorded earliness in seed germination and enhanced
seedling vigour compared to the fungicidal treatments and control.
The results of field experiment under polyhouse and rain shelter conditions
showed that all the treatments are superior to control in early blight disease management,
of which, spraying of iprodione + carbendazim (0.2%) and propineb (0.2%) were the best
among fungicides and PGPM mix application was the most efficient among bioagents.
Moreover, the highest yield was recorded from iprodione + carbendazim treated plants.
Biocontrol treated plants showed better performance in overall plant vigour of which
PGPM mix application was the most effective. Residue analysis showed that degradation
rate of fungicides was more under polyhouse condition.
Analysis of population of phylloplane and endophytic microflora proved that there
was drastic reduction in microbial population after spraying with chemical fungicides
whereas population increased after bioagent application. The study on survival of
bioagents on tomato phylloplane revealed that both Pseudomonas fluorescens and T.
viride, survived on leaf surface up to 15 days after foliar application. Analysis of
fungicidal residue on tomato fruits revealed that, the degradation of fungicides was faster
in polyhouse compared to rain shelter.
Metagenomic analysis of microbial diversity on tomato leaves revealed that
spraying of chemical fungicides reduces microbial population and diversity while
bioagent application enhances the same. However, microbial community structure was
changed in both cases. This study also enlightened the new mode of action for fungicides
and bioagents besides their direct effect that is shifting the microbial community structure
so that it provides greater resistance against the pathogen. Interestingly, metagenomic
results also showed association of Cladosporium, Corynespora, Pseudocercospora along
with early blight pathogen Alternaria on tomato leaves that otherwise remain undetected.
Another important observation was Clostridium in tomato leaf samples except in PGPM
mix treatment, suggesting the possibility of plants as alternate host for major human and
animal bacterial pathogens.
Hence, considering the effects of treatments on per cent disease severity both under
polyhouse and rain shelter condition, residue analysis, phylloplane and endophytic
microbial enumeration study and metagenomics analysis of microbial diversity, the present
study recommends spraying of propineb (0.2%) as the best treatment among the tested
fungicides and spraying of PGPM mix among biocontrol agents for the management of
early blight disease of tomato under protected cultivation. Further system-level analysis of
the complex interaction that governs outcomes among community members in the context
of the plant host is required, in order to identify microbial interaction and selection
processes for beneficial communities at different concentrations of fungicides and
pathogen pressures.
Description
PhD