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2015 - 20202
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Subject: Plant Pathology × Author: KAU × Author: Sumbula, V × Thesis Type: Ph.D ×
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    ThesisItemOpen Access
    Management of early blight disease of tomato (Solanum lycopersicum L.) under protected cultivation
    (Department of Plant Pathology, College of Horticulture, Vellanikkara, 2020) Sumbula, V; KAU; Sainamole Kurian, P
    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.
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    ThesisItemOpen Access
    Phenotypic and molecular characterisation of phytophthora sp. inciting leaf fall of nutmeg
    (College of Horticulture, Vellanikkara, 2015) Sumbula, V; KAU; Sally, K Mathew
    The study on ‘Phenotypic and molecular characterisation of Phytophthora sp. inciting leaf fall of nutmeg’ was conducted in the Department of Plant Pathology, College of Horticulture,Vellanikkara during 2014-2015. The major objectives were to study the cultural, morphological and molecular characters and variability of different isolates of Phytophthora sp. associated with leaf fall of nutmeg and also to study the host range of the pathogen and to chalkout suitable management strategies Isolation of the pathogen from 18 samples from different locations revealed the association of the fungus, Phytophthora sp. and its pathogenicity was established under lab and in vivo conditions. Inoculation of the pathogen with culture disc on injured lower leaf surface showed early infection than that with zoospore suspension. Symptoms observed on leaves, shoot and fruits were almost same under both natural and artificial conditions. Variation in virulence was noticed among the isolates collected from different locations. The isolates, PPaL-1 and PPaF-17, from Parakkadavu, Thrissur and PSrL-10 from Sreemoolanagaram, Ernakulam were highly virulent. PKoL-2, the isolate from Kodissery was less virulent and other 14 were moderately virulent. Cultural and morphological characters of the isolates of pathogen were studied with different media viz. carrot agar, potato dextrose agar, oat meal agar, coconut water agar and V8 juice agar. Variation in cultural characters among the isolates was observed only in carrot agar and the variation in growth rate was noticed among the different media. Morphologically, mycelia of Phytophthora isolates from nutmeg were branched, coenocytic and hyaline and the sporangia were borne terminally /laterally on the sporangiophore in sympodial fashion, caducous, semi papillate, ovoid/elongated-ovoid/ellipsoid in shape with average size of 31.9-49.5 x 22.3 – 27.9 μm, L/B ratio of 1.4 – 1.8 and pedicel length of 10.21 – 20.24 μm. Early sporangial production was noticed in carrot agar and potato dextrose agar and the maximum count was in oat meal agar and carrot agar. Numerous chlamydospores were observed in all media. Comparison on the cultural and morphological characters of Phytophthora isolates of nutmeg with other Phytophthora spp. such as P. meadii, P. palmivora, P. capsici, P. colocasiae, P. citrophthora and P. ramorum revealed that, Phytophthora isolates from nutmeg could not be completely fitted into the phenotypic description of any of these known Phytophthora species. However, they showed some similarity to P. meadii, P. colocasiae and P. ramorum. In molecular characterisation, out of 18 isolates of nutmeg Phytophthora, 15 showed maximum homology with P. colocasiae and three viz. PPoL-3, PMaL-4 and PVaL-15 with P. meadii. Isolates PPaL-1, PKoL-2, PVeL-5, PKtL-6, PKaL-7, PMtL-8, PThL-9, PTuL-11, PKnL -12, PPaF-17 and PMoF-18 also showed homology with P. citrophthora and PMaL-4 and PSrL-10 with P. botryosa. Host range of Phytophthora isolate of nutmeg includes, rubber, vanilla, rose, Coreopsis, Eucalyptus and Citrus. Nutmeg is also a host of P. meadii of vanilla and P. citrophthora of Citrus and non host of P. palmivora, P. capsici, P. colocasiae, P. meadii of arecanut, rubber and cardamom. Rose is also found to be a host of P. meadii isolates of arecanut, rubber, cardamom, vanilla and P.citrophthora of Citrus. The cultural, morphological and molecular characters, host range and cross infectivity studies of various Phytophthora isolates could not revealed the exact identity of these isolates, as the distinguishing features overlapped among the various Phytophthora species. In vitro evaluation of chemicals / bioagents showed complete inhibition of the pathogen with 1% Bordeaux mixture, copper hydroxide (2g/l), copper oxychloride (2.5g/l), potassium phosphonate (3ml/l), combination fungicides, iprovalicarb + propineb (1.5 and 2.0g/l), cymoxanil + mancozeb (2g/l) and Trichoderma viride -1, T. harzianum and T. viride -2, the isolates from nutmeg and the reference cultures viz. T. viride (KAU) and T. harzianum (IISR). In in vivo experiment, all treatments were superior to control of which, spraying of 1% Bordeaux mixture + soil drenching of copper hydroxide (2g/l) and spraying of 1% Bordeaux mixture and soil application of T. viride showed maximum reduction of the disease. In addition, spraying and drenching of copper hydroxide and copper oxychloride were also found equally effective.