Characterization and documentation of diseases of passion fruit (Passiflora edulis Sims.)
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Department of Plant Pathology, College of Horticulture,Vellanikkara
Abstract
Passion fruit (Passiflora edulis Sims.), a native of Brazil is an emerging high value crop with fruits of great export potential. It is a woody, climbing vine which is valued for its fruits with unique flavor and aroma as well as for its nutritional and medicinal properties. During the recent past, there is tremendous increase in the cultivation of passion fruit, particularly in the state of Kerala. This is because of its importance as an assured source of income to small and marginal farmers. But, the longevity and productivity of passion fruit vines are often challenged by many biotic stress especially diseases. Even though many diseases are reported internationally, no systematic study has been conducted in the country particularly in Kerala due to the recent introduction of this crop in the agricultural scenario of the state. Hence, the present study was undertaken to identify the diseases affecting passion fruit occurring in Kerala, documentation of the same and characterization of associated pathogens. Purposive sampling surveys were conducted in the passion fruit growing tracts of five districts viz. Thrissur, Ernakulam, Palakkad, Wayanad and Ernakulam during the period of March, 2019 to January, 2020. Eight leaf spots (VKALS, MDKLS, NELLS, NENLS, MNPLS, MVPLS, THNLS and AMBLS), two leaf blights (NENLB and MVPLB), four fruit rots (MDKFR, NELFR, MVPFR and PAZFR), one bud blight (KLYBB), two wilts (MNPWt and KANWt) and one leaf malformation (VKALM) were observed during the survey. Among the foliage diseases, leaf blight (MVPLB) recorded the highest per cent disease incidence (PDI) of 78 per cent with a severity of 63.9 per cent. Among the fruit diseases, PAZFR recorded the highest incidence of 53.59 per cent disease with a severity of 26.47 per cent. The disease incidence of wilt; MNPWt and KANWt were 37.64 and 33 per cent respectively. Isolation and pathogenicity studies of the associated microorganisms yielded 15 fungal isolates (leaf spots, leaf blights and fruit rots), two bacterial isolates (wilt) and one virus isolate (leaf malformation). Symptoms of the different diseases were studied both under natural and artificial conditions. The fungal isolates were characterized and identified by studying the cultural and morphological characteristics. Based on these characters, Colletotrichum sp., Alternaria sp. and Lasiodiplodia sp. were identified as leaf spot pathogens, Rhizoctonia sp. as leaf blight pathogen, Phomopsis (Diaporthe) sp. as the bud blight pathogen and Colletotrichum sp., Alternaria sp. and Sclerotium sp. as fruit rot pathogens infecting passion fruit. The bacterial isolates (MNPWt and KANWt) causing wilt disease in passion fruit was identified as Ralstonia solanacearum Yabuuchi et al. based on cultural, morphological and biochemical characters. The differential host inoculation test conducted for race identification of the bacteria revealed that it belongs to Race 1. Morphological and biological characterisation of virus associated with leaf malformation revealed the association of Potyvirus as the causal organism of leaf and fruit malformation. Molecular characterization was carried out by genomic DNA isolation and amplification of specific region of fungal genome by Polymerase Chain Reaction (PCR) for the species level identification of the isolates. Amplification of Internal Transcribed Spacer (ITS) region of the fungal isolates followed by ITS sequencing and in silico analysis confirmed that the pathogen causing VKALS, NELLS, MVPLS, THNLS, AMBLS, NELFR and MVPFR as Colletotrichum brevisporum, MDKLS and MDKFR as Alternaria porri, NENLS as Colletotrichum queenslandicum, MNPLS as Lasiodiplodia theobromae, MVPLB and NENLB as Rhizoctonia solani, KLYBB as Diaporthe phaselorum and PAZFR as Sclerotium rolfsii. The bacterial isolates (MNPWt and KANWt) causing wilt disease in passion fruit was further confirmed as R. solanacearum through the amplification of 16 S rRNA region of the genome followed by sequencing and in silico analysis. The molecular characterization of Potyvirus associated with passion fruit was carried out using RNA extraction followed by cDNA synthesis and amplification of gene corresponding Cylindrical Inclusion (CI) protein using degenerate primers (Ha et al., 2008). The PCR yielded the expected band size of 680 bp, thus confirming Potyvirus as the causal organism of fruit and leaf malformation. An in vitro experiment was conducted to study the efficacy of fungicides and biocontrol agents against major fungal pathogens viz. C. brevisporum and S. rolfsii. Fungicides viz. copper hydroxide, hexaconazole, propineb, difenoconazole, carbendazim 12% + mancozeb 64%, azoxystrobin and Bordeaux mixture were found to be very effective against C. brevisporum at recommended dose and showed cent per cent inhibition of fungal growth. In the case of S. rolfsii, fungicides viz. hexaconazole and azoxystrobin were found to be effective even at a lower dose. Dual culture studies revealed that the biocontrol agent, Trichoderma sp. was effective against both the pathogens viz. C. brevisporum and S. rolfsii whereas Pseudomonas fluorescens was effective only against C. brevisporum.
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174975