Identification of major fungal pathogens of dragon fruit and management of its major disease

Abstract

The research study entitled "Identification of fungal pathogens of dragon fruit and management of its major disease" was carried out at the Department of Plant Pathology, College of Agriculture, Vellayani, from 2021 to 2023 to identify and characterize the prevailing fungal pathogens of dragon fruit plants grown in Southern Kerala and develop an integrated management strategy against its major diseases. Two dragon fruit cultivating regions, AEU 09 (south-central laterite) and AEU 12 (southern and central foothill) in Kerala, were surveyed to identify the major fungal pathogens and associated symptoms on dragon fruit plants. Stem canker, characterized by yellow spots that further progressed into orange-brown lesions with yellow haloes, was observed in plantations of Palode and Vembayam regions of Thiruvananthapuram district. In the Vellavoor block of Kottayam district (AEU 12), a widespread outbreak of a wilt disease caused by Rigidoporus spp. was recorded. In Palode region (AEU 09), Fusarium infection was prevalent, where soft rot symptom was observed near the soil line and brownish-red lesions on the stems/cladodes. Neoscytalidium spp, Fusarium spp, and Rigidoporus spp were isolated from the infected samples collected from various locations. Three isolates of Neoscytalidium spp (I1, I3, I5) were obtained from Palode, Vembayam, and Pala regions respectively. Among them, the isolate (I1) was found to be more virulent, and hence selected for further studies. The pathogenicity of these isolates was confirmed by artificial inoculation on healthy dragon fruit cladodes and fruits, and Koch's postulates were established.

The morphological and cultural characteristics of each isolate was examined on potato dextrose agar (PDA) medium. Neoscytalidium isolates (I1, I3, I5) exhibited greyish colonies with dark grey to black pigmentation. The average growth rate of these isolates was about 1.06 cm per day. Their hyphae were characterized by brown colour, branching pattern, and septation. Conidia of various shapes and colours were formed in chains (arthroconidia). The Isolate I2 had white aerial mycelium with concentric rings and sickle-shaped conidia. The Isolate I4 produced dense cottony white mycelia with an average growth rate of 0.87 cm per day. Molecular characterization with ITS primers indicated that isolates I1, I3, and I5 had 97.21 Per cent similarity with Neoscytalidium spp reported from Malaysia, and China respectively. The isolates I2 and I4 showed 95.29 Per cent and 100 Per cent similarity with Fusarium spp and Rigidoporus spp reported from India and Malaysia respectively. DNA of isolate I2 on amplification in PCR with ITS and TEF primers revealed 100 per cent similarity to F. fujikuori Potential of biocontrol agents, including Bacillus amyloliquefacians, Pseudomonas fluorescens, Trichoderma asperellum (KAU isolate), and T. harzianum (IISR isolate), were tested against the above pathogens. T. harzianum significantly inhibited the hyphal growth of Rigidoporus sp. whereas T. asperellum showed 75.62 Per cent inhibition against Fusarium spp and 66.85 Per cent inhibition against Neoscytalidium spp. In vitro studies with fungicides revealed that even lower concentrations of the Bordeaux mixture (0.25%) completely inhibited the growth of Neoscytalidium spp. Among systemic fungicides, Difenoconazole (0.2%) showed 100 Per cent inhibition, whereas carbendazim + mancozeb (0.05%) was identified as the best combination fungicide against the pathogen. Against Fusarium spp and Rigidoporus spp, given lower concentrations of fungicides i.e., BM (0.25%), Difenoconazole (0.025%), and Trifloxystrobin + Tebuconazole (0.013%) gave 100 Per cent inhibition. Among the chemicals, all concentrations of potassium phosphonate, calcium chloride, and sodium carbonate showed 100 Per cent inhibition of Rigidoporus spp. All doses of calcium chloride showed cent percent inhibition for Fusarium spp, while recommended and double

doses of calcium chloride and potassium phosphonate were found effective against Neoscytalidium spp. Compatibility of promising fungicides and chemicals with potent bioagents was carried out based on the results of an in vitro study. Calcium chloride and potassium phosphonate showed maximum compatibility with B. amyloliquefacians and T. asperellum respectively. A pot culture study was conducted to find out the effective treatments against stem canker disease of dragon fruit which is identified as the most prevalent and devastating disease during the study period. Prophylactic application of T. asperellum enriched FYM @100 g plant -1 as well as combined application of calcium chloride (0.7 g/l) and B. amyloliquefacians (600 ml/1.5 ×10 8 CFU) significantly reduced disease severity under in vivo condition. Thus, the present study identified successful management strategies for the fungal pathogens of dragon fruit through proper employment of bioagents, chemicals, fungicides and their combinations. These findings contributed valuable insights for the development of an economical and environment-friendly strategy against the dragon fruit diseases prevailing in southern Kerala.