Allium cepa is used as a condiment and reduction of yield due to infectious diseases is a major economical constraint. The present study was aimed at isolation and identification of fungal pathogens associated with damping-off disease of onion in Sri Lanka. Trichoderma spp. present in the soil of the same onion fields were isolated with a view to evaluating them as possible biocontrol agents of damping-off pathogen(s). The diseased seedlings were collected from fifty-five onion fields in Matale and Anuradhapura districts during the yala season. Soil collected from the same onion fields and soil fungi isolated using the Warcup method. Fusarium sp. isolated from diseased seedlings was confirmed to be the causative agent of damping-off disease of big onions by following Koch's postulates. The pathogenic Fusarium sp. was identified as Fusarium solani based on the similarity matches of the Internal Transcribed Spacer region using Basic Local Alignment Search Tool. Two Trichoderma spp. showing significantly high (p ≤0.05) reduction of growth of F. solani in dual culture assay, higher sporulation capacity and growth rates were identified as T. asperellum (Tr.3) and T. virens (Tr.1). Polymerase Chain Reaction (PCR) using two primer pairs i.e. ITS 1 and ITS 4, FR 1 and NS 1 were used to characterize the seven Trichoderma spp. while ITS 1 and ITS 4 were used to characterize Fusarium spp. Although a lesser degree of polymorphism was detected using these primers, the random amplified polymorphic DNA analysis had the ability to differentiate T. asperellum, T. virens and F. solani. The capability of two Trichoderma spp. to suppress F. solani is through formation of loops and coils and attachment of hyphal tips. They also had the ability to produce Chitinase and volatile metabolites that controlled the growth of F. solani.

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Substantial yield losses have been reported during the storage of Allium cepa L., and effective methods to minimize postharvest losses are currently lacking. Therefore the objectives of this study were to investigate the impact of pre-treatment with Trichoderma asperellum and Trichoderma virens on post-harvest losses during storage and to evaluate factors associated with these losses. The storage behaviors of Allium cepa L. bulbs after different field treatments with Trichoderma spp. were examined using a complete randomized design. The common onion bulb diseases encountered during storage were basal rot and black mould diseases, with Fusarium sp., Mucor sp., Penicillium sp., Aspergillus niger and Aspergillus flavus being associated with diseased bulbs. The results regarding the percentage of diseased bulbs showed that pre-harvest treatments with Trichoderma asperellum and Trichoderma virens did not have a significant effect on disease control during storage. Furthermore, storage losses of onions were attributed to sprouting, rooting and wilting, which were exacerbated by the prevailing temperature (~30-32°C) and relative humidity (~77%-79%) during the storage period.