Damping-off disease of big onion (Allium cepa L.) in Sri Lanka and evaluation of Trichoderma asperellum and Trichoderma virens for its control

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.