CONTROL OF DAMPING OFF AND BASAL ROT DISEASES OF BIG ONION (Allium cepa L.) USING T richoderma asperellum and Trichoderma virens

Abstract

Big onion (Allium cepa L.) is a condimcnt grown for its flavorful bulbs in Sri Lanka as well as a number of other countries in thc world. One of the problcms associated with big onion cultivation is yield reduction due to infectious diseases that occur during various stages of growth. Some of the morc economically important diseases are eauscd by fungal pathogcns and thus studying these diseases will enable the development of strategies for their managcmcnt. With this in vicw, one of the objcctivcs of the present study was survcying the diseases prcvalcnt in onion cultivations in Sri Lanka at diffcrent stages of growth and isolation and identification of the causative fungi. Using biological agents in discasc managemcnt practices as opposcd to use of hannful fungicides offers an environmentally friendly solution. As Trichoderma spp. are widely used bio control agents of many fungal pathogens. Trichoderma spp.lIere isolated from the soil in local onion fields and characterized with a view of using thc more effective isolatcs as biological control agcnt(s) of the more important fungal pathogens of onion.Onion fields in the Matale and Anuradhapura districts wcrc selected for the study.Diseased onion samplcs were collected from the fields and nurseries and the causative fungi were isolated and identificd. Soil was collected from onion fields and soil fungi were isolated using the Warcup mcthod. Preliminary identification of all fungal SPCCICS isolated was carried out based on their morphological fcatures and sporulating structurcs.A Fusarium sp.and Collelolrichum g/oelJ.lporioides wcre isolated and identified from infected bulbs,leaves and flower stalks. Fusarium, Curvu/aria, Alternaria and Sclerotium spp. Were isolated and identified from seedlings showing damping off symptoms and the Fusarium so/ani was con finned to be the causative agent of seedling damping off disease using Koch's postulates. Fourteen Trichoderma spp. were isolated from the soil samples.Molecular characterization of Trichoderma spp. and Fusarium spp. was carried out using Polymerase Chain Reaction (PCR), using Random Amplified Polymorphic DNA (RAPD) and using Restriction Fragment Length Polymorphism (RFLP) for higher accuracy.The Trichoderma isolates were tested for their ability to control the growth of Fusarium solani under in lilro conditions using dual culture assays.All Trichoderma spp. isolated showed inhibition of mycelial growth of the Fusarium solani under in vilro conditions.Two of the more effective isolates were cultured in a low cost medium containing molasses and yeast. The cultured Trichoderma spp.were then added to a low cost carrier medium consisting of talc. The Trichoderma inocula prepared in this manner were applied using two methods i.e. soil application prior to planting of seeds and priming of onion seeds with the Trichoderma inoculum before planting.Both methods of introducing the Trichoderma inocula reduced disease incidence.Sever,tjof damping off disease significantly (p <=¬ 0.05) in pot and field experiments indicating that the Trichoderma inocula prepared in the present study can be used effectively against damping off disease of onion in the field.Subsequent Trichoderma spp. treatments de seedling root dipping or soil application at the time of transplanting using T. asperellum alone or T. asperellum together with T. virens are efficient means to control basal rot disease of A. cepa L.This is the first record of an effective locally produced Trichoderma inoculum for the control of oman diseases in Sri Lanka.