Biocontrol Potentiality of Native Microbial Isolates Against Collar rot Disease of Crossandra P Arunasri, P Madhusudhan and M Johnson Department of Plant Pathology, S V Agricultural College, Tirupati 517 502 ABSTRACT In the present study, we have isolated nine fungi and three bacteria from the rhizosphere of Crossandra by serial dilution method. Of various mycoflora, Trichoderma viride (T 1 ), Trichoderma spp (T 2 , T 3 and T 4 ), Penicillium spp, Rhizopus spp, Aspergillus flavus and A. niger and three rhizobacterial isolates including Pseudomonas spp were obtained. In dual culture studies, Trichoderma viride (T 1 ) was highly effective in inhibiting mycelial growth of Sclerotium rolfsii . upto 70% and sclerotial production by 91% followed by Trichoderma species (T 2 ). Pseudomonas spp was also effective among rhizobacterial isolates with mycelial and sclerotial inhibition upto 43.1% and 71% respectively. Key words : Biocontrol, Crossandra, Invitro, Rhizosphere. Crossandra (Crossandra infundibuliformis (L.) Nees) is a major revenue generating ornamental crop of India. In Andhra Pradesh, the crop acerage is to an extent of 719 ha with production of 503 tonnes during 2003. Crossandra cultivation is profitable, to farmer however the crop production is hampered by several production constraints, of which soil-borne diseases play a major havoc. Mainly, Crossandra cultivation in our country is plagued by collar rot disease causing significant yield losses ranging about 40 to 50% (Harinath Naidu, 2000) which is caused by Sclerotium rolfsii Sacc. Presently, the pathogen is managed through application of chemical fungicides in various crops (Anitha Chowdary, 1997) and their usage is having several concerns. Among different alternative methods of managing the collar rot disease of Crossandra, biological control is a viable option that can be effective on a long term basis. Trichoderma spp. are abundantly found in the crop rhizosphere and contribute to disease control of various soil-borne pathogens through various mechanisms. Growth promotion due to Trichoderma spp. is also reported in several crop species including ornamentals (Manoranjitham et al., 1999). Among bacterial bioagents, plant growth-promoting rhizobacteria (PGPR) were widely employed in controlling several plant pathogens besides enhancing crop yields (Pathak et al., 2004). The present experiment was conducted to identify potential fungal and bacterial antagonists that can be used in managing the collar rot disease as an alternative or supplement the existing chemical control practices. The Andhra Agric. J 60(1): 85-88, 2013 MATERIAL AND METHODS Isolation and Maintenance of Sclerotium rolfsii Collar rot affected crossandra plants were collected from different crop growing areas of chittoor district of Andhra Pradesh and the pathogen was isolated using tissue segment method (Rangaswami and Mahadevan, 1999). The pathogen was identified as Sclerotium rolfsii based on its mycelial and sclerotial characters (Barnett and Hunter, 1972). The isolated pathogen was maintained on PDA at 28±2 0 C. Pathogenicity was proved prior to taking up the in vitro efficacy tests with fungal and bacterial antagonists. Isolation, Identification and maintenance of native fungal and bacterial antagaonists Serial dilution technique (Johnson and Curl, 1972) was used to isolate fungal and bacterial antagonist from Crossandra rhizosphere. Composite soil sample collected from rhizosphere of healthy plants and collar rot infected crossandra plants was shade dried and then used for serial dilution. Antagonistic mycoflora were isolated on rose bengal agar medium by using a dilution of 10 -4 . Antagonistic bacteria were isolated on soil extract agar medium by using a dilution of 10 -6 . One ml of soil suspension was poured into sterilized petriplates, then the melted and cooled media was poured. Then the plates were incubated at 28±2°C for the development of colonies. Isolations were also carried out on potato dextrose agar for fungi/ and on nutrient agar for bacteria simultaneously. Three days old colonies of mycoflora were picked up and purified by single hyphal tip method, while one day old colonies of