Plant Archives Vol. 19 No. 2, 2019 pp.2476-2480 e-ISSN:2581-6063 (online), ISSN:0972-5210 *Author for correspondence : E-mail : reelee.pat@gmail.com OCCURRENCE, VIRULENCE AND PATHOGENICITY OF PYTHIUM APHANIDERMATUM CAUSING TOMATO DAMPING OFF Karmel Reetha A.* and Muthukumar A. Depart. of Plant Pathology, Faculty of Agriculture, Annamalai University, Annamalainagar-608002, Chidambaram, Tamil Nadu, India. Abstract An intensive survey for the occurrence of tomato damping off (Pythium aphanidermatum) was carried out during the year 2018–19 in order to assess the incidence of tomato damping off disease in different districts of Tamil Nadu. The maximum incidence of damping off (pre and post emergence - 38.65 & 60.98 per cent) was recorded at Sivapuri village in Cuddalore district followed by 37.55 and 59.69 per cent at Pollachi in Coimbatore district. Radial growth (mm/day) over 4 day incubation and visual assessment of colony morphology indicated variability among the isolates. Among the twenty isolates, maximum mycelia growth and fast sporulent germination at an early stage of incubation 24 h interval for 4 days (1–4 days after incubation), there were maximum mycelia growth was observed in I 4 (33.56 to 90.00 mm) followed by I 7 recorded 32.25 to 89.00 mm radial growth (mm/day) 1-4 days after incubation. Further, various inoculum levels tested, application of 100g/kg of soil inoculum load of P. aphanidermatum registered the maximum incidence of 37.68 per cent pre-emergence damping-off and 69.78 per cent of post-emergence damping-off. Key words : Tomato, Pythium, Incidence, Radial growth, Inoculum level. Introduction Tomato (Lycopersicon esculentum Mill.) is one of the most popular and widely grown vegetable crops in the world. It is one of the important food and cash crops for many low-income farmers in the tropical countries. (Prior et al., 1994). Tomato is a solanaceae family and belongs to the genus Lycopersicon, grown for its edible fruits (Jones, 1999). The ripened fruits are good source of vitamin A, B and C which add wide varieties of colour and flavour to the food (Dias, 2012). In India, the tomato- growing in an area of approximately 894 ha of the total vegetable-growing area with total production accounting for approximately 19167 of total vegetable production (Anonymous, 2017). Among the pathogens that affect the tomato crop, soil-borne fungal pathogens, including genus belonged to Pythium, Sclerotium, Fusarium, Rhizoctonia, and Verticillium genera causing the root rot or damping- off and wilt which affect the quality with yield reduction (Mandal et al., 2017). Stanghellini (1974) found that worldwide distribution of Pythium spp. cause damping- off of many plants. Martin and Loper (1999) reported that Pythium species are ubiquitous soil-borne oomycetes that rank from opportunistic up to highly virulent pathogens on many plant species. Elshahawy et al., (2018) Symptoms observed on affected tomato plants developed symptoms of wilting, dead plant, root rot with crown and stem rot on above ground plant parts. Damping off incited by Pythium aphanidermatum causes more than 60 per cent losses in seedlings both in nursery and main field (Manoranjitham et al., 2000). Jukte et al., (2016) proved that in tomato pre- emergence damping off caused seed, any young seedlings to rot before they emerge from the growing in soil. In post-emergence damping off the pathogen cause water soaked soft brown lesion at base of the stem near soil line, pinches off stem causing the seedling to topple over and die. (Elshahawy et al., 2018) reported that Pythium isolates recovered from diseased tomato plants were morphologically similar to each other. Based on their morphology, all isolates were identified as P. aphanidermatum. Therefore, the objectives of this research were to isolate and identify the Pythium species that cause tomato damping off, to characterize their relative occurrence, virulence and pathogenicity.