Onion productivity is affected by the diseases like Colletotrichum (twister) blight, Purple blotch, Stemphylium blight and basal rot. Since Onion twister blight cause heavy loss this investigation was carried out to assess the prevalence of disease in different districts of Tamilnadu to report the disease severity in terms of PDI (Percent disease index) along with AUDPC (Area under disease progress curve) value. Based on the AUDPC mean values, the districts surveyed were classified into four divisions. Amongst, Perambalur hold under severity division. Chettikulam village in Perambalur district recorded the highest AUDPC value and the highest PDI (1908.52, 52.6 percent) and Pavitram village in Trichy district recorded the lowest PDI (31.30 percent) among the districts surveyed, with a AUDPC score of 1117.35. During the Rabi (September-November) season, different isolates in pot culture was assessed with variety Co -5. The greatest PDI was observed from Perambulur district, Chettikulam village isolate 51.3%, followed by Rasipuram 48.9% PDI from Nammakal, and the lowest PDI was recorded from comibatore district, Pollachi village isolate 26.9% and Chettikulam with highest AUDPC value 1269 and Pollachi with lowest AUDPC value 809.25. During the Kharif (Feb-May) season the greatest PDI was observed from Perambulur district, Chettikulam village isolate 25.8%, followed by odichattiram 23.3% PDI from Nammakal, and the lowest PDI was recorded from comibatore district, Pollachi village isolate 15.9%, Chettikulam with highest AUDPC value 801.75 and Pollachi with lowest AUDPC value 578.25 .These also revealed size diversity, with conidia sizes ranging from (9.52 x 3.85 µm to 23.20 × 5.15 µm). The smallest conidial size is 9.52 x 3.85 µm in isolate from V.P kovil village, and the largest conidial size is 23.20 x 5.15 µm in isolate from tiruppur village.