Onion (Allium cepa L.) is one of the important bulb crops and most important commercial vegetable crops cultivated extensively in India and it accounts for 90 per cent of the exported vegetables from India in terms of value. The production of bulbs and seeds is limited by certain diseases. The most serious one is the purple blotch caused by Alternaria porri (Ellis). The disease causes extensive damage to bulbs as well asseed crop and also a major limiting factor in cultivation of onion. In view of the destructive nature of purple blotch of onion the present investigation was conducted through survey to know the disease incidence collection of infected samples. A survey was conducted during Rabi 2018-2019 in major onion growing areas of central part of Bihar viz. Nalanda, Muzaffarpur, Vaishali, Patna and Samastipur districts. The highest (37.17) per cent disease index was noticed in field of Pariaunna village of Nalanda district. While, the lowest (17.62) per cent disease index was recorded in Azampur village of Vaishali district. Among the districts, severity of disease was more in Nalanda (34.42) and less in Vaishali (23.98). Isolation was made from onion leaves showing typical purple blotch symptoms. Pure culture of Alternaria porri was obtained and its pathogen city to onion plants was proved. On the basis of isolation and morphological studies, the pathogen was identified as Alternaria porri (Ellis). Key words: Onion; Purple blotch; Alternaria blight; PDI; Pathogen city;

Keyword: Studies, vegetable crops, isolation, morphological studies