Vol. 9, Special Issue 12, Part J (2025)

Onion (Allium cepa L.) indeed holds significant importance globally, not just as a culinary staple but also for its numerous health benefits. As a member of the Alliaceae family, it has been cultivated for centuries and is widely recognized for its medicinal properties. The fungus associated with naturally infected onion leaves was isolated by adapting standard tissue isolation technique on potato dextrose agar medium. Similarly, different isolates of Alternaria porri were obtained from diseased plants from different onion growing area of Konkan region and rest of the Maharashtra. Pathogenic ability of the fungus isolated from diseased plant tissue was carried out on susceptible variety of onion (Cv. Alibag Local) in pots under glass house conditions by following Koch’s postulates and the pathogenic fungus was identified as A. porri (Ellis) Cif. Identification of the causal organism inciting purple blotch disease in onion was done based on morphological characters of the isolated fungus. The mycelium was composed of smooth, septate, short, simple or branched, sub fasciculate hyphae and light brown to dark brown in colour. The conidia of the pathogenic fungus were generally solitary, straight or curved, obclavate or with the body of conidium ellipsoidal tapering to the beak with 4-10 transverse and zero to few longitudinal septa and with beak flexuous, pale, thick and tapering. The conidia were pale to dark brown and muriform with size ranged from 67.3 – 131.4 x 17.7 - 22.3 μ. All isolates of A. porri exhibited a wide range of variability in respect of the cultural characteristics viz., colony diameter, colony colour, growth rate, colony elevation and topography, colony margin and zonation. All isolates of Alternaria porri exhibited a wide range of variability in respect of the morphological characteristics viz., hyphal width, conidial shape, conidial size, beak length and number of transverse and longitudinal septa.