Morphological and Pathogenic Characterization of Alternaria solani (Sorauer) Isolate Causing Early Blight Disease of Potato

Potato (Solanum tuberosum L.) is an important food and vegetable crop worldwide and in India, where Uttar Pradesh contributes a substantial share of national production. Early blight, caused by Alternaria solani, is a major fungal disease of potato and is associated with considerable yield reduction under favourable conditions. The present study was conducted at the Department of Plant Pathology, Chandra Shekhar Azad University of Agriculture and Technology, Kanpur, Uttar Pradesh, to isolate, purify, characterise and establish the pathogenicity of the causal agent associated with early blight of potato. Diseased potato leaves showing small, dark brown necrotic spots with concentric target-board lesions were collected from the experimental field. The pathogen was isolated from infected leaf tissues by the standard tissue isolation method on potato dextrose agar medium and purified through the hyphal-tip method. Morphological observations of the 7-day-old culture showed greyish-brown to black mycelium and brown to olivaceous-brown conidiophores. The conidia were olive green to brown, ellipsoid, straight to slightly curved, 35.15-24.85 micrometres long and 15.60-10.90 micrometres wide, with 3-4 transverse septa and 1-3 longitudinal septa. Pathogenicity was assessed on 55-day-old potato plants under controlled conditions by spraying a conidial suspension of 1 x 10^5 conidia per ml. Yellow flecks and early blight symptoms developed 5-10 days after inoculation, followed by sunken necrotic lesions with concentric rings, whereas control plants sprayed with sterile distilled water remained symptomless. Re-isolation from inoculated plants yielded cultures morphologically similar to the original isolate, thereby fulfilling Koch's postulates and confirming A. solani as the causal agent of early blight in the tested potato plants.