Physiological studies on Alternaria porri causing purple blotch disease in white onion

The investigation on cultural and morphological characters of Alternaria porri exhibited fluffy, lanose to loose, cottony, aerial and greenish grey to black colonies on oat meal agar medium. Hyphae were septate and irregularly branch, conidiophores were arose singly or in group, straight, pale to light brown, measured 90–100 x 9–12 μm. Conidia were solitary, straight to curved, obclavate and measured 52.7–126.4 x 9.26–16.20 μm having long beak with 6–11 transverse and 0–3 longitudinal septa. Among the various media tested, maximum mycelial growth of A. porri was recorded on oat meal agar followed by host extract agar, Richard's agar and Asthana & Hawkar's agar. Similarly, host extract broth supported maximum vegetative growth, followed by oat meal and Richard's broth. Maltose supported maximum growth of A. porri, followed by sucrose, dextrose and fructose as carbon sources and urea followed by potassium nitrate and sodium nitrate as sources of nitrogen. The pH level 4.0 -6.0 was found ideal for the optimum growth of A. porri.