Variability of Stemphylium vesicarium on Onion (Allium cepa L.)

The onion (Allium cepa L.) crop is affected by a number of diseases in the field as well as in storage which cause a significant reduction in yield leading to heavy losses each year depending on weather conditions. Leaf blight and leaf spot caused by Stemphylium vesicarium is one such disease that affects onion plants in field causing purple coloured spots and blighting of leaves, particularly in seed production plots. During investigations of the pathogen, it was found that there was a considerable variation in the conidial characteristics of Stemphylium species compared to the available literature. Stemphylium vesicarium was invariably found associated with typically blighted onion leaves throughout the growing period (82.75 per cent in January to 77.40 per cent in February), though its relative occurrence declined slightly in March (49.15 per cent) and April (37.75 per cent). The average conidial dimensions of Stemphylium vesicarium recorded were 43.54 ± 0.44 µm X 25.57 ± 0.36 µm with l/w ratio ranging from 1.40-2.13. It can be concluded that there are significant variations in the conidial dimensions of S. vesicarium obtained from host and grown under in vitro conditions. Differences also exist in the conidial dimensions of isolates from different geographical regions which indicates that further studies are required at molecular level to decipher the reasons and implications.