Cultural, morphological and molecular characterization of Stemphylium vesicarium isolates causing onion blight

Stemphylium blight caused by Stemphylium vesicarium (Wallr.) is one of the important diseases of onion which causes considerable losses in seed as well as bulb crops, particularly in Northern India. Morpho-molecular characterization for variability among 11 isolates of S. vesicarium isolated from different onion cultivars of four states namely, Delhi, Punjab, Karnataka and Maharashtra was done. Variable colony growth was observed as either cottony or velvety and pigmentation was recorded as whitish, light to dark grey to brownish with a filiform margin. The mean colony diameter ranged between 44.53 and 71.64 mm. Conidial colour among different isolates varied from light brown to brown with the mean conidial length ranging between 17.96 and 30.99 μm and the breadth between 11.63 and 17.95 μm. The longitudinal septation of conidia ranged from 0 to 4 and transverse septation varies from 0 to 5. Molecular detection by PCR assay using ITS1F/ITS4R primer amplified about ~ 550 bp amplicon, whereas, β-tubf1 and β-tubr1 primer amplified ~ 1400 bp amplicons. The study revealed that accurate identification of S. vesicarium based on morphological observation supplemented with molecular characterization will be helpful in understanding the variability among isolates of S. vesicarium prevalent in a wide range of geographical conditions.