Abstract
The incidence of onion purple blotch disease was investigated at Assiut Governorate, Egypt during 2011. Lesions with typical symptoms of the disease were colonized by Alternaria porri, Stemphylium vesicarium or mixtures of the both pathogens comprising 2.6, 39.8 and 57.6 % of samples, respectively. Four isolates were selected for virulence evaluation according to the nature of isolation, whereas two isolates were individually associated with the diseased samples and two isolates were collectively recovered. Individually, A. porri AUMC9301 and AUMC10453 showed high virulence contributing 81.25 and 78.13 %, respectively. Also, S. vesicarium AUMC10512 and AUMC10519 exhibit severity comprising 37.50 and 25.00 %, respectively. On the other hand, mixture of A. porri AUMC10453 and S. vesicarium AUMC10519 exhibited highest virulence (91.50 %). In conclusion, this investigation proved that onion plants showing purple blotch symptoms mainly colonized by both A. porri and S. vesicarium. Consequently, increase in the disease may be attributed to the association between these pathogens.