الفهرس 
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Abstract
The present study was carried out to survey diseases t prevail onion nurseries of Assiut Governorate and to tify their incitants. Role of onion seeds in
smission of these diseases as well as effectiveness of ii solarization and seed treatment with fungicides in :rolling them were also investigated.
Results obtained from this investigation can be rarized as follows
Damping—off, smut, pink root and white rot diseases of onion frequently occurred in surveyed onion nurseries planted with Giza 6 cultivar during 1987/88 and 1988/89 seasons. Percentages of infection with these diseases in the first season were 11, >1, 35 and 8.7, respectively, while they were 10.7, >1, 36.3 and 11, respectively, in the second season.
Pathogenicity tests of fungi isolated from infected onion seedlings proved that Fusarium oxysporum Schlecht. and Rhizoctonia solani Kuhn were the incitants of damping—off disease. However, Sclerotiuzn cepivorurn Bez-k., Pyrenochaeta terrestris Hansen and Urocystis cepulae Frost. were the incitants of white rot, pink root and smut diseases, respectively. S. cepivorum caused also damping—off.
Aggressiveness of fungi and their isolates on Giza 6 onion cultivar was different. F. oxysporum Schlecht. was the highest followed by R. solani Kuhn, S. cepivorum