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In order to isolate the antagonistic and pathogenic mycoflora of onion rhizosphere in the fields of onion in East Azarbaidjan province, Rose Bengal Malt - Agar, Nash ¬
Snyder Medium (NM) and selective medium Davet were used. Root and bulb samplings were made from onion fields affected with root rot (Fusarium spp.,) and white rot (Sclerotium cepivorum Berk) as well as from unaffected ones. The cultures were made by using the suspension obtained from washing the infected tissues of root and bulb with sterile water, using a flask to shake for half an hour in an electric shaker that ran at
75 revolutions per minute. Two droplets of the obtained suspensions of microorganism's propagules (as 10% concentrates) were placed in the center of petri ¬
dishes containing the medium culture. After scraping the medium surface, samples were incubated at 240 c temp. and 10: 14 (L: D) light re:gime. The species isolated from mycoflora were identified as follows: Fusarium oxysporum Schletcht., Fusarium acuminatum Ell. Et Ev., Fusarium so/ani (Mard). Fusarium equiseti (Corda) Sacco Sclerotium cepivorum Berk. Trichoderma viride Pres, Trichoderma harzianum Raifi.
The mechanism by which the antagonists. (T vir ide and T harzianum) affected the pathogens were hyphal contact, coiling, deformation and lysis. Non - volatile metabolites of Trichoderma spp. Were more effective: in prohibition of mycelial growth of Fusarium spp. and sclerotial germination of S. cepirovum (P= 1 %). Volatile metabolites of the Trichoderma spp. inhibited mycelial growth only when Trichoderma was cultured 24 hours before Pathogenic fungi (P := 1 %) but exhibited no fungicidal effects on sclerotia of S. cepirovum, as compared with control.