Abstract Onion ( Allium cepa L.) is threatened with infection caused by different pathogenic microorganisms. Soil born fungi that infect roots and bulbs of this crop are one of the most important pathogens in Iran which cause severe damage annually . Pink root and Fusarium basal rot are important diseases of cultivated alliums. During 2007-2008 sampling was carried out from fields in northeast of Iran . After morphological and molecular studies and Pathogenicity test, the most frequently isolated fungi were Fusarium axysporum , F. solani, F. proliferatum , Pyrenochaeta terrestris and Rhizopycnis vagum respectively. In host range study all isolates of Fusarium oxysporum caused basal rot only in onion and proved to be F. axysporum f.sp cepea. The results showed that all the fusaria that isolated from roots , could induced pink root and R. Vagum caused necrosis of onion roots. Under greenhouse trail , no significant correlation between Fusarium species and their Pathogenicity power was observed . The results are thought to be first report of R. vagum and P. terrestris from Khorasan province , Iran.

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