This study was carried out, at the greenhouse of plant protection department of college of agriculture university of Baghdad, to test the pathogenicity of 75 isolates of Fusarium spp causative agent of wheat crown rot. Wheat plants showing yellowing and brown coloring of lower internodes were collected, from several governorates of Iraq. The lower internodes were cutting to small pieces of 1 cm length, sterilized in sodium hypochlorite 1% for 3 min., rinsed in sterile distilled water and dried on filter papers. The pieces were cultivated on PDA amended with mixture of Ampicillin and Tetracycline at 100 mg/L, in petridishes of 9 cm diameter. The petridishes were maintained at 25±2Cº for 7 days and the growing Fusarium spp were identified by polymerase chain reaction (PCR). The Fusarium spp were grown on sterile wheat seeds in petridishes of 11 cm diameter at 25±2Cº for 21 days. Surface sterile wheat seeds were sowing in mixed soil contaminated with Fusarium spp, grown on wheat seeds, at 0.5% in pots of 6 cm diameter (3 seeds/ pots). The pots were maintained at the greenhouse and the disease incidence, disease severity, rate of fungus aggressiveness and DON toxin production in plant tissues were determined after 35 days of sowing. Results of isolation showed that Fusarium graminearum was more prevailing at 92% compared with 8% of Fusarium pseudograminearum. The isolates showed obvious variation in pathogenicity on wheat seedling under greenhouse conditions. The highest of disease severity, disease incidence and fungus aggressiveness rate, 0.856, 100% and 5.67 were found for the isolate 28 of F.graminearum isolated from Baghdad. Also the highest of disease severity, disease incidence and fungus aggressiveness rate 0.362, 100% and 4.94 were found for the isolate 151 of F. pseudograminearum isolated from Erbil. The concentration of mycotoxin produced by the isolates was found to be ranging from 0.002 to 0.807 µg/g. A significant correlation between DON mycotoxin concentration with disease severity, disease incidence and fungus aggressiveness were registered, r= 0.431, 0.409 and 0.556 respectively. Negative effects in most growth parameters of wheat seedling including plant highs, leaves number and dry weight upon treating with the fungus isolates were manifested.