The Comparative growth and determination of the isolates reaction of Sclerotinia sclerotiorum on oil seed rape cultivars

White stem rot, caused by Sclerotinia sclerotiorum, is a major canola disease in northern Iran and throughout the world that limits the canola yield. Nineteen isolates of S.sclerotiorum were collected from infected canola plants from different areas of northern Iran and were used to study their comparative growth on culture media. The mycelial growth rate and sclerotium formation were used to compare the fungal isolates. The reactions of the plants to the fungal isolates were evaluated by recording the average size of the stem lesion length (SLL) with selected isolates (Ss2-Ss5-Ss18-Ss8) on sixteen cultivars of canola in the field condition from 2004 to 2005.There was significant differences in the growth rate and sclerotial production of most of the isolates. The isolates Ss5 and Ss18 had the most and the least growth, sclerotial production, and virulence respectively. Significant differences were observed between different isolates (P=0.05) in relation to pathogenicity. Differences were also observed between the different canola cultivars (P=0.05) in their responses to different isolates of S. sclerotiorum and there was also a significant host × pathogen interaction in this experiment. Among the oil seed rape cultivars Sarigol(18cm,SLL) and Hyola 330(28.7cm, SLL) were the most resistant and susceptible cultivars respectively. The wide range in cultivar reactions indicated that the breeding for resistance to this pathogen is possible. It is recommended to sow the resistant cultivars RGS 3006 and Sarigol rather than Option 500 and Hyola 330, which are common sensitive cultivars.