Sclerotium rolfsii is a soil borne pathogen responsible for root and stem rot on a wide range of crops. This study was conducted to identify the virulence of different S. rolfsii isolates on a susceptible local peanut germplasm and determine the resistance of 20 peanut genotypes to the most virulent isolate and also the relationship between virulence and mycelial compatibility groups (MCGs). Seventy eight isolates of this fungus from 10 host plants and six known MCGs were used in the experiment. The experiment was done in greenhouse conditions (25±5°C) using a complete randomized block design with three replications. Pots containing sterile soil (pH=6.7) were inoculated with barley seeds colonized by each isolate separately before being seeded with the peanut germplasm. Disease severity was assessed by scoring the wilting, yellowing or death of plants, mycelia or sclerotia production on the soil surface or on plant stem, stem area affected (%) and stem lesion length, at the stage of plant maturity. Also, shoot wet weight and plant height were recorded at this stage. According to the results of the pathogenicity tests, all of the isolates were virulent on the susceptible peanut germplasm and the virulence diff ered signifi cantly between the isolates (P≤0.01). There was no relationship between the virulence of the five groups of isolates identified in the present study and the MCGs. The peanut genotype 140, which was better than the others based on seed size, plant height and the canopy size, was also the most resistant one
