Population variability of S. sclerotiorum, the causal agent of Sclerotinia stalk rot of sunflower, was determined by mycelial compatibility grouping (MCG) and pathogenicity variation comparison. To study mycelial compatibility grouping and pathogenicity variability, isolates of S. sclerotiorum were collected from sunflower fields in East, West Azerbaijan and Ardebil provinces of Iran. Among 186 isolates tested, 26 MCGs were identified and 46% were represented by single isolates. There were differences among MCGs comparing mycelial growth rate, sclerotial production on PDA and aggressiveness cause disease. Significant differences were detected in number of sclerotia, dry weight of sclerotia, mycelial growth rate and aggressiveness among MCGs (p < 0.001) regardless of their geographic origins. There was generally a poor correlation (r = 0.21, p ≤ 0.05) between sclerotia weight and number of sclerotia produced on PDA and also to the mycelial growth rate at 24 (r = 0.35, p ≤ 0.05) and 48h (r = 0.39, p ≤ 0.05). Our studies in comparison of the detached leaf and cut-stem methods showed that the highest rank correlations (r = 0.78 p ≤ 0.01), while aggressiveness of two inoculation methods (stem and leaf detached) were not correlated to colony diameter growth or the other two factors. Variation in isolates aggressiveness may be important considerations in disease management systems.
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