Population structure ofSclerotium rolfsii in peanut fields

Sclerotium rolfsii isolates from peanut fields in Ibaraki were classified into mycelial compatibility groups (MCGs) based on the barrage zone formation. A total of 132 isolates collected from four fields within a 120 m radius in 1994 comprised four MCGs; MCG A (71 isolates), B (34 isolates), C (26 isolates) and D (one isolate). Fields 1 and 2 were occupied exclusively by MCG A. MCG A also predominated in field 3. In field 4, MCGs A, B and C were dominant. Population structure in 3 additional fields was determined in 1997. All 11 isolates from Field 5, which was 400 m distant from field 1, belonged to MCG C. A total of 42 isolates from fields 6 and 7, 2.5 km distant from other fields and 100 m distant from each other, were all MCG A. These results suggested that the population structure ofS. rolfsii was simple. RAPD fingerprintings showed that most isolates of the same MCG were clonal.