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Sixty-seven isolates of the southern blight fungus from Japan were divided into five groups based on ITS-RFLP analysis of nuclear rDNA. Morphological characters of sclerotia varied between groups. Three groups were reidentified as Sclerotium rolfsii, and two resembled S. delphinii in RFLP patterns and/or in having large sclerotia and relatively low optimal growth temperature (28~ Sclerotia of the latter, however, varied in size according to temperature and became indistinguishable from those of S, rolfsii at high temperatures. Hyphal anastomosis (imperfect fusion) was observed between different ITS-RFLP groups, as well as between different isolates belonging to the same groups. These results indicate that populations of this fungus in Japan consists of several different subgroups, although morphological differences are not always evident.

The process of teleomorph development in the white root rot fungus Rosellinia necatrix is described on diseased roots of Japanese pear. Stromata were also found on dead plants in nonagricultural lands such as yards and forests. The stroma of R. aquila is also described. Key WordsRosellinia aquila; Rosellinia necatrix; white root rot.The causal fungus of white root rot of fruit trees and other crops in Japan has been referred to as Rosellinia necatrix Prillieux. Identification of the fungus was based solely on vegetative criteria, such as its white, fanshaped mycelia on diseased roots and pyriform swellings adjacent to septa in the hypae (e.g., Ito and Nakamura, 1984;Kanadani et al., 1994). The only available study of its conidial stage is that of Watanabe (1992), who described its anamorph, Dematophora necatrix Hartig.

Sclerotium rolfsiiisolates 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 of S. rolfsiiwas simple. RAPD fingerprintings showed that most isolates of the same MCG were clonal.

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