1Hokkaido National Agricultural Experiment Station, 1 Hitsujigaoka, Sapporo, 062, Japan; 2Department of Tohoku University, 1-1 Amamiya, Sendai, 982, Japan

Background and objectives
Rhizoctonia solani is well known as a root-infecting fungus, consisting of genetically distinct groups, such as anastomosis groups (AGs), their subgroups, and vegetative compatibility populations (VCPS) or clones. A better understanding of R. solani complex population biology is important for the development of environmentally friendly control measures against pathogens of this groups. Cubeta and Vilgalys [1] reviewed current knowledge regarding the population biology within a same AG and a subgroup of R. solani via VC and molecular approaches. Many studies on R. solani (anamorphs) population dynamics and diversity have been reported in the literature. However, not much is known how the population dynamics and diversity develop, genetically and ecologically in nature. When conditions are favourable, hymenia (of the R. solani teleomorph) are occasionally formed on a wide variety of plant hosts, either on the stem bases, or on other plant and soil surfaces in fields [2]. However, little is known about the role of basidiospore isolates in VCP diversity in nature. The present study focuses on the analysis of VCP of single basidiospore isolates within the same generation and over several generations and theirassociation with the population diversity evaluated as colonization or infection of host plants.

Materials and methods
A total of 10 isolates, consisting of six of AG 2-21V from sugar beets (except for one isolate from soil), three of AG 2-3 from soybeans and one of AG 5 from sugar beet, were all tested as parent isolates. T. cucumeris basidiospores were formed by the soil covered method. A germinated single spore deposited on a water agar plate (WA) was transferred to PDA supplemented with a drop of lactic acid ( pH 5.2). A total of 1115 single basidiospore isolates were cultured on PDA. Pairings of the single-spore isolates on WA were done to determine the VC reaction within the same and different generations. Paired isolates with the C3 anastomosis reaction (perfect fusion) were considered as the same VCP, whereas pared isolates showing the Cl reaction (hyphal contact only) or the C2 (killed contacted cells) considered as different a VCP.

Results and discussion
All of the progenies from each parent four out of six parent AG 2-2IV isolates belonged to the same VCP (gave C3 reaction) even at the first generation of the teleomorph stage (G1). When one of them (isolate Rh 509) formed basidiospores Gl through G10, all of its 723 isolates within and among generations belonged to the same VCP. However, molecular analysis of the nuclei is required to find out whether the parent isolate bearing the homogenous VCP is homokaryotic. Conversely, two parent AG 2-2IV isolates gave rise to two more than the six VCPs in Gl, some of which produced homogenous VCPs while others produced heterogenous VCPs in the next G2. Two parent isolates of AG 2-3 were homogenitic and one parent isolate heterogenetic based on their progeny analysis. One isolate of AG 5 was heterogenetic. The heterogenetic parent developed homogenous progenies by repeating hymenia formation. Occasionally, R. solani on sugarbeet leaves infected by basidiospores develop in soil, and thus, can overwinter [2]. This may support the idea that basidiospores may be associated, at least in part, with population diversity in fields by infecting its host plants.

1. Cubeta MA, Vilgalys R, 1997. Phytopathology 87, 480-484. 2. Naito S, 1977. In: Sneh et al., eds. Rhizoctonia Species: Taxonomy, Molecular Biology, Pathology and Disease Control, 197-205.