Characterization of populations of Rhizoctonia solani in paddy rice fieldsin Cote d'Ivoire

Isolates of Rhizoctonia solani were obtained from plant and soil samples th at had been systematically collected in a field experiment in Cote d'Ivoire to study the diversity of the pathogen and the influence of three differen t rice rotations on the pathogen population. Characterization by morphology , anastomosis testing, pathogenicity testing, and restriction fragment leng th polymorphisms (RFLPs) of AT-rich DNA (AT-DNA) showed that there were no differences in isolates from different experimental plots, suggesting that the soil as well as the plant population of the fungus was indistinguishabl e throughout the experiment and was not influenced by crop rotation. Analys is of AT-DNA showed that the isolates obtained from plant material and one from soil shared a distinct banding pattern, identical with the AT-DNA RFLP obtained for the reference strain of anastomosis group 1 (AG-1). The remai ning soil isolates produced a consistent RFLP pattern that was distinct fro m that of the plant isolates. Morphological characterization of isolates pr oduced two major clusters consisting of the same groups of isolates as foun d by AT-DNA RFLP. Diversity in morphological characters was much higher in plant than in soil isolates and indicated that the population might consist of several clones. Anastomosis testing revealed that soil as well as plant isolates were able to fuse with the tester strain of AG-1. Significant dif ferences in disease severity were observed between the two groups of isolat es in pathogenicity tests on rice plants, with plant isolates being distinc tively more virulent.