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.