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The rates of in vitro hyphal growth of Rhizoctonia solani isolates,and their pathogenicity were evaluated to identify R.solani isolates that are suitable to detect quantitative resistance in rice.The isolates ofR.solani were purified from the infected rice and two grass species in Arkansas over three years.Among 200 Rhizoctonia-like isolates,102 isolates were identified as R.solani,and confirmed using a ribosomal DNA intal transcribed spacers’ marker.The rates of in vitro hyphal growth of the 102 R.solani isolates ranged from 1.17 to 1.89 mm/h,of which only 13.7% were significantly different from each other.The rates of in vitro hyphal growth of eight selected isolates were correlated with lesion lengths (r=0.86 at P = 0.005 9 and r = 0.93 at P = 0.000 1) on the detached leaves of rice cultivars of Jasmine 85 (resistant) and M202 (susceptible),respectively.The eight isolates were selected based on the mean values of the maximal (1.89),median (1.54) and minimal (1.17) rates of hyphal growth.Two isolates that consistently exhibited significant differences in the rates of the hyphal growth were selected to examine the aggressiveness of isolates in microchambers.Using a micro-chamber,the slow growing isolates separated susceptible cultivars from moderately resistant cultivars better than the fast growing isolates.In contrast,the differences in disease reactions between both R.solani isolates were undetected using a standard field evaluation method.We suggest that the slow growing isolates are more useful than the fast growing isolates for detecting quantitative resistance with the micro-chamber method.