Submitted to: Frontiers of Agriculture in China
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/22/2007
Publication Date: 11/1/2007
Citation: Wamishe, Y.A., Jia, Y., Singh, P., Cartwright, R.D., Eizenga, G.C., Lee, F.N. 2007. Studies on isolates of Rhizoctonia solani from Arkansas for management of rice sheath blight. Frontiers of Agriculture in China. 1(47):361-367. Interpretive Summary: Complete resistance to rice sheath blight caused by Rhizoctonia solani is not known and detection of resistance in rice has been difficult. Studying field isolates of the pathogen was considered to facilitate screening for resistance, and 102 isolates were identified as R. solani using ribosomal DNA internal transcribed spacers’ marker. Fourteen isolates were tested for anastomosis grouping and all were in AG1-IA, a group that causes rice sheath blight disease. Hyphal growth rate was studied in a Petri dish and ranged from 1.17 to 1.89 (mm/h) representing a range in aggressiveness. Selected isolates were tested on detached rice leaves, rice plants at V-11, and 18-day-old rice seedlings and were correlated with the field results. The fastest growing isolates caused longer lesion length than the slowest growing isolates. However, the slow growing isolates produced disease levels in the greenhouse tests on rice seedlings that corresponded to the field disease reactions of 90.9% of the cultivars. However, 71.4% of the cultivars known to be tolerant to sheath blight in the field were highly affected by the fast growing isolates indicating resistance in the seedling tests can better be detected using the slow growing isolates. The overall results showed the significance of combining evaluation techniques with the pathogen isolates to be able to detect the moderate level of resistance present in some rice cultivars and germplasm.
Technical Abstract: Because resistance to sheath blight disease in rice is considered quantitatively inherited and the incidence of the pathogen,Rhizoctonia solani, in the field is erratic, screening for genetic resistance is difficult. Studying the attributes of common R. solani field isolates should facilitate the screening of rice cultivars for sheath blight resistance. Over three years, 200 isolates of Rhizoctonia-like fungi were obtained from infected rice and two grass species from 19 counties in Arkansas. Among 200 sclerotia-forming fungi 103 were identified as R. solani using ribosomal DNA (rDNA) internal transcribed spacers (ITS) marker. Anastomosis grouping (AG), hyphal growth rate, and pathogenicity of the R. solani isolates were studied. Fourteen isolates randomly selected and tested for AG from collections made in all 2001 belonged to AG1-IA. In vitro hyphal growth rate ranged from 1.17 to 1.89 (mm/h) however most of the isolates representing the field population in Arkansaswere not signatively different. Three fastest growing isolates caused longer lesion length on inoculated detached rice leaves than the three slowest growing isolates. Lesion lengths on detached leaves of Jasmine 85 (tolelrant) and M-202 (susceptible) were positively correlated (r = 0.86 at P= 0.0059 and r = 0.93 at P= 0.0001, respectively) to in vitro hyphal growth rates; however results from detached leaves of nine other cultivars were inconsistent. At V-11, the two fastest growing isolates showed significant difference in percent lesion length between Jasmine 85 and M202 contrary to the two tested slow growing isolates. When18-day old seedlings of 11 rice cultivars were inoculated with one of the fastest growing isolates 71.4% of the cultivars known to be tolerant to sheath blight in the field showed percentage lesion lengths not significantly different from the susceptible control, M202. However, when inoculated with one of the slowest growing isolates, 90.9 % of the cultivars showed disease reactions similar to their field response, indicating the importance of combining evaluation techniques with the pathogen isolates in order to be able to detect differences in resistance among rice cultivars.