Submitted to: Experiment Station Bulletins
Publication Type: Experiment Station
Publication Acceptance Date: 6/15/2006
Publication Date: 7/13/2006
Citation: Pinson, S.R., Fjellstrom, R.G., Shank, A.R., Oard, J., Groth, D., Jia, Y., Jia, M.H. 2006. Incorporating foreign sheath blight resistance genes into us rice gemplasm. Texas Rice, Highlighting Research 2006. pp.VI-VII. Interpretive Summary:
Technical Abstract: Sheath blight disease, caused by the Rhizoctonia solani fungus, has been the most economically significant rice disease throughout Texas, Louisiana, and Arkansas since the early 1970s. While diseases such as blast and straighthead can also devastate yield, sheath blight disease occurs more consistently with several hundred thousand acres of US rice requiring fungicide treatment each year to control this disease. Chemical control is costly and incomplete. While several fungicides can slow the progression of R. solani infection, multiple applications repeated every 10 to 14 days are generally required. The cost of chemical control can be as much as $25 per acre. The most economical and effective method for controlling crop disease is to plant genetically resistant varieties. Sheath blight resistance, however differs from that of blast resistance in several key ways. Unlike rice blast where complete resistance to several races or forms of the fungus can be conferred by individual impact and even the most SBR rices known to scientists can be infected by R. solani and develop mild to severe symptoms, depending on the environmental conditions. To achieve field-useful levels of sheath blight resistance, breeders must combine several SBR genes into a single rice line. Some varieties, such as Jasmine 85 and TeQing, contain enough resistance genes, however, to allow them to develop only minor sheath blight symptoms and no measurable yield losses under typical Texas field conditions. Earlier studies showed that TeQing, a highly SBR variety from China, contains 15 SBR genes. Interestingly, these same studies showed that Lemont, a highly susceptible US variety grown widely in the 1990s, also contains three SBR genes. The desired SBR genes in foreign varieties like TeQing and Jasmine 85 are unfortunately intermingled and/or physically linked with genes coding for undesirable traits such as late maturity, tall height, a propensity to lodge, poor milling quality, and atypical grain shape and cooking quality. It is difficult for breeders to disentangle the desired SBR genes from the undesired genes when using the foreign rice lines as breeding parents. Three SBR genes from TeQing were fortuitously maintained, though not purposely selected for, during the development of the variety Saber. Saber is indeed less susceptible to sheath blight disease thanks, in part, to these three introgressed SBR genes. Six additional TeQing SBR genes have since been isolated from the undesirable foreign genes making them more available to US rice breeders. Molecular evaluation of three newly-developed germplasm lines show that they each contain from one to four TeQing SB QTLs now introgressed into a genetic background similar to that of Lemont. The foreign SBR genes are now associated with acceptable levels of plant height, heading, and grain qualities giving breeders a ‘head start’ in efforts to incorporate these SBR genes into improved US rice varieties. The single TeQing SBR gene contained in TIL:642 is of particular interest in that the introduction of this one foreign SBR gene imparts SBR equal to that seen in Saber, which contains three foreign SBR genes.