The way to a more precise sheath blight resistance QTL in rice

Rice sheath blight (ShB), caused by Rhizoctonia solani, leads to severe yield losses in many rice production areas worldwide. Resistance to rice ShB is a typical quantitative trait controlled by quantitative trait loci (QTLs). To identify the QTLs responsible for ShB resistance, phenotypes for ShB resistance have been surveyed in different individuals/lines in various mapping populations. The 0-9 rating system, based on the percentage of plant height (PH) above the water line with ShB symptoms, is extensively used in the evaluation of ShB in rice communities. By reviewing previous ShB-QTL-mapping studies, we found that the ShB disease score attained using the 0-9 system was inevitably affected by PH and resulted in the colocalization of QTLs for PH and ShB resistance, i.e., some of the PH-QTLs were mistaken as "ShB-QTLs" and had pleiotropic effects. These colocalizing "ShB-QTLs" are irrelevant to physiological ShB resistance and should not be targeted for utilization. We analyzed the ways in which PH affects ShB resistance through the 0-9 rating system and suggest solutions for improving the accuracy of QTL mapping for rice ShB resistance. We also developed the first physical map of the ShB-QTLs and PH-QTLs detected since 1995. This map will be useful in the marker-assisted selection of ShB-resistant QTLs. Under current circumstances, we believe that conventional breeding remains an effective approach for developing ShB-resistant varieties.