Evaluation of Resistance to Sheath Blight (Rhizoctonia solani) in Partial Indica Hybrid Rice Combinations from Southern China

Resistance to sheath blight is generally controlled by polygenes in rice (Oryza sativa L.) and varies greatly among cultivars. At present, very few germplasms can be used as resistant parents in rice breeding. A total of 166 indica hybrid rice combinations collected from 11 provinces in southern China were evaluated for the resistance to sheath blight at seedling stage using artificial inoculation method. According the disease indexes (DIs) to 5 isolates of Rhizoctonia solani, the 166 hybrid combinations were classified into 5 types based on dynamic clustering analysis (DCA), with the ratios of 1.2% for resistance (R), 13.9% for moderate resistance (MR), 36.1% for moderate susceptibility (MS), 43.4% for susceptibility (S), and 5.4% for high susceptibility (HS); but no combination was immune or highly resistant to the disease. K-you 88 and Zhongyou 9801 showed relatively high resistance at seedling stage, but their adult resistance should be further evaluated. The discriminant function for each resistance type was calculated using Bayes method, and the accuracy rate for discrimination was 96.39%. The average DI ranged from 2.84 to 7.64 with an average of 5.27. The concept of synthetic disease index (SDI) was also introduced to classify the 166 hybrid combinations, which is the overall resistance performance to total isolates tested. Based on SDI grading criteria, the 166 hybrid combinations were also divided into R, MR, MS, S, and HS types with the ratios of 1.2%, 13.3%, 63.3%, 21.7%, and 0.6%, respectively. The SDI classification system was significantly correlated with the DCA system (r = 0.81, P < 0.01), showing that both methods can be used for evaluating the disease resistance. The DCA method is suitable for seedling screening under the uniform growth condition. The SDI method is independent on test time, place, and batch, and thus can be applied in relatively complicated conditions. Twenty-six combinations were identified as resistant (R or MR) indica hybrid rice using both methods. The genetic distances among these combinations ranged from 0.04 to 0.71. Twenty-two of them were grouped in the same clade after cluster analysis using unweighted pair group method with arithmetic mean (UPGMA) method. This result and pedigree analysis showed a narrow genetic background of the resistant hybrid combinations.