Inheritance of Resistance to Sheath Blight in Long Grain Rice (Genetics, Fungus Disease).

The inheritance of resistance to the sheath blight disease of rice (Oryza sativa L.), caused by Rhizoctonia solani Kuhn, was studied using as parent lines four susceptible commercial varieties--Lebonnet, Labelle, Starbonnet and Leah, and three resistant lines: L201, RU7902185 and RU7902191. The parental lines, F(,2) and F(,3) progeny of each cross were inoculated when the seedlings were 35 days old using the isolate LR172, and then kept in a humidity chamber for two weeks. The infection level of each plant was evaluated using a 0-9 rating system where 0 indicates no symptoms and 9 indicates the most severe infection. Infection height as a percentage of the total sheath height was also estimated. The frequency distribution of the F(,2) progeny of susceptible x resistant crosses showed a bimodal distribution with the modal classes at 5 and 7. The resistant parents had a modal class at 4 and the susceptible parents had their modal class at 7. The distribution of the F(,2) progeny of most of the crosses appears to fit a 9:7 resistant to susceptible ratio. This suggests that two pairs of complementary genes with a high level of dominance controlled resistance to sheath blight. The analysis of variance indicates a high level of dominance and epistasis. Rating disease reaction by measuring the height of the highest lesions was not a satisfactory method for rating sheath blight infection since lesions sometimes occurred around the uppermost leaf collar on otherwise symptom-free plants. Heritability estimates were low. The highest estimates were 16.09% and 24.62% using respectively the regression and correlation of F(,2) on F(,3) line means. Since sheath blight resistance appears to be an effect of complementary dominant genes, the epistatic interactions may be partly responsible for the low heritability. However the low heritability suggests a need for progeny testing of individual plants selected in segregating populations in order to effectively identify resistant genotypes