Measurement and Inheritance of Resistance to Sheath Blight Caused by Rhizoctonia Solani Kuhn in Rice.

Abstract

Sheath blight (ShB), caused by Rhizoctonia solani Kuhn, has been the most destructive rice disease in Louisiana. To breed ShB resistant varieties, an effective procedure for screening progenies of segregating populations is required and detailed knowledge of the mode-of-inheritance of partial ShB resistance is also necessary. In this study four inoculation methods; rice hull/grain mixture (MIX), rice straw, toothpick, and brown rice, along with five assessment systems; 0-9 rating scale (RAT9), relative lesion height (RLH), disease severity (DS), disease incidence, and lesion height were compared on nine rice genotypes showing different resistance levels to ShB. Progenies of segregating populations from crosses between the ShB resistant rice genotypes H$\sb4$/CODF, LB86-30344, Jasmine 85, Teqing, Gui Chao, and Yangdao 4 and the susceptible cultivar Lemont and crosses among the resistant genotypes were field screened for ShB resistance using the highly virulent R. solani isolate LR172 from 1995 to 1997. The best separation between rice genotypes with different levels of partial ShB resistance was obtained by the MIX method of inoculation and the RAT9, RLH, and DS assessment systems. Inheritance studies showed that H$\sb4$/CODF, LB86-30344, and Jasmine 85 had a common dominant gene designated as Rh-2 for partial ShB resistance, which was nonallelic to the common dominant gene Rh-3 in Teqing and Gui Chao. Both Rh-2 and Rh-3 were inherited independently from the recessive gene rh-1 in LSBR-5. The broad-sense heritabilities estimated from F$\sb3$ and F$\sb{2:4}$ lines from the Jasmine 95 x Lemont and LB86-30344 x Lemont crosses ranged from 73.2% to 82.1%. Two of three undesirable agronomic traits; tall plants, pubescent foliage, and red pericarp, found in the partial resistance sources LB86-30344 and H$\sb4$/CODF, were monogenically inherited and inherited independently from the Rh-2 gene, but Rh-2 was loosely linked to the gene for red pericarp with a crossover value of 0.45. The Rh-2 and Rh-3 genes appeared to have an additive effect for partial ShB resistance when combined in the same breeding lines.