Sheath blight, caused by Rhizoctonia solani, is one of the most important diseases of rice. Despite extensive searches of the rice germ plasm, the major gene(s) which give complete resistance to the fungus have not been identified. However, there is much variation in quantitatively inherited resistance to R. solani, and this type of resistance can offer adequate protection against the pathogen under field conditions. Using 255 F4 bulked populations from a cross between the susceptible variety 'Lemont' and the resistant variety 'Teqing', 2 years of field disease evaluation and 113 well-distributed RFLP markers, we identified six quantitative trait loci (QTLs) contributing to resistance to R. solani. These QTLs are located on 6 of the 12 rice chromosomes and collectively explain approximately 60% of the genotypic variation or 47% of the phenotypic variation in the 'Lemont'x'Teqing' cross. One of these resistance QTLs (QSbr4a), which accounted for 6% of the genotypic variation in resistance to R. solani, appeared to be independent of associated morphological traits. The remaining five putative resistance loci (QSbr2a, QSbr3a, QSbr8a, QSbr9a and QSbr12a) all mapped to chromosomal regions also associated with increased plant height, three of which were also associated with QTLs causing later heading. This was consistent with the observation that heading date and plant height accounted for 47% of the genotypic variation in resistance to R. solani in this population. There were also weak associations between resistance to R. solani and leaf width, which were likely due to linkage with a QTL for this trait rather than to a physiological relationship.
Rice blast, caused by the fungus Pyricularia grisea (Cooke) Sacc., is a serious rice (Oryza sativa L.)disease causing considerable economic damage worldwide. DNA markers for rice blast resistance have been developed, but most are not suitable for routine use in a marker‐assisted selection breeding program involving large numbers of progeny. After identifying candidate microsatellite markers from public database sources, we have mapped these markers near the blast resistance genes Pi‐b, Pi‐k, and Pi‐ta2 on rice chromosomes 2, 11, and 12, respectively, using segregation information from hundreds of progeny in several crosses. Two microsatellite markers, RM208 and RM224, were found to cosegregate with the Pi‐b and Pi‐k genes, respectively, while additional microsatellites were found to closely flank these two genes and the Pi‐ta2 gene. The new markers are polymorphic in the narrow crosses characteristic of applied breeding programs and appear to be ideally suited for marker assisted selection for blast resistance in rice because of their tight linkage with resistance genes and ease of use through analysis of amplification products. A dominant marker indicating the presence of the Pi‐b gene, Pibdom, has also been developed on the basis of the sequence of the cloned Pi‐b gene. These markers should facilitate the introgression and pyramiding of these three blast resistance genes into new rice cultivars and elite lines.
An accurate greenhouse screening method has not been developed previously to identify host response to sheath blight disease caused by Rhizoctonia solani Kühn that causes significant economic losses in rice yield worldwide. The unavailability of a robust screening system in the greenhouse has made it difficult to quantify disease reactions to R. solani, and has hampered studies on the genetics of resistance and plant breeding efforts to improve resistance. In an effort to develop a standardized laboratory micro-chamber screening method to quantify resistance to R. solani in rice, five rice cultivars, representing a wide range of observed disease reactions under field conditions, were examined in a blind inoculation test at three locations (Arkansas, Texas, and Colombia). Rice seedlings were inoculated at the three- to four-leaf stage with potato dextrose agar plugs containing mycelium and then covered with a 2- or 3-liter transparent plastic bottle for maintaining high humidity after inoculation. Two cultivars, Jasmine 85 and Lemont, that consistently have shown the highest and lowest levels of resistance, respectively, in previous field and greenhouse studies, were used as standards. Concurrent field experiments in Arkansas and Texas also were performed to compare the greenhouse disease ratings with those observed under field conditions. Overall, the relative disease ratings of the seven test cultivars were consistent between test locations and with field evaluations. Thus, the micro-chamber screening method can be used as an effective approach to accurately quantify resistance to the sheath blight pathogen under controlled greenhouse conditions and should help expedite the selection process to improve resistance to this important pathogen.
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