Resistance to rice yellow mottle virus and performance of selected improved rice genotypes in Central Uganda
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Rice yellow mottle virus (RYMV) causes yield losses up to 100 % although yield losses of 19% have been recorded in selected areas of northern Uganda yet resistant rice varieties with farmer and consumer preferences are not available to farmers. The aim of this study was to contribute towards development of RYMV resistant rice genotypes for Uganda through: (i) Identifying new sources of resistance to RYMV among improved genotypes and (ii) Assessing performance of RYMV resistant genotypes. One hundred twelve genotypes were screened for RYMV resistance in an alpha lattice design with two replicates, using 3 isolates of the virus in screen houses of NaCRRI-Namulonge during two rounds in 2017. These isolates were collected from Iganga, Lira and Namulonge. RYMV disease severity was assessed using a scale of 1-9, with 1 being highly resistant and 9 being highly susceptible. Analysis of variance (ANOVA) revealed highly significant differences for RYMV severity among genotypes. Relative area under disease progress stairs, plant height reduction, delay of days to flowering, reduction in chlorophyll content and the reduction of grain weight were positively correlated and their heritability was high (H2 ≥ 0.65). Findings revealed 17 resistant genotypes, among which six were highly resistant [ARS126-3-B-1-2 (11), ARC36-2-P-2 (2), ARC39-145-P-2 (5), Gigante, IRL 2 (GP 54) and IRL 4 (69 GP 54)] and could be used as sources of RYMV resistance genes. RYMV resistant genotypes, together with ten checks, were evaluated for their performance in field trials for two seasons at NaCRRI-Namulonge using an alpha lattice design. Data were collected on diseases, yield, yield components and agronomic traits at different growth stages using a standard evaluation system of IRRI. ANOVA revealed high diversity among genotypes for RYMV disease incidence and severity, bacterial leaf streak, brown spot, narrow brown leaf spot, gall midge, number of tillers per plant, days to 50 % flowering, culm strength, plant height, leaf senescence, flag leaf length and width, panicle exertion, maturity period, tillering efficiency, panicle length, number of panicles per m2, spikelet fertility, panicle threshability, weight of 1000 filled grains and grain yield. Heritability was low (H2 = 0.48) for traits influenced by environment such as yield. Correlation and principal components analysis helped to determine traits to use in calculation of selection index. Genotypes IRL 53 (GP 54) and Namche 2 had high selection indices (SI = 27.75 and 25.99 respectively) suggesting that they performed better in most of traits. However, the most yielding genotypes were IRL 2 (GP 54) and IRL 4 (69 GP 54). These genotypes should be screened further for other traits of interest for use in the rice breeding program in Uganda. Multi-locational evaluation should also be done to identify the most promising genotypes for development of improved varieties with farmer-preferred traits.