Assessment of genetic diversity of rice based on SNP markers for selection of parents for sheath rot (Sarocladium oryzae) resistance breeding

Sheath rot of rice, caused by Sarocladium oryzae, is an important emerging rice disease not only in Rwanda, but also in other rice-growing countries. Given that cultivar resistance is a sustainable management strategy for small-scale farmers, the aim of this study was to identify genetically distant parental materials for sheath rot resistance breeding. Ten resistant and fifteen susceptible accessions were analysed using 94 single nucleotide polymorphism (SNP) markers. The number of alleles amplified per locus ranged from 1 to 4 with a mean of 2.01 and a total of 189 alleles detected from the 25 genotypes. The number of observations per marker locus ranged from 11 to 25 with an average of 23. The mean major allele frequency was 76.2%, whereas the mean polymorphic information content was 0.263, and gene diversity was estimated at 0.325. Consequently, the markers were highly informative and revealed good estimates of genetic diversity among the studied accessions. Genetic distances ranged from 0 to 0.63 and a UPGMA dendrogram distinguished resistant and susceptible genotypes. This study revealed the possibility of improving resistance to sheath rot with minimum risk of genetic depression or reduced variability among progenies through hybridisation of locally adapted germplasm

Genetic analysis of mechanisms associated with inheritance of resistance to sheath rot of rice

Understanding genetic mechanisms controlling inheritance of disease resistance traits is essential in breeding investigations targeting development of resistant genotypes. Using North Carolina design II, 32 F1 hybrids were generated by crossing eight susceptible to four resistant parents and submitted for field evaluation. The analysis of general and specific combining ability (GCA and SCA) indicated involvement of additive and non-additive gene action controlling inheritance of horizontal resistance to sheath rot of rice. High GCA/SCA ratio and high heritability estimates revealed additive effects and were more predominant than none additive ones. The level of dominance indicated dominant genes was more important than recessive genes. Estimates of GCA and SCA analysis suggested that crop improvement programmes should be directed towards selection of superior parents or good combiners, emphasizing on GCA. As far as source of resistance is concerned, most promising genotypes were Cyicaro, Yunertian and Yunkeng. The predominance of additive genetic effects together with the relevance of dominant genes suggested possibilities of improving the resistance by introgression of resistance genes through recurrent selection coupled with phenotypic selection

Data from: Wheat nutrient response functions for the East Africa highlands

Wheat (Triticum æstivum L.) is an important East Africa highland crop but yields are low. Information is scarce for optimization of fertilizer use. Research was conducted to determine yield response functions for N, P and K, and to diagnose Mg–S–Zn–B deficiencies. The average grain yield increase in Rwanda due to N application was 1.5 Mg ha−1 with a mean economically optimal rate (EOR) of 68 kg ha−1 N. In Kenya and Tanzania, yield was increased by 29% with EOR N for two SY but unaffected by N rate for four other SY which on average had 50% of the soil organic C (SOC) as the N-responsive SY. Yield was increased, on average, with application of P and K by 0.47 and 0.23 Mg ha−1, respectively, at EOR in Rwanda but effects were inconsistent for other SY where soil test K was higher than in Rwanda. Application of Mg–S–Zn–B resulted in 0.46 Mg ha−1 more yield in Rwanda but did not affect yield at other SY where the average soil test values for these nutrients was 35% higher than in Rwanda. If the financially constrained farmer opts to apply the affordable fertilizer to twice as much land at 50% EOR compared with 100% EOR, the mean yield increase is reduced by 27% but production and PCR are increased by 43 and 72%, respectively. Nutrient effects were relatively consistent and positive in Rwanda, but less and less inconsistent elsewhere with generally less SOC, more K–Mg–S–Zn–B availability, and often lower yields