| dc.description.abstract | Rice blast caused by Magnaporthe oryzae is one of the most serious diseases that jeopardize rice production in Uganda. In fact, it causes up to 100% yield loss in severe cases. In order to overcome the effect of this disease, resistant cultivars must be developed. However, to design appropriate breeding schemes that can be adopted by the rice breeding programmes, it is important to understand the genetic basis of resistance in these cultivars. The objectives of this study were; 1) to determine the reaction of selected rice genotypes with monogenic resistance to the M. oryzae isolate from Namulonge, 2) determine the mode of inheritance governing resistance to rice blast disease among local resistant genotypes, 3) determine the allelic relationship between broad spectrum resistant genes Pi7t and Pi54 and 4) determine the segregation pattern of SSR markers in a population created from a resistant local source (Namche2) and a susceptible adapted local genotype (Supa Soroti). In study 1, 83 genotypes including 73 monogenic differential lines and 10 local varieties were evaluated in the screen house in a 10 x 8 alpha lattice design in two replications with the susceptible and resistant checks placed every four blocks in a replication. Results revealed a significant difference among the tested genotypes at a confidence level of 0.1% (P<0.001) at 7, 14 and 21 days after inoculation for the evaluated traits including; disease severity, severity percentage, disease incidence as well as rAUDPS in the first, second and across experiments. This is an indication of genetic variation amongst the tested genotypes. Nineteen (22.9%) genotypes with R-genes Pi3, Pi5 (t), Pi7 (t), Pi-b, Pik, Pi54, Pik-m, Pit, Pita, Pita-2, Piz, Piz-4 and Piz-5 had consistently low disease severity scores during the two experiments of a range between 0-3 and hence their genes were considered to be effective against the isolate of M. oryzae from Namulonge. Seven resistant varieties (including two lines with monogenic resistance and five resistant varieties) and five susceptible locally adapted varieties were selected from the screening work. These were crossed in a North Carolina design II. Twenty-six (26) crosses were planted in the screen house alongside 17 crosses from the Diallel and 12 parents to make a total of 56 entries were planted in the screen house in an 8x7 alpha lattice design. However, these designs were analysed differently as RCBD. Results revealed that inheritance of resistance to rice blast among these genotypes is mainly additive. Crosses and GCA of the male parents were highly significant (P<0.001) while both SCA and GCA of the female parents were non-significant which showed that the female parents were genetically similar in transmitting their susceptibility while for non-significant SCA showed that there were low non additive effects involved in the inheritance of rice blast resistance. From the screen house data analysis, broad sense (0.85), narrow sense (0.74) coefficients of genetic determination and Baker’s ratio (0.86) were relatively high which confirms the importance of additive effects in the inheritance of resistance among the susceptible x resistant crosses. Segregation pattern analysis showed single dominant gene (3:1) for some crosses, other crosses showed duplicate recessive epistasis (9:7), some showed dominant epistasis (13:3) while others showed a three dominant gene complimentary epistasis (27:37). This showed that resistance to rice blast among the tested genotypes could be influenced by one, two or more genes. The seven resistant varieties were then crossed with each other to carry out an allelism test on 100 individuals from each cross. This test indicated that the cross between Namche2 and monogenic differential line WH13-3203 did not segregate and hence giving a ratio of 1:0 (R: S) which indicates that Namche2 could be carrying the same resistance gene as the monogenic line which is Pi54 or alleles of this gene. Therefore, Namche2 can easily be used as a resistant parent for gene pyramiding since at least one of the resistant loci is now known. This resistance gene can be easily pyramided with other resistance genes in order to improve resistance durability to rice blast disease
Lastly, eleven SSR markers were tested for polymorphism out of which nine were polymorphic between Namche2 (resistant) and Supa Soroti (susceptible). The chi square goodness of fit analysis showed that all the nine markers segregated in a normal Mendelian fashion of 1:2:1. However, both the regression and chi square test of independence were significant for SSR marker RM21 (R2= 0.046, P<0.05 and χ2 = 8.30, P<0.05). This showed that this specific marker could effectively be utilized by the breeding programme for MAS and has the potential to accelerate introgressing of resistance gene into susceptible commercial varieties.
Rice blast caused by Magnaporthe oryzae is one of the most serious diseases that jeopardize rice production in Uganda. In fact, it causes up to 100% yield loss in severe cases. In order to overcome the effect of this disease, resistant cultivars must be developed. However, to design appropriate breeding schemes that can be adopted by the rice breeding programmes, it is important to understand the genetic basis of resistance in these cultivars. The objectives of this study were; 1) to determine the reaction of selected rice genotypes with monogenic resistance to the M. oryzae isolate from Namulonge, 2) determine the mode of inheritance governing resistance to rice blast disease among local resistant genotypes, 3) determine the allelic relationship between broad spectrum resistant genes Pi7t and Pi54 and 4) determine the segregation pattern of SSR markers in a population created from a resistant local source (Namche2) and a susceptible adapted local genotype (Supa Soroti). In study 1, 83 genotypes including 73 monogenic differential lines and 10 local varieties were evaluated in the screen house in a 10 x 8 alpha lattice design in two replications with the susceptible and resistant checks placed every four blocks in a replication. Results revealed a significant difference among the tested genotypes at a confidence level of 0.1% (P<0.001) at 7, 14 and 21 days after inoculation for the evaluated traits including; disease severity, severity percentage, disease incidence as well as rAUDPS in the first, second and across experiments. This is an indication of genetic variation amongst the tested genotypes. Nineteen (22.9%) genotypes with R-genes Pi3, Pi5 (t), Pi7 (t), Pi-b, Pik, Pi54, Pik-m, Pit, Pita, Pita-2, Piz, Piz-4 and Piz-5 had consistently low disease severity scores during the two experiments of a range between 0-3 and hence their genes were considered to be effective against the isolate of M. oryzae from Namulonge. Seven resistant varieties (including two lines with monogenic resistance and five resistant varieties) and five susceptible locally adapted varieties were selected from the screening work. These were crossed in a North Carolina design II. Twenty-six (26) crosses were planted in the screen house alongside 17 crosses from the Diallel and 12 parents to make a total of 56 entries were planted in the screen house in an 8x7 alpha lattice design. However, these designs were analysed differently as RCBD. Results revealed that inheritance of resistance to rice blast among these genotypes is mainly additive. Crosses and GCA of the male parents were highly significant (P<0.001) while both SCA and GCA of the female parents were non-significant which showed that the female parents were genetically similar in transmitting their susceptibility while for non-significant SCA showed that there were low non additive effects involved in the inheritance of rice blast resistance. From the screen house data analysis, broad sense (0.85), narrow sense (0.74) coefficients of genetic determination and Baker’s ratio (0.86) were relatively high which confirms the importance of additive effects in the inheritance of resistance among the susceptible x resistant crosses. Segregation pattern analysis showed single dominant gene (3:1) for some crosses, other crosses showed duplicate recessive epistasis (9:7), some showed dominant epistasis (13:3) while others showed a three dominant gene complimentary epistasis (27:37). This showed that resistance to rice blast among the tested genotypes could be influenced by one, two or more genes. The seven resistant varieties were then crossed with each other to carry out an allelism test on 100 individuals from each cross. This test indicated that the cross between Namche2 and monogenic differential line WH13-3203 did not segregate and hence giving a ratio of 1:0 (R: S) which indicates that Namche2 could be carrying the same resistance gene as the monogenic line which is Pi54 or alleles of this gene. Therefore, Namche2 can easily be used as a resistant parent for gene pyramiding since at least one of the resistant loci is now known. This resistance gene can be easily pyramided with other resistance genes in order to improve resistance durability to rice blast disease
Lastly, eleven SSR markers were tested for polymorphism out of which nine were polymorphic between Namche2 (resistant) and Supa Soroti (susceptible). The chi square goodness of fit analysis showed that all the nine markers segregated in a normal Mendelian fashion of 1:2:1. However, both the regression and chi square test of independence were significant for SSR marker RM21 (R2= 0.046, P<0.05 and χ2 = 8.30, P<0.05). This showed that this specific marker could effectively be utilized by the breeding programme for MAS and has the potential to accelerate introgressing of resistance gene into susceptible commercial varieties. | en_US |
