Abstract:
Russian wheat aphid (Diuraphis noxia Kurdjumov) (RWA) and the emergence of stem rust (Puccinia graminis Pers. f.sp. tritici ) race TTKSK (‘Ug99’) in Kenya are currently some of the most binding constraints to wheat production in Kenya. Severe infestation by RWA may result in yield losses of up to 90% in commercial wheat cultivars while ‘Ug99’ infected plants may suffer up to 100% loss. The two pests combined have seriously affected wheat farmers forcing them to heavily rely on pesticides. This has increased the cost of production making wheat an expensive crop to produce. There is therefore great need to come up with cheap and durable solutions to both RWA and ‘Ug99’. This research work sought to develop wheat germplasm that is resistant to both RWA and ‘Ug99’ by pyramiding two major resistance genes. The work was done in a breeding cage and green house at Kenya Agricultural Research Institute (KARI)-Njoro. The RWA and ‘Ug99’ resistant material were obtained from the breeding and biotechnology departments of KARI-Njoro. Three varieties of wheat were used in this experiment; they include ‘Kwale’, a Kenyan commercial variety known to be high yielding but susceptible to both RWA and ‘Ug99’, ‘Cook’, an Australian variety carrying stem rust resistance gene Sr36 and is known to confer immunity to ‘Ug99’ at both seedling and adult plant stages, and ‘KRWA9’, a Kenyan line known to be resistant to RWA but has poor agronomic attributes. The F1 of the double cross (DC F1) was obtained by crossing the F1 of ‘Kwale × Cook’ and the F1 of ‘Kwale × KRWA9’. The DC F1 population was subjected to sequential screening for both RWA and ‘Ug99’ resistance. The surviving DC F1 progenies were left to self pollinate in the field in order to obtain the F2 of the double cross (DC F2). The DC F2 progenies were sequentially screened against RWA and ‘Ug99’ to obtain a population that is resistant to both RWA and ‘Ug99’. Genotypic characterization of the DC F2:3 families was later done to select only homozygous dominant plants to the two resistance genes. Data collected was subjected to chi-square “goodness of fit” using GenSTAT 12th edition to determine the mode of inheritance of RWA and ‘Ug99’ resistance genes. The results indicated that the RWA resistance gene in ‘KRWA9’ and ‘Ug99’ resistance gene Sr36 were successfully pyramided. These genes proved to be dominant with single crosses exhibiting ratios of 3:1. The genes were simply inherited and easy to transfer into ‘Kwale’, a Kenyan commercial variety. The fact that the two genes were not linked, they were inherited independently. It was recommended that though races with virulence for Sr36 have been reported, the gene is immune to the race ‘Ug99’ and could still be used effectively as a component for ‘Ug99’ resistance breeding together with other Sr genes. This study has clearly demonstrated that it is possible to get one population that is resistant to both RWA and stem rust. This population can be advanced to early generations and selections made within preliminary and advanced yield trials for future variety release.
