Demand-Driven Technologies for Sustainable Maize ... - IITA

28Overview of the strategy for developing Striga resistantextra-early maturing maizeA major emphasis of the IITA-WECAMAN maize-breeding programhas been on the development of extra-early drought and Strigaresistant/tolerant populations using adapted and exotic germplasmidentifi ed through several years of extensive testing in WCA. Inaddition, Striga resistance/tolerance genes have been introgressed intoextra-early maize populations and varieties using the STR inbred linesfrom IITA (9030 STR, 1368 STR, and 9450 STR) and CIMMYT (Pop22 STR) as the resistance sources. Recurrent selection, backcrossing,inbreeding and hybridization methods have all been successfully usedin the breeding program to increase the frequency of favorable allelesfor resistance to Striga. In this paper, tolerance is defi ned as the abilityof the host plant to withstand the effects of the parasitic plants that arealready attached. On the other hand, resistance refers to the ability ofthe host plant to prevent the attachment of the parasite to its roots, orkill the attached parasite thus resulting in reduced Striga emergence(Kim 1994; Badu-Apraku et al. 2006). During the early stages of thebreeding program, the emphasis was on the selection for tolerance,which allowed the pathogen to reproduce, thus increasing the buildupof the Striga seed bank in the soil. Since 1999, the emphasis has beenon the selection for reduced Striga emergence and several inbred lines,which combine reduced Striga emergence counts and high grain yield,have been developed. Introgression of the sources of Striga resistanceinto existing extra-early maize populations and elite varieties, and theuse of recurrent selection methods for improvement have led to thedevelopment of two extra-early Striga resistant populations. Theseare TZEE-W Pop STR (extra-early white endosperm) and TZEE-Y PopSTR (extra-early yellow endosperm). In addition, several extra-earlymaturing Striga resistant varieties with moderate levels of resistance/tolerance to Striga have been developed from the populations andcrosses between various materials in the program.The S 1family selection scheme has mainly been used in thebreeding program under artifi cial Striga infestation to concentratefavorable alleles for resistance to Striga. The S 1family selection programwas initiated in 1996 in each of the two source populations for Strigaresistance. Since then each population has gone through four cyclesof S 1recurrent selection. S 1progenies from each cycle of improvementwere screened under artifi cial infestation with S. hermonthica and noninfestedconditions at Ferkessédougou (hereafter called Ferké) in Côted’Ivoire from 1996 to 2001 and at Abuja and Mokwa since 2002.Genotypes were evaluated using lattice designs with two replications.The number of progenies screened in each cycle ranged from 196 to256, with a selection intensity of 25–30%. Based on the data across