Witchweed, Striga gesnerioides (Willd.), continues to be a major menace to production in West and Central Africa, where cowpea is an important crop.
This parasitic weed feeds on cowpea plants, leading to severe chlorosis or yellowing, wilting, stunting, and even the death of susceptible hosts. Annual yield losses are estimated in millions of tons.
A collaborative project funded by the Generation Challenge Program has taken a close look at the Striga problem and is using molecular tools to identify new sources of resistance. Marker-assisted selection or breeding (MAS or MAB) is being used to facilitate the selection of lines with resistance to Striga. In 2008 and 2009, the Institut National de lâ€™Environnement et des Recherches Agricoles (INERA) of Burkina Faso and IITA undertook a study to identify potential sources of Striga resistance, worked with farmers to articulate their problems and preferences, and tested new Striga-resistant improved lines developed using MAS.
Prior to the breeding activity, participatory rural appraisal (PRA) and farmersâ€™ participatory variety selection (FPVS) sessions were organized in seven Striga hot-spots in Niger and Burkina Faso, which are major cowpea-producing areas. During the 2 years, in Niger, 403 farmers contributed to the FPVS and several of their preferred cowpea lines were selected.
To determine farmersâ€™ preferences in Niger, germplasm of 24 cowpea varieties with various characteristics (seed color, seed size, plant type, maturity, and Striga resistance) were planted in farmersâ€™ fields and used for the PRA and FPVS. The same 24 accessions were also planted in two fields at IITAâ€™s Minjibir Experimental Farm as checks.
In Niger, researchers and local agricultural agencies worked with farmers in their fields. Based on interviews and responses to questionnaires, farmers acknowledged that Striga is a serious problem that seems to be increasing, said IITA scientists Satoru Muranaka and Ousmane Boukar, and Jean Baptiste Tignegre of INERA, who collaborated on the project. Farmers, they added, also suggested that the use of resistant varieties could be a solution. This reconfirmed the importance of Striga resistance as a breeding goal.
The project has also confirmed farmersâ€™ preference for IT00K-1148 and IT90K-372-2-1 because of their agronomic traits. These lines are susceptible to the dominant race of Striga in Niger. In the FPVS, farmers preferred new lines, such as KVX30-309-6G and TN256-87, which also lacked resistance to Striga.
Of the top five genotypes selected by the farmers in the seven locations, only twoâ€”IT99K-573-2-1 and IT98K-205-8â€”were picked by farmers because these met their preferences for Striga resistance, early maturity, and high yield potential.
Farmers preferred the white-seeded variety for consumption but genotypes with brown seed color that are early maturing and high yielding were also acceptable. The surveys and FPVS activities showed that farmers use consistent selection criteria based on various traits.
Because of unstable rainfall and other problems in 2009, farmers did not get good grain yields, although in a few cases, Striga-resistant IT99K-573-2-1 showed 4âˆ’6 times more grain yield (average 214 kg/ha compared with 37âˆ’51 kg/ha for local varieties).
Both IT99K-573-2-1 and IT98K-205-8 showed resistance to Striga in all the locations used for the trial. These varieties could be recommended for cowpea production in Southeastern Niger where Striga and drought are major constraints, and for use as sources of resistance genes in breeding other varieties.
Using MAB, pot and field experiments in 2008 and 2009 evaluated backcrossed varieties (crossed to their parents) for various traits, such as resistance to Striga, flowering and maturing dates, disease resistance (to bacterial blight, virus, and leaf rust), and seed characteristics.
Of the 60 genotypes tested in a pot screening trial in the Maradi station of the Institut National de Recherche Agronomique du Niger (INRAN) from October 2008 to November 2009, 18 showed Striga resistance. Results confirmed that the Striga race (SG3) dominant in two locations in northern Nigeria is also dominant in the four trial locations in southeastern Niger. Field trials had been conducted earlier in Kano and Borno States in Nigeria. Hence, the same Striga-resistant genetic resources could be used for breeding varieties for these areas.
Two existing sequence characterized amplified region (SCAR) markers, 61R and MahSE2, were earlier identified to have the potential for use in MAB for SG3 Striga resistance. To confirm this, pot experiments were conducted using Striga-resistant lines that had been developed using MAS. Results showed a higher percentage of resistant plants in the MAB-developed populations than in the control (those that did not use MAB). SCAR marker MahSE2 showed 88% and 96% marker efficiency for evaluating Striga resistance in backcrossed populations.
Pyramiding or building up Striga resistance in these breeding lines via MAB is important. However, further exploration of appropriate markers is needed to develop efficiently the varieties preferred by farmers. Likewise, more markers linked to various traits that meet farmersâ€™ preferences identified in the PRA and FPVS also need to be converted to SCARs for use in MAB.
This project was able to identify germplasm lines with resistance to Striga races predominant in the Niger Republic; identify farmersâ€™ constraints and preferences to aid in selecting for important traits to combine with Striga resistance; and conduct with farmers the participatory field testing of the new Striga-resistant improved lines developed via the MAB method. It also confirmed the efficiency and effectiveness of MAB for Striga resistance