The root parasitic weed Striga, also known as ‘witch weed’ or ‘violet vampire’ due to its deceptively beautiful purple flowers, is a pervasive and recalcitrant problem of cereal-based systems in many parts of Africa. As a root parasite, Striga causes significant damage by attaching itself to a maize root, where it extracts the water and nutrients it needs, stunting or killing its host. Indeed, Striga does most of its damage underground before emerging above the surface. As such, farmers cannot do much to save their crop when they see the weed and its flowers appear.

Developed by the CGIAR Research Program on Maize (MAIZE), Striga-resistant maize varieties (SRMVs), such as SAMMAZ 11 or Across 97, SAMMAZ 15 and SAMMAZ 16, are easy for farmers to adopt and have already been released and widely disseminated in Nigeria. Adoption rate of these cultivars in the Federal Capital Territory area of Nigeria stands at about 41 percent. Adoption of SRMVs has led to both higher maize yields and increased household income of adopters by about US$ 110 per capita. Adoption also reduced the incidence of poverty among adopters by 9 percentage points (Hassan et al., 2016). There is also significant adoption of Integrated Striga Management (ISMA) practices in the northern part of Nigeria. The study by Hassan et al. (2016) highlighted the need for policies and programs aimed at enhancing adoption of SRMVs in Nigeria and beyond.

Although ISMA practices work well, there are reports of variability in the extent of control achieved by Striga control options, particularly host plant resistance, in different agro-ecological zones. For example, maize varieties that show tolerance in a specific geographical location, when taken to another test location do not perform as well as the landraces in these locations. Therefore, there is a need to further understand the basis of Striga diversity and host-parasite relationships in order to deploy suitable control methods that will remain effective over time and across different locations.

As a first step towards understanding the interaction between the host plant and the parasitic weed, MAIZE researchers performed genetic diversity and population structure tests on Striga populations. Approximately 1500 high quality genotyping by sequencing (GBS)-derived single nucleotide polymorphism (SNP) markers were used to characterize over 1000 Striga populations collected from several locations and different host crops in Kenya and Nigeria. In both populations, high levels of genetic diversity were observed. The populations from northern Nigeria formed three distinct subpopulations based on their collection location, while those collected from western Kenya were largely undifferentiated. Striga populations parasitizing rice in Kenya showed some genetic distinctness, but clear evidence of host specificity was not observed. In summary, the findings highlight the importance of developing and testing Striga control technologies in diverse locations as the existence of potential Striga ecotypes (a genetically distinct geographic population within the species, adapted to specific environmental conditions) has been brought to light. These findings pave the way for breeding maize varieties with broad spectrum or ecotype specific resistance to Striga. MAIZE is also working with the University of Sheffield in the United Kingdom to study the genetic basis of Striga resistance.