Striga hermonthica resistance in maize also warrants low Striga asiatica infections and contributes to yield performance across environments

Background Parasitic weeds Striga hermonthica (Benth.) and Striga asiatica (L) Kuntze are important yield reducing factors to maize in Africa. Varietal resistance offers a feasible and effective solution but Striga-resistant maize varieties are scarce and their broad applicability is uncertain. Objectives A lab and multi-location, multi-season field study was conducted to establish whether Striga resistance observed in maize is attributable to lower strigolactone production, is sustained among Striga species and ecotypes and ensures yield performance across environments and seasons. Methods Twenty-two maize genotypes derived from Malawian public or private breeding endeavors and from an international organization, IITA, based in Nigeria, were grown in four S. hermonthica infested sites in Kenya (2017), and on three S. asiatica infested sites in Malawi (2017 and 2018). Striga numbers and maize grain yields (in Malawi only) were assessed from fields and strigolactone profiling of root exudates of these genotypes was done in the lab. Results Striga hermonthica and S. asiatica resistance of the Nigerian genotype group was significantly higher than that of the two Malawian genotype groups. Maize genotypes with resistance against S. hermonthica also showed effective resistance against S. asiatica . Resistance was only partly attributed to low strigolactone production, and some genotypes combined very low Striga infection levels in the field with high strigolactone production. Nigerian genotypes had lower yields than Malawian genotypes in five of the six Malawian site-seasons. The only site-season where Nigeria genotypes performed better was when high Striga infection coincided with very low rainfall. Conclusion Striga resistance in maize is effective across the two main species, S. hermonthica and S. asiatica, but can only partly be attributed to low production levels of strigolactones. When high Striga infection coincides with low rainfall, resistance helps mitigate negative impacts on maize yields. In seasons of low Striga-infection or more favorable rainfall, other factors appear more important determinants for maize yield. Implications Genotypes bred for Striga resistance should not be blindly promoted for use in farmers’ fields, as they are not necessarily adapted to conditions that differ from those of their breeding origin. Introgression of resistance genes from these genotypes in locally adapted genotypes will likely result in varieties with improved yield performance. None of the yield determining traits by itself would ensure yields in any given year or site.