A parasitic weed, Striga hermonthica (Del.) Benth., infects millions of hectares of arable land in sub‐Saharan Africa, and it threatens production of cereal crops. The objectives of this study were to investigate inheritance in maize (Zea mays L.) inbred lines of tolerance and resistance to S. hermonthica, based on visible host plant symptoms and Striga emergence counts. A diallel cross involving 10 inbreds was tested under Striga infested fields at Mokwa, Nigeria, for 5 yr. Host plant response was rated on a scale of 1 (no symptoms) to 9 (plants dead or dying). Ratings for the crosses ranged from 3.6 (TZi 11 × TZi 12) to 8.3 (TZi 9 × TZi 10). General combining ability (GCA) mean square was roughly twice specific combining ability (SCA) mean square. Data on S. hermonthica emergence counts were taken from the 45 F1 crosses for 2 yr. Average Striga emergence of the 45 crosses ranged from 7.5 (TZi 9 × TZi 12) to 41.0 (TZi 2 × TZi 10). The SCA mean square was roughly 3.5 times greater than the GCA mean square. The results of this study reveal that genetic control for tolerance and resistance of maize genotypes tested to S. hermonthica is polygenic and that the inheritance is quantitative.

Purple witchweed [Striga hermonthica (Del.) Benth.], here called just striga, parasitizes cereal crops in the savanna zone of sub‐Saharan Africa. The objectives of this study were to investigate the expressions of a tolerant and a susceptible cultivar of maize (Zea mays L.) to striga as affected by timing (0, 2, 4, and 6 wk after maize planting) and rates (60 and 120 kg N ha−1) of N application under striga infestation. The experiment was designed as a split‐split plot with four replications. Timing of N application and N rates significantly affected striga emergence, host‐plant damage scores, agronomic traits, and grain yield. Nitrogen rate x application time interaction was highly significant for striga emergence. Time of N application was more important than N rate in suppressing striga emergence and host‐plant damage. Nitrogen application at 2 wk after planting and 120 kg N ha−1 gave the best result in terms of maize performance and reduction of striga emergence. Host‐plant damage symptoms were more useful in differentiating response of host genotypes to striga than striga emergence values. The tolerant cultivar (hybrid 8322‐13) produced 188% higher grain yield than the susceptible cultivar (hybrid 8338‐1) across all treatments. Grain yield of the tolerant cultivar at 60 kg N ha−1 was 88% higher than that of the susceptible cultivar at 120 kg N ha−1. The tolerant cultivar produced an average 157% more ears at 60 kg N ha−1 and 51% more ears at 120 kg N ha−1 than the susceptible cultivar. Among all the factors studied, the most important component for striga management was genetic tolerance, the ability of a host plant to withstand the parasite.

The parasitic flowering plants, Striga species, represent the largest biological constraint to cereal and legume crop production in sub-Saharan Africa. Eighty-three percent of Striga worldwide (35 species) occurs in Africa. Among them, Striga hermonthica causes the greatest damage. The IITA's scientists began research on breeding maize for horizontal resistance to Striga in 1982. By 1995 a comprehensive approach to combat Striga on maize had been developed and demonstrated. This included the development of a simple field infestation technique, the discovery of durable resistance genes, genetic studies of resistance genes and the formation of many resistant varieties (hybrids and synthetics) with high grain quality, high grain and stover yields and a combined resistance to major biotic and abiotic stresses. Multilocation testing and subsequent seed multiplication of the resistant varieties was carried out by national programmes in Benin, Burkina Faso, Cameroon, the Ivory Coast, Ethiopia, Ghana, Nigeria, and Togo. Striga-resistant maize varieties show horizontal resistance not only to S. hermonthica, but also to another species, Striga asiatica. Based on the results of a 15year research, an integrated approach using resistant varieties and cereal-legume intercropping or rotation is recommended as a sustainable and permanent solution to combat Striga in Africa. This horizontal resistance package, with a combined resistance to other biotic stresses, could be applicable not only to Striga problems in other crops such as sorghum, millet, rice and cowpea, but also to other parasitic weeds, such as Orobanche species. This paper reviews and discusses why, approximately a century's research work on parasitic weeds, has not led to a major research breakthrough.

Maize streak virus (MSV) transmitted by Cicadulina leafhoppers causes severe yield loss of maize (Zea mays L.) in Africa. This paper summarizes our studies on gene action for resistance to MSV using generation mean analysis. Crosses were made between a resistant inbred (IB32) and four susceptible inbreds (BI4, B68, B73, and Moi7). A total of II 255 plants from the four sets of biparentul crosses, their F2, backcross, and F3 progenies were grown in four experiments under severe artifically induced MSV epiphytotics at the International Institute of Tropical Agriculture (IITA), Ibadan, Nigeria. Severity of streak symptoms on test plants was classified on a five‐point scale based on the amount of streaking of leaves. Under severe artificial infestation inside a screenhouse, average ratings of resistant and susceptible parents were 1.3 and 5.0, respectively. The average F1 rating was 3.13, which was close to the mean midparent value (3.15). The average F2 rating was 2.83, which was 0.3 units more resistant than the F1 value. The average rating of four F1 crosses in the field was 2.22, 0.81 units more resistant than the mean midparental value (3.03). Average rating of the F2 populations was 2.22, which was similar to that of the F1 (2.27). Backcross ratings were close to mean of the F1 and recurrent parent. Individual F3 plants derived from same rating of F2 plants showed considerable variation in ratings. Resistance to MSV from IB32 appeared to be inherited quantitatively with relatively small numbers of genes involved. The symptoms of MSV varied with the genetic background of the susceptible parent. Simple recurrent selection or modified backcross breeding methods could be used to breed for MSV resistance in Africa.

A phytoparasite, Striga hermonthica (Del.) Benth., infests millions of hectares of cultivated fields of cereal crops in sub‐Saharan Africa. Yield losses are often 70 but can be as high as 100% and farmers often abandon infested fields and move to new areas. The objectives of this study were to investigate the effects of nitrogen on S. hermonthica infestation and the subsequent grain yield and agronomic traits of tolerant and susceptible maize hybrids (Zea mays L.). Two tolerant and two susceptible hybrids were grown under six levels of N (0–150 kg ha−1), with and without Striga seed infestation. The trials were conducted for 3 yr at Mokwa, Nigeria. Each plant was infested with approximately 3000 germinable Striga seeds, and the density of Striga that emerged, Striga damage on the host plant, plant height, stalk lodging, and grain yield were measured. Striga infestation (emergence and host damage), maize grain yield, and plant height were significantly affected by N rates. Two tolerant hybrids showed significantly lower Striga emergence and host plant damage symptoms than the susceptible hybrids (P< 0.001). Interactions between infested and uninfested plots and hybrid (tolerant vs. susceptible) for grain yields and stalk lodging were significant (P < 0.001). Striga infestation reduced grain yields of two susceptible hybrids by 49%, and of two tolerant hybrids by 24%. The two tolerant hybrids produced on average 87% greater grain yields than the two susceptible hybrids under low N rates (0– 60 kg ha−1) and 51% greater yields under high N (90–150 kg ha−1). Among the plant traits measured, Striga damage score had the highest correlation with grain yield (r = − 0.60**).