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Identifying efficient testers for separating maize (Zea mays L.) inbred lines into heterotic groups can facilitate the development of superior hybrids. Striga-resistant yellow-maize inbred lines de...

Rice yellow mottle virus (RYMV) is the most harmful virus that affects irrigated and lowland rice in Africa. The RBe24 isolate of the virus is the most pathogenic strain in Benin. A total of 79 genotypes including susceptible IR64 (Oryza sativa) and the resistant TOG5681 (O. glaberrima) as checks were screened for their reactions to RBe24 isolate of RYMV and the effects of silicon on the response of host plants to the virus investigated. The experiment was a three-factor factorial consisting of genotypes, inoculation level (inoculated vs. non-inoculated), and silicon dose (0, 5, and 10 g/plant) applied as CaSiO3 with two replications and carried out twice in the screen house. Significant differences were observed among the rice genotypes. Fifteen highly resistant and eight resistant genotypes were identified, and these were mainly O. glaberrima. Silicon application did not affect disease incidence and severity at 21 and 42 days after inoculation (DAI); it, however, significantly increased plant height of inoculated (3.6% for 5 g CaSiO3/plant and 6.3% for 10 g CaSiO3/plant) and non-inoculated (1.9% for 5 g CaSiO3/plant and 4.9% for 10 g CaSiO3/plant) plants at 42 DAI, with a reduction in the number of tillers (12.3% for both 5 and 10 g CaSiO3/plant) and leaves (26.8% for 5 g CaSiO3/plant and 28% for 10 g CaSiO3/plant) under both inoculation treatments. Our results confirm O. glaberrima germplasm as an important source of resistance to RYMV, and critical in developing a comprehensive strategy for the control of RYMV in West Africa.

Striga hermonthica (Del.) Benth parasitism, low soil N, and nutritional deficiencies of normal‐endosperm maize (Zea mays L.) threaten maize yield and exacerbate nutritional problems in sub‐Sahara Africa (SSA). This study was conducted (a) to evaluate genetic variation among extra‐early maturing maize hybrids with provitamin A and quality protein characteristics, (b) to investigate gene action governing the inheritance of Striga resistance, grain yield, low N tolerance, and other measured traits under low‐N, high‐N, and Striga‐infested environments, and (c) to identify hybrids with high yield and stability across environments. One hundred and fifty hybrids developed using North Carolina Design II were evaluated with six checks under low‐N, high‐N, and Striga‐infested environments in Nigeria. Mean squares for hybrids were highly significant (P

The development of maize varieties with increased concentration of Provitamin A (PVA) is an effective and affordable strategy to combat vitamin A deficiency in developing nations. However, the considerably high cost of carotene analysis poses a major challenge for maize PVA biofortification, prompting the use of marker-assisted selection. Presently, two types of genotyping with PVA trait-linked functional markers have been developed and extensively used in breeding programs. The two systems are low throughput gel-based genotyping and genotyping with Kompetitive Allele-Specific PCR (KASP) single nucleotide polymorphism (SNPs) markers. Although the KASP SNPs genotyping was developed to replace the gel-based genotyping, studies have not been conducted to compare the effectiveness of the KASP SNPs markers with the gel-based markers. This study was conducted to assess the carotenoid content of 64 tropical PVA biofortified maize inbred lines containing temperate germplasm in their genetic backgrounds and screen them with both gel-based and KASP markers of PSY1, LCYE and crtRB1 genes. Many of the 64 inbred lines had PVA concentrations surpassing the 15 µg/g provitamin A breeding target set by the HarvestPlus Challenge Program. Favorable alleles of crtRB1, crtRB1 and the KASP SNPs markers were detected in 25 inbred lines with high PVA concentrations. Inbred lines with the favorable alleles of LCYE had the highest concentrations of non-PVA carotenoids, whereas those with the favorable alleles of crtRB1 had high levels of PVA carotenoids. Data from the sequenced region of LCYE revealed one SNP in the first intron that clearly differentiated the high and low β-carotene maize inbred lines. The results of our study demonstrate that the automated KASP SNPs markers can replace the gel-based genotyping for screening a large number of early generation maize inbred lines for PVA content.

A hemiparasitic plant, Striga hermonthica (Del.) Benth and soil nitrogen stress are the key constraints to maize (Zea mays L.) productivity in sub-Saharan Africa, where commonly cultivated maize is the normal endosperm type that is deficient in provitamin A, tryptophan and lysine (PVATL). Seventy-six extra-early maize inbreds with provitamin A, tryptophan, and lysine qualities (TZEEIORQ) were developed to address these constraints, and four checks were assessed under Striga, low and high nitrogen conditions at three locations in Nigeria. The inbreds were further genotyped with two beta-carotene hydroxylase 1 (crtRB1) markers, and their seeds were quantified for provitamin A content. Significant (P &lt, 0.01) genetic variations were observed for grain yield and other agronomic attributes of the inbreds under varying environmental conditions. Levels of PVATL for the inbreds ranged from 2.21–10.95 µg g−1, 0.04–0.08%, and 0.19–0.39%, respectively. Beta-carotene marker, crtRB1-3′TE, was polymorphic and grouped the inbreds into two. The marker was effective in identifying inbreds with moderate provitamin A content. Inbreds TZEEIORQ 5, TZEEIORQ 52, and TZEEIORQ 55 exhibited resistance to Striga, tolerance to nitrogen stress with moderate levels of PVATL and could be invaluable sources of favorable alleles for breeding nutritionally improved maize varieties with resistance/tolerance to Striga and soil nitrogen stress.

This study determined the gene effects involved in the inheritance of pod length and other yield-related traits and relationships among traits in crosses between two cowpea lines (TVu 2280 and TVu 2027) and a yard-long bean (TVu 6642) line with long pods. Plants of six generations (P1, P2, F1, F2, BC1P1, and BC1P2) derived from TVu 2280 × TVu 6642 and TVu 2027 × TVu 6642 were evaluated under field conditions. Data collected on 14 yield components of each cross were used for generation mean analysis. Gene effects and their magnitudes varied with the crosses, digenic epistatic gene effects were detected for 10 traits in TVu 2280 × TVu 6642 and 11 traits in TVu 2280 × TVu 6642. Only additive gene effect was significant for pod length in TVu 2280 × TVu 6642 while additive, dominant, and two of the three digenic epistatic gene effects were significant in TVu 2027 × TVu 6642. Models that incorporated only significant additive, dominant, and digenic epistasis were adequate for all 14 traits in TVu 2280 × TVu 6642 and eight of the 12 traits in TVu 2027 × TVu 6642 for which model-fitting was possible. Across segregating generation of the two crosses, pod length (PodLNT) was significantly (p &lt, 0.001) correlated with three major yield components viz. pod weight (0.84, 0.77), number of seeds per pod (0.41, 0.30) and seed weight per pod (0.61, 0.29). Significant correlation of PodLNT with seed yield per plant was moderate and significant (p &lt, 0.01–0.001) in the BC1P1 of the two crosses (0.31 and 0.41). An improvement in cowpea seed yield is feasible through selection for long pods in segregating generations involving crosses with yard-long bean.

The choice of an appropriate tester is important for success in resistance hybrid breeding programs. Limited information is available on the most suitable testers that allow the selection of yellow endosperm maize inbred lines with good combining ability for resistance to witchweed (Striga hermonthica) and superior agronomic performance in hybrids targeted to areas infested with the parasite. Testcrosses of 30 Striga-resistant yellow endosperm maize inbred lines with three testers having varying levels of resistance to S. hermonthica were evaluated at Abuja and Mokwa in Nigeria under Striga-infested and non-infested conditions in 2018 and 2019. The lines &times, tester interaction was significant for grain yield under Striga infestation, indicating that the performance lines in testcrosses varied with the testers. The respective average general combining ability effect of lines and testers was 1.5 and 32.4 times greater than the corresponding specific combining ability effects under infestation, showing the greater importance of additive gene action controlling the performance of testcrosses under Striga infestation. The different testers exhibited a different capacity to discriminate among the test lines with the susceptible tester being the least effective. Most of the criteria used to determine the best tester favored the selection of the tolerant and resistant testers as promising candidates to identify superior Striga-resistant yellow endosperm inbred lines for the hybridization and development of resistant hybrids. The results of this study demonstrated that testers with a high frequency of desirable alleles were superior to the tester with a low frequency of favorable alleles in hybrid breeding programs for resistance to S. hermonthica.

Molecular-based characterization of open-pollinated varieties (OPVs) in maize is useful to define their unique profiles. A total of 58 SSR markers selected from a panel of 70 were used for genotyping three samples of 30, 50 and 100 plant bulks for each of 32 OPVs. The SSR markers detected a total of 253 alleles in the 32 maize OPVs across the three bulk samples. The number of alleles per marker varied from 2 to 7, with an overall mean of 4.36. The genetic distance among the OPVs varied from 0.17 to 0.93 with an average of 0.70 ± 0.007 for bulk samples of 30 plants, 0.20 to 0.91 with an overall mean of 0.70 ± 0.007 for bulk samples of 50 plants and from 0.30 to 0.95 with an average of 0.75 ± 0.006 for bulk samples of 100 plants. Cluster analysis separated the 32 maize OPVs into two major groups, which were further separated into two sub-groups for each type of bulk sample. The groupings of the OPVs into two major groups and their corresponding sub-groups was consistent with known breeding history (common parentage) and common target traits during development of the OPVs irrespective of the sample size used. However, in the bulk samples of 50 and 100 plants, some sub-groups were composed of OPVs with mixed maturity classes and diverse genetic backgrounds. Of the three independent bulk samples, the smallest sample size of 30 plants was found to be optimal for characterizing heterogeneous and heterozygous maize populations and OPVs owing to its cost-effectiveness and relative ease of sample processing.

Yam (Dioscorea spp.) is a monocotyledonous herbaceous vine plant grown in the tropics and subtropics. It is a multi-species plant with varied intra- and interspecific ploidy levels. Of the 600 species, 11 are cultivated supporting the livelihood of over 300 million people. The paucity of information on ploidy and the genomic constitution is a significant challenge to the crop’s genetic improvement through crossbreeding. The objective of this study was to investigate the ploidy levels of 236 accessions across six cultivated and two wild species using chromosome counting, flow cytometry and genotyping-based ploidy determination methods. Results obtained from chromosome counting and genotyping-based ploidy determination were in agreement. In majority of the accessions, chromosome counting and flow cytometry were congruent, allowing future rapid screening of ploidy levels using flow cytometry. Among cultivated accessions, 168 (71%) were diploid, 50 (21%) were triploid, and 12 (5%) were tetraploid. Two wild species included in the study were diploids. Resolution of ploidy level in yams offers opportunities for implementing successful breeding programmes through intra- and interspecific hybridization.

Maize streak virus (MSV) is responsible for streak disease of maize and poses a serious threat to maize production in sub-Saharan Africa. Polygenic resistance to MSV has become an essential requirement in modern maize cultivars to mitigate yield losses. Many single nucleotide polymorphism (SNP) markers linked to putative MSV resistance loci have been identified for use in forward breeding. This study aimed to validate, using the high-throughput kompetitive allele specific PCR (KASP) assay, the diagnostic ability of the three SNP markers linked to the loci for the Msv1 resistance trait in 151 early generations inbred lines with diverse genetic backgrounds, together with nine MSV-resistant elite lines and a susceptible check (cv. Pool-16). The phenotypic responses were determined by MSV inoculation using viruliferous leafhoppers (Cicadulina triangular) under screenhouse conditions. Based on an established MSV disease rating system, the maize lines were categorized into resistant, moderately resistant, susceptible, and highly susceptible. The three SNPs associated with MSV resistance were detected in 133 lines, which were categorized as resistant (54), moderately resistant (76), and susceptible (1). The 18 early generation lines without these SNPs were classified as moderately resistant (10), susceptible (5), and highly susceptible (3). This study confirms the strong association of SNPs with MSV resistance and their usefulness for forward breeding in maize while emphasizing the need to identify additional markers to screen lines for MSV resistance without any ambiguity.

SUMMARYMaize (Zea maysL.), a major staple food crop in West and Central Africa (WCA), is adapted to all agro-ecologies in the sub-region. Its production in the sub-region is greatly constrained by infestation ofStriga hermonthica(Del.) Benth. The performance and stability of the extra-early maturing hybrids, which are particularly adapted to areas with short growing seasons, were assessed underStriga-infested andStriga-free conditions. A total of 120 extra-early hybrids and an open-pollinated variety (OPV) 2008 Syn EE-Y DT STR used as a control were evaluated at two locations each underStriga-infested (Mokwa and Abuja) andStriga-free (Ikenne and Mokwa) conditions in 2010/11. TheStriga-resistant hybrids were characterized by higher grain yield, shorter anthesis–silking interval (ASI), better ear aspect, higher numbers of ears per plant (EPP), lowerStrigadamage rating, and lower number of emergedStrigaplants at 8 and 10 weeks after planting (WAP) compared with the susceptible inbreds. UnderStrigainfestation, mean grain yield ranged from 0·71 to 3·18 t/ha and 1·19 to 3·94 t/ha underStriga-free conditions. The highest yielding hybrid, TZEEI 83×TZEEI 79, out-yielded the OPV control by 157% underStrigainfestation. The hybrids TZEEI 83×TZEEI 79 and TZEEI 67×TZEEI 63 were the highest yielding under bothStriga-infested andStriga-free conditions. The genotype main effect plus genotype×environment interaction (GGE) biplot analysis identified TZEEI 88×TZEEI 79 and TZEEI 81×TZEEI 95 as the ideal hybrids across research environments. Twenty-three pairs of simple sequence repeat (SSR) markers were used to assess the genetic diversity among the inbred lines. The correlations between the SSR-based genetic distance (GD) estimates of parental lines and the means observed in F1hybrid underStrigainfestation and optimum growing conditions were not significant for grain yield and other traits except ASI under optimum conditions. Grain yield of inbreds was not significantly correlated with that of F1hybrids. However, a significant correlation existed between F1hybrid grain yield and heterosis underStrigainfestation (r=0·72,PStrigaendemic areas in WCA.

Striga hermonthica causes signifi cant yield loss in maize (Zea mays L.) and other cereals. Limited studies have been conducted to determine the relationship between marker-based genetic distance among maize inbred lines expressing varying levels of fi eld resistance to S. hermonthica and the reaction patterns of the resulting crosses to S. hermonthica. Forty-fi ve diallel crosses of 10 parental lines were evaluated with and without S. hermonthica infestation at two locations each in Benin Republic and Nigeria for 3 yr. Canonical discriminant analysis using nine traits separated the crosses into three distinct groups in each country depending on doses of resistance obtained from their parental lines. Hybrids formed from two resistant parents had the highest levels of resistance, while those formed from two susceptible parents exhibited the lowest levels of resistance to S. hermonthica. Crosses that involved one resistant line as a parent showed intermediate levels of fi eld resistance to S. hermonthica. Assessment of genetic divergence among the 10 parental lines using 18 amplifi ed fragment length polymorphism (AFLP) primer combinations generated a total of 1176 polymorphic AFLP fragments. The genetic distance (GD) estimates among all pairs of parental lines varied from 0.44 to 0.93, with an average of 0.63 ±0.023. The correlation between the AFLPbased GD estimates of parental lines and the means observed in diallel crosses under S. hermonthica infestation were not signifi cant for grain yield and other traits. Some hybrids formed from inbred lines with GD estimates both below and above 0.50 exhibited good performance under S. hermonthica infestation. The observed broad range of genetic divergence detected with AFLP markers indicates the presence of a signifi cant reservoir of diversity among resistant lines that can be exploited in breeding.

Maize (Zea mays) is a staple food crop in sub-Saharan Africa, but its production is threatened by Striga hermonthica (Del.) Benth. Transfer of resistance genes from wild relatives may increase resistance to S. hermonthica in tropical maize. The objective of this study was to determine the combining ability of resistance to S. hermonthica among lines containing Zea diploperennis and tropical germplasm. Forty-five diallel crosses of 10 inbred lines were evaluated in an alpha-lattice design with and without artificial Striga infestation at two locations each in the Republic of Benin and Nigeria for 3 years. Results of analyses showed that only general combining ability (GCA) mean square was significant (P = 0.01) for number of emerged Striga plants (NESPP), while both GCA and specific combining ability (SCA) mean squares were significant for host damage score (HDS) and grain yield under Striga infestation. The ratio of GCA to SCA mean squares for the three traits varied from 3.5 to 57.5. Although GCA x environment interaction was significant for the three traits, two inbred lines containing Z. diploperennis (ZD 551) and tropical (TZL TC 87) germplasm had negative and significant GCA effects for NESPP and HDS and positive GCA effects for grain yield under Striga infestation in the two countries. Correlation between NESPP and HDS was strong and significant (r = 0.87, P = 0.01). Our results highlight the importance of harnessing useful genes from wild relatives to improve resistance to S. hermonthica in adapted maize germplasm.

Low field emergence and early field vigour are major problems in shrunken-2 maize (sh-2). The genetic variability for stand-ability and early field vigour in a shrunken-2 maize population previously improved for adaptability to a tropical environment was investigated using a Design I mating system in which 40 randomly sampled males were crossed to four randomly sampled plants that served as females. Plants of the 160 crosses, representing full- and half-sib relationships, were evaluated in four sets, each composed of 40 crosses derived from 10 males during two growing seasons. Experimental design was randomized complete block with three replications. Difference in Emergence Index (EI) was not significant between the two seasons. Emergence Percentage (E%), Emergence Rate Index (ERI), and early field vigour traits viz. vigour score and seedling height determined at 28 days after planting (DAP) were, however, significantly (P

Seventeen supersweet Shrunken-2 (sh-2) maize lines and progenies of their crosses to two sh-2 hybrid testers were investigated for factors responsible for differences in field emergence in Korea in studies carried out in the field, greenhouse and laboratory. The lines and their testcrosses showed significant genetic variation for emergence. Two lines showed significant positive general combining ability (GCA) for emergence while two other lines had significant negative GCA effects. Because none of the specific combining ability effects was significant, emergence was primarily influenced by additive gene action, and could, therefore, be improved by recurrent selection. Seed weight was significantly (Pr=0·63) and testcross progeny involving the tester with the lower mean seed weight (r=0·54). Two lines also showed significant positive GCA effects for seed weight. The lines with positive GCA effects for emergence were different from those with positive GCA for seed weight, indicating that factors other than seed weight influence field emergence. The severity of Fusarium infection of seeds of the lines was significantly correlated to germination (r=−0·61, Pr=−0·67, Psh-2 inbred parents did not give an indication of emergence in hybrid combinations.

The incidence of the parasitic weeds, Striga spp. in the savanna and the mid-altitude ecology of Africa is on the increase. Eight inbred lines of maize developed for the mid-altitude ecology and a susceptible check (TZi 10) were evaluated in Mokwa, Nigeria for two seasons to determine their performance under Striga hermonthica, the most widespread species of Striga in Africa. The experiment was a split-plot with three replications; infestation treatments (Striga-infested and non-infested) and maize inbreds constituted the main and sub-plots, respectively. There were no significant differences among the inbred lines for number of underground attachments of Striga seedlings to maize roots. However, differences in number of emerged Striga plants were significant (P < 0.01). Damage scores at 10 WAP and reductions in height and biomass at 12 WAP varied significantly (P < 0.01) among the maize lines tested. Damage score was significantly related to the severity of height and biomass reduction by Striga (r = 0.70 − 0.73). Differences in response of the lines to Striga manifested fully after silking. Thus, resistance was expressed at adult plant stage. Damage scores and number of emerged Striga plants were lower (P < 0.05) for TZMi 102, TZMi 302 and TZMi 306 than the susceptible check. These lines demonstrated potential as sources of resistance to Striga. Genetic variation for resistance to S. hermonthica is therefore present among the mid-altitude inbreds tested.

A split-plot experiment was conducted in 1992 and 1993 at Mokwa in the southern Guinea savanna zone of Nigeria to determine the responses of maize yield components to infestation by parasitic weed Striga hermonthica (Del.) Benth. and identify the most important yield components under Striga parasitism. Infestation treatments (infested v. non-infested) and hybrids [9022-13 (highly tolerant), 8644-31 (moderately tolerant), 8321-21 (moderately susceptible) and 8338-1 (highly susceptible)] were the main and subplots, respectively.Striga significantly reduced grain yield and all the yield components considered. Pre-flowering stress due to Striga parasitism was higher than post-flowering stress and resulted in higher reduction for ears per plant (44%) than reductions for other yield components (12–29%). The numbers of yield components for which the hybrids exhibited the highest percentage reduction increased with the degree of susceptibility to Striga. Thus, 8338-1, the highly susceptible hybrid, suffered more severe reductions in a larger number of yield components than the other hybrids. Number of ears per plant was a major yield component and accounted for an overwhelming proportion (R2 = 0·95) of the variation in grain yield under Striga. Reductions in maize grain yield and its components were more severe under the moisture stress that occurred in 1992.

Twelve maize genotypes with different degrees of resistance were evaluated in plots inoculated with seeds of the phytoparasite Striga hermonthica (Del.) Benth. and in Striga-free (control) plots for three seasons between 1991 and 1993. Resistant genotypes, although showing similar levels of underground infection as susceptible 9 weeks after maize planting (WAP), had significantly fewer emerged parasitic plants and sustained lower damage. Host damage was not determined by the severity of infection. Percent height and dry matter reductions increased from 3 to 6 WAP for resistant and susceptible genotypes. While percent height and dry matter reductions for resistant genotypes declined at 9 and 12 WAP, susceptible genotypes either maintained or increased their levels of damage. Correlation between the severity of height and dry matter reductions at the vegetative (3 and 6 WAP) and reproductive (9 and 12 WAP) stages of maize growth were not significant, indicating that damage at early stages of growth cannot be used to predict mature plant response. Maize damage score (1–9) at 8 WAP, a non destructive and composite shoot indicator of host performance under Striga infestation, was significantly correlated (r = 0.88, r = 0.82; p < 0.01) to the level of shoot reduction at mature plant stages. Emerged Striga count at 8 WAP was significantly correlated (r = 0.98, p < 0.01) to the count at 10 WAP, the time when parasite emergence was highest. Under high and uniform infestation, mature plant resistance can be detected at 8 WAP, the onset of flowering in maize.

A parasitic plant, Striga hermonthica (Del.) Berth.,infests millions of hectares of arable land in sub-Saharan Africa, and it threatens production of major cereal crops including maize (Zea mays L). Erratic natural infestation is considered a major constraint to assess host plant resistance to the parasite. The objective of this study was to evaluate the effects of different S. hermonthica seed inoculum rates on a tolerant and a susceptible maize genotype. Individual maize plants of the tolerant (8322-13) and the susceptible maize hybrid (8338-1) were inoculated with six rates of germinable Striga seeds: 750, 1500, 2250, 3000, 3750, and 4500. Uninfested plots served as the control. The experiment was conducted at two locations, Mokwa (two seasons) and Abuja (one season), in Nigeria. Increasing Striga inoculum level increased Striga emergence and the amount of damage to the host plant, plant height, and grain yield of the host crop. The tolerant hybrid showed less host plant damage (P < 0.01) and that fewer S. hermonthica plants emerged (P < 0.05) than the susceptible hybrid. Among the four traits studied, host plant damage score was most correlated with grain yield (r = -0.65**). Average grain yield of the tolerant hybrid, 8322-13, was 97% higher than that of the susceptible hybrid, 8338-1, across the six inoculum rates. The tolerant hybrid produced similar grain yields under all levels of the inoculum; the yield reductions ranged from 27% (2250 seeds) to 35% (4500 seeds), while the yield reduction of the susceptible hybrid ranged from 43% (750 seeds) to 74% (3750 seeds). Based on the results of this study, we conclude that host plant tolerance is the key element for stabilizing crop yield under S. hermonthica infestation.

A phytoparasite, Striga hennonthica (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. hennonthica 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**).

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.

Seeds of the obligate parasite Striga hermonthica , germinate in response to stimulants in the root exudate of maize and other host crops. Under laboratory conditions, conditioning (defined as treatment with moisture) of Striga seeds for one to three weeks before exposure to germination stimulant is required for optimum germination. The objective of this study was to determine if maize performance and Striga emergence, both indices of resistance, were influenced by the interval between Striga seed infestation and maize planting, under high parasite inoculum (3000–6000 germinable seeds) as practiced in maize resistance breeding field nurseries. Four conditioning treatments were investigated: maize planted immediately after Striga seed infestation, and maize planted after one, two and three weeks after infestation. Experimental design was randomized complete block with four replications. Differences among the four conditioning treatments were not significant for maize damage score, height, stalk lodging, number of ears harvested, grain yield and number of emerged Striga plants. Thus, maize planted on the same day as Striga infestation showed similar response as maize planted one to three weeks after infestation. Using high seed inoculum, maize can be planted immediately after Striga infestation in maize breeding nurseries. Combining Striga seed infestation and maize planting into a single field operation is convenient and reduces cost of screening.