Striga Emergence Research Articles

Increasing imazapyr-resistant maize yield by increasing plant density under natural Striga hermonthica infestation

Maize (Zea mays L.) is one of the most important crops in East Africa serving both as a staple food and cash crop to millions of people. As a vegetable, it is produced as green maize cobs, sweet corn or baby corn. Yields under farmers’ conditions in the Lake Victoria Basin average 1.3 t ha -1 which is less than 25% of the potential yield. This is attributed to several factors; the greatest being Striga (Striga hermonthica) which is a parasitic weed attacking several crops in the grass family. A medium term technological breakthrough in form of a herbicide (imazapyr) resistant maize variety has been developed to help reduce Striga infestation. The optimum maize plant population for effective seedbank reduction and optimum grain yield has however not been determined. The main objective of this study was therefore to evaluate the performance of imazapyr-resistant (IR) maize at different plant densities on Striga weed incidence and crop growth. The study was conducted for two seasons on a naturally infested Striga field in Western Kenya. The experiment was laid out as a split plot with three replications. Maize variety (treated IR maize, untreated IR maize and WH505/H516 commercial checks) was the main plot factor and density (44,444, 66,666 and 88,888 plants ha -1 ) as the sub-plot factor. Data was collected on maize stand, days to first Striga emergence, Striga incidence, Striga biomass, maize biomass and its yield components. Treated IR maize delayed Striga emergence on maize thus suppressing Striga parasitism. Any late Striga attachments had little or no effect on maize growth. Increasing the plant density of treated IR maize from 44,444 to 88,888 plants ha -1 did not affect Striga incidence but increased maize yield from 1.60 to 3.48 t ha -1 . It is therefore recommended that treated IR maize should be planted in Striga infested fields at 75cm x 15cm to hasten Striga seed bank reduction and maximize on maize yield to improve staple food availability.

Effects of Recurrent Selection for Grain Yield and Striga Resistance in an Extra‐Early Maize Population

ABSTRACTStriga hermonthica (Del.) Benth threatens maize (Zea mays L.) production in sub‐Saharan Africa. In an extra‐early population, S1 families derived from four cycles of recurrent selection for grain yield and Striga resistance were evaluated with and without Striga infestation at three locations in Nigeria for 2 yr to determine relative changes in genetic variances, heritabilities, and genetic correlations for yield and other traits. Under infestation, yield was not correlated with other traits at C0 but was significantly correlated with ears per plant (EPP), Striga damage, and emerged Striga plants in advanced cycles. Genetic correlations between yield and most traits were significant in C0 when Striga‐free but was reduced in advanced cycles. Genetic variances and heritabilities for yield and ear aspect increased in advanced cycles. Genetic variances decreased with selection for emerged Striga counts and EPP under infestation. A response to selection for improved yield, Striga emergence and EPP is expected in subsequent cycles. Low genetic variances and heritabilities for Striga damage and low predicted gain cycle−1 for all traits except numbers of emerged Striga plants suggest a need to introgress novel resistance genes into the population for faster progress from selection for increased yield.

Registration of Striga‐Resistant Tropical Extra‐Early Maize Population

An extra‐early maturing white‐grained, flint/dent maize (Zea mays L.) population, TZEE‐W Pop STR C4 (Reg. No. GP‐576; PI 657704), has been developed at the International Institute of Tropical Agriculture. The population has moderate levels of resistance to Striga hermonthica (Del.) Benth. It was released in 2004 in west and central Africa as source germplasm for the development of synthetic varieties, parental inbred lines, and hybrids with improved resistance to Striga and superior grain yield. It is also drought tolerant with good levels of resistance to maize streak virus and southern corn leaf blight [caused by Bipolaris maydis (Nisikado & Miyake) Shoemaker]. Results of trials at three locations in Nigeria and Benin Republic, 2006 and 2007, showed that under Striga‐infested conditions, the cultivars 2004 TZEE‐W Pop STR C4, derived from C4 of TZEE‐W Pop STR, and TZEE‐W Pop × LD (Set A2), were the most promising in terms of grain yield, low levels of Striga damage and Striga emergence. TZEE‐W Pop STR C4 outyielded the Striga‐susceptible check by 43% under Striga infestation and 33% when Striga‐free. In another trial with 16 normal and quality protein maize (QPM) varieties evaluated at 10 Striga‐free locations, TZEE‐W Pop STR C4 outyielded the normal local check by 24% and the QPM reference check by 18%.

Resistance screening of sorghum and effects of varieties and herbicide application on reproduction of Striga hermonthica

The obligate parasitic witchweed, [Striga hermonthica (Del.) Benth] is the greatest biological constraint to cereal production in sub-Saharan Africa. Sorghum [Sorghum bicolor (L.) Moench], a widely grown cereal in the region is an important host crop that has coevolved with the parasite. Several methods to control striga have been proposed, but with limited success. Genetic control of striga is effective, although sources of resistance are limited in most crops. Studies were initiated on striga–infested fields in view to identify sources of resistance to Striga hermonthica and to investigate the response of selected sorghum cultivars and some herbicides on the emergence of striga. In one experiment, eleven sorghum cultivars were screened for resistance to Striga hermonthica. Cultivars of sorghum differed significantly with respect to number of emerged striga plants at 60 days after planting and at harvesting. The varieties S 35 and CS 54 constantly recorded low number of striga plants than the popular resistant variety Framida. In the herbicide application trials, the numbers of emerged striga per square meter were much reduced following spraying with 2,4-D. Although the urea treatmenthad high grain yield, the number of emerged striga plant per unit area was also very high. Application of the appropriate herbicide at the right stage of striga emergence and growth, combined with the use of resistant/tolerant varieties may inhibit striga development. The results reported in this study offer promise in simultaneously sustaining high grain yields and suppressing the development of striga in farmers’ fields.Key words: Herbicides, Sorghum bicolor, host resistance, Striga hermonthica, sustainable production

Fusarium oxysporum and post-emergence herbicide for the control of the parasitic plant Striga hermonthica in maize

Two trials were conducted to evaluate the efficacy of a granular mycoherbicide formulation based on Fusarium oxysporum and post-emergence herbicide for the control of the parasitic plant, Striga hermonthica in the Nigerian Savanna. Four fungal treatments were used: F. oxysporum followed by 2,4-D, F. oxysporum followed by supplementary hoe weeding, F. oxysporum followed by Triclopyr and a control (No F. oxysporum but hoe-weeded). The experiments were laid out in a split plot design with three replications in the two locations. The two varieties (Across 97 TZL and farmer's local variety) formed the main plot treatments, while the Striga fungal treatments formed the sub-plot treatments. At the Lafia location, the emergence of Striga was delayed by 7 days (46 days) as compared to the Makurdi location, which germinated earlier 39 days after sowing. Maize variety Across 97 TZL similarly delayed the time to Striga emergence when compared to the farmer's local variety. However, the different Striga control methods did not have any significant effect on the time of Striga emergence. Generally, number of maize plants infected with Striga was highest with the farmer's local variety throughout the period of observation, while in the Striga control treatments, hoe-weeded check recorded the highest; the minimum was obtained with plots treated with F. oxysporum followed by postemergence application of Triclopyr at the rate of 0.36 kg a.i./ha at 6 weeks after sowing. Highest maize grain yields were obtained at Lafia with Across 97 TZL and plots treated with F. oxysporum followed by either post-emergence 2,4-D or Triclopyr at 0.36 kg a.i./ha each. The results demonstrate the high potentiality of using F. oxysporum as a spot application at planting followed by post-emergence herbicide (2,4-D or Triclopyr) application at 6 weeks after sowing for the control of the parasitic plant S. hermonthica in the Nigerian Savanna.

Efficacy of Herbicide Seed Treatments for Controlling Striga Infestation of Sorghum

Witchweed (Striga spp.) infestations are the greatest obstacle to sorghum [Sorghum bicolor (L.) Moench] grain production in many areas in Africa. The objective of this study was to evaluate the efficacy of herbicide seed treatments for controlling Striga infestation of sorghum. Seeds of an acetolactate synthase (ALS) herbicide–tolerant sorghum hybrid were treated with two ALS‐inhibiting herbicides. Treatments included three rates of imazapyr (IMI), three rates of metsulfuron‐methyl (MET), and an untreated control group. In greenhouse trials, observations at 32, 46, and 60 d after planting showed that seeds treated with the highest herbicide rates had the fewest Striga attachments and the greatest delay in attachment. All plants in the untreated group died at or before sorghum flowering; however, herbicide seed treatments, particularly metsulfuron, reduced Striga emergence and significantly increased sorghum grain yield and dry matter production. Field studies comparing seed treatments produced similar results with delayed Striga emergence and fewer emerged Striga plants in herbicide‐treated plots. These studies indicate that herbicide seed treatments may provide a highly effective tool for managing Striga in sorghum.

Integrating Fusarium oxysporum f. sp. strigae into cereal cropping systems in Africa

Striga hermonthica (Del.) Benth. (witchweed) poses the greatest biological constraint to food production in sub-Saharan Africa (SSA). Control options for Striga are currently largely ineffective or unavailable to farmers, and other management possibilities are urgently needed. Biological control obviates some of the problems of several of the other techniques and provides a management option that is durable and environmentally responsive. The efficacy of S. hermonthica control using different formulations of three isolates of Fusarium oxysporum Schlecht. emend. Synder & Hans f. sp. strigae was tested on Striga-resistant and Striga-susceptible varieties of sorghum and maize under African field conditions for the first time. Isolates PSM197 and Foxy 2 were effective in witchweed repression, especially when applied as pesta granules. Isolate M12-4A was less effective under the field conditions investigated. Application of the fungi was generally more beneficial in maize than in sorghum for the varieties tested. Application of the biocontrol agent caused significant decreases in the number of flowering Striga plants, and hence deposition of seeds with impact of enhancing future crop yield. Synergistic effects between the Striga-resistant maize line and Fusarium oxysporum f. sp strigae led to over 90% reduction in Striga emergence. These results will further encourage the distribution of the isolates tested or selection of country-specific relatives as viable and environmentally safe biocontrol agents to be used against Striga. Pesta was the most effective formulation, while seed coating may be more cost effective.

Improvement of Yield and other Traits of Extra‐Early Maize under Stress and Nonstress Environments

Striga, drought, and low soil fertility constrain maize (Zea mays L.) productivity in West and Central Africa (WCA). Two Striga‐resistant extra‐early populations, TZEE‐W Pop STR (white) and TZEE‐Y Pop STR (yellow) were improved by four cycles of S1 recurrent selection, and the products were evaluated in field trials in Nigeria from 2005 to 2007. The objective was to assess progress in improvements for grain yield, Striga resistance, and other traits when Striga‐free, Striga‐infested, and under low‐ and high‐N management. Under Striga infestation, yield gain from C0 to C4 was 136 kg ha−1 for the yellow population and 71 kg ha−1 cycle−1 for the white. The yield gain in advanced cycles of selection in the yellow population was associated with decreases in Striga emergence at 8 and 10 wk after planting (WAP) and Striga damage at 8 WAP, while that of the white was accompanied by increased days to silking (DS) and ears per plant (EPP) as well as a decrease in Striga emergence at 8 WAP. Under Striga‐free conditions, yield gain from C0 to C4 was 164 kg ha−1 cycle−1 for the white and 221 kg ha−1 cycle−1 for the yellow. The yield gain in advanced cycles of selection was more pronounced under high‐N (157 kg ha−1 cycle−1) than low‐N conditions (144 kg ha−1 cycle−1) in the yellow, and higher under low N than high‐N conditions in the white. Selection in the yellow was accompanied by increased days to anthesis (DA) and DS, plant and ear heights under low N and increased DS under high N. In the white, selection induced increased DA, ear height, and decreased anthesis–silking interval (ASI).

Imazapyr (herbicide) seed dressing increases yield, suppresses Striga asiatica and has seed depletion role in maize (Zea mays L.) in Malawi.

The parasitic weed species, Striga asiatica (L.) Kuntze, also known as witchweed, is one of the major constraints in maize production in Malawi. Most of the control measures do not protect a current crop from damage. In 1998/99 season, a trial was initiated at Chitedze Research Station under artificial infection, to evaluate the effects of seed dressing with imazapyr (an acetolactate synthase {ALS} inhibiting herbicide) using three seed treatment methods (coating, priming or drenching) and three herbicide rates (15, 30 and 45 g active ingredient ha-1) on S. asiatica suppression, maize growth and yield. The maize hybrid IntA/IntB//Pioneer325irMZ98F2, bearing target site resistance to imazapyr (IR maize), was used as test crop. In the subsequent season, normal or non-IR maize was planted on the same plots of 1998/99, to assess the residual or spill-over effects on Striga emergence, maize growth and yield. In the first season, results showed that imazapyr seed dressing suppressed (P emergence to 6.7 plants m-2 compared to 14.7 plants m-2 in untreated control. At 106 DAP, the number of Striga that flowered in untreated plots was 6.2 plants m-2, compared to significant (P > 0.05) yield differences. In the subsequent season, imazapyr treatments gave no residual or spill-over effects on maize growth and yield (P > 0.05. There were significant (P Striga emergence similar to the first season. The results therefore suggest that the use of ALS inhibiting herbicides not only suppresses Striga emergence, but also has a seed depletion role in integrated management of Striga without any spill-over or, herbicide injury in subsequent unprotected maize. This technology would be simple for farmers to adopt.

A study on the susceptibility of rice cultivars to Striga hermonthica and mapping of Striga tolerance quantitative trait loci in rice

Striga is a parasitic weed attacking mainly maize, sorghum, millet and cowpea. Studying the interaction between rice and Striga is valuable since rice is a model monocot. In this paper, the susceptibility of different rice cultivars to S. hermonthica was tested and quantitative trait loci (QTL) for Striga tolerance mapped on the Bala x Azucena F(6) population. Seven rice cultivars were grown with and without S. hermonthica for 14 wk. For the mapping experiment, 115 recombinant inbred lines (RILs), along with Azucena and Bala, were grown with and without Striga for 11 wk. Rice cultivars tested had different susceptibilities to Striga, ranging from highly susceptible to completely resistant. Azucena and Bala differed in the speed of Striga emergence and the impact on host growth. A genomic region between positions 139 and 166 cM on chromosome 1 was identified containing strong QTL (LOD = 4.9-15.7) for all traits measured. This indicates that genes for Striga tolerance exist in rice germplasm and the mapped QTL can be further studied to promote understanding of the nature of resistance/tolerance and breeding for Striga-resistant crop plants.

S1 Family Selection in Early‐Maturing Maize Populations in Striga‐Infested and Striga‐Free Environments

Striga and drought are major constraints to maize (Zea mays L.) production in the savannas of west and central Africa (WCA). A study was conducted in 2005 and 2006 in Nigeria to determine the level of resistance to Striga hermonthica for two early‐maturing maize populations, one white and one yellow, and to assess the progress made in improving each for grain yield and level of resistance after four cycles of S1 recurrent selection. Percentage yield gain per cycle of the yellow population when Striga infested was 13% with r2 = 82% and 5% with r2 = 97% in a Striga‐free environment. Corresponding values for the white population were 15% with r2 = 40% and 1% with r2 = 9% for the two Striga environments. For the two populations, therefore, realized gain in grain yield was higher under Striga‐infested than Striga‐free conditions. Yield gains in the advanced cycles of selection were accompanied by increased number of ears per plant in both populations. Striga damage at 10 wk after planting (WAP) and Striga emergence at 8 WAP decreased by about 5 and 4% cycle−1, respectively in the yellow population. For the white population, Striga damage decreased also by about 5 and 4% per cycle−1 at 8 and 10 WAP, respectively, while Striga emergence decreased by about 5% cycle−1 at 8 WAP. In conclusion, S1 recurrent selection was effective in improving grain yield and Striga resistance in the two early maize populations.

Granular formulations and seed coating: delivery options for two fungal biological control agents of Striga hermonthica

A two-seasons outdoor pot experiment was conducted in Sudan to study the efficacy of two Fusarium isolates on controlling Striga hermonthica. “Pesta” and alginate formulations with different amendments were deployed, in addition to seed coating treatments as an alternative delivery option. Alginate granules generally gave higher numbers of colony forming units (cfu) per g of formulated material compared to “Pesta”. For the diverse fungal isolates, different formulations proved to be most suitable. The best Striga control efficacy was obtained by Fusarium Abuharaz (FA) formulated in “Pesta” which almost completely inhibited Striga emergence in both seasons resulting in a highly significant increase of sorghum plant height (by 80 and 59%) and sorghum shoot dry weight (391 and 240%), respectively. Fusarium species in alginate pellets also significantly delayed Striga emergence, reduced the total number of Striga throughout the growing seasons, and generally also significantly increased sorghum plant height and sorghum shoot dry weight compared to the control. Sorghum seed coating with the bioagents also showed a potential to control Striga although its efficacy was generally lower than that obtained by granular formulations.

Integrated management of Striga hermonthica, stemborers, and declining soil fertility in western Kenya

Striga hermonthica (Delile) Benth., stemborers, and declining soil fertility are serious threats to sustainable food production in the Lake Victoria zone of Kenya. To address these constraints, promising integrated crop management technologies were evaluated, using a multi-locational design in four sub-locations in Siaya and Vihiga district (western Kenya) for six cropping seasons. Technologies evaluated consisted of the traditional maize ( Zea mays L.) – bean ( Phaseolus vulgaris L.) intercrop, maize – Desmodium (Desmodium uncinatum (Jacq.) DC.) push–pull intercrop, Crotalaria ( Crotalaria ochroleuca G. Don) – maize rotation, and soybean ( Glycine max (L.) Merr) – maize rotation. Within each of these systems, imazapyr-coated herbicide-resistant maize (IR-maize) and fertilizer were super-imposed as sub-plot factors. The push–pull system was observed to significantly reduce Striga emergence and stemborer damage from the second season onwards. IR-maize reduced and delayed Striga emergence from the first cropping season. Differences in Striga emergence and stemborer damage between the other systems were not significantly different. After five cropping seasons, the Striga seedbank was significantly higher in the maize-bean intercrop system than in the push–pull system under both maize varieties while the rotational systems had intermediate values not different from the day zero values. Under IR-maize, the Striga seedbank was significantly lower than under local maize for all cropping systems. Maize yields varied between seasons, districts, and cropping systems. Yields in the push–pull system were higher than in the maize-bean intercrop after two seasons and in the absence of mid-season drought stress. Both maize and soybean responded significantly to fertilizer application for both districts and for most seasons. The various interventions did not substantially affect various soil fertility-related parameters after five seasons. In the short term, IR-maize integrated in a push–pull system is the most promising option to reduce Striga while the rotational systems may need a longer timeframe to reduce the Striga seedbank. Finally, farmer-led evaluation of the various technologies will determine which of those is really most acceptable under the prevailing farming conditions.

Imazapyr (herbicide) seed dressing increases yield, suppresses Striga asiatica and has seed depletion role in maize (Zea mays L.) in Malawi

The parasitic weed species, Striga asiatica (L.) Kuntze, also known as witchweed, is one of the major constraints in maize production in Malawi. Most of the control measures do not protect a current crop from damage. In 1998/99 season, a trial was initiated at Chitedze Research Station under artificial infection, to evaluate the effects of seed dressing with imazapyr (an acetolactate synthase {ALS} inhibiting herbicide) using three seed treatment methods (coating, priming or drenching) and three herbicide rates (15, 30 and 45 g active ingredient ha-1) on S. asiatica suppression, maize growth and yield. The maize hybrid IntA/IntB//Pioneer325irMZ98F2, bearing target site resistance to imazapyr (IR maize), was used as test crop. In the subsequent season, normal or non-IR maize was planted on the same plots of 1998/99, to assess the residual or spill-over effects on Striga emergence, maize growth and yield. In the first season, results showed that imazapyr seed dressing suppressed (P emergence to 6.7 plants m-2 compared to 14.7 plants m-2 in untreated control. At 106 DAP, the number of Striga that flowered in untreated plots was 6.2 plants m-2, compared to significant (P > 0.05) yield differences. In the subsequent season, imazapyr treatments gave no residual or spill-over effects on maize growth and yield (P > 0.05. There were significant (P Striga emergence similar to the first season. The results therefore suggest that the use of ALS inhibiting herbicides not only suppresses Striga emergence, but also has a seed depletion role in integrated management of Striga without any spill-over or, herbicide injury in subsequent unprotected maize. This technology would be simple for farmers to adopt.

CO2-assimilation and chlorophyll fluorescence as indirect selection criteria for host tolerance against Striga

Striga hermonthica (Del.) Benth. is a parasitic weed on tropical cereals causing serious yield losses in Africa. The use of host crop varieties with improved resistance and tolerance against this parasite is a key component of an integrated control strategy. Breeding for tolerance is however seriously hampered by the absence of reliable and yet practical selection measures. The observation that the photosynthetic rate of tolerant genotypes is less sensitive to Striga infection was used as a starting point to search for suitable selection measures. In a greenhouse pot experiment the effect of Striga infection on the photosynthesis of four sorghum (Sorghum bicolor [L.] Moench) genotypes, differing in Striga tolerance level, was measured at three moments in time (26, 48 and 75 days after sowing). Genotypes were CK60-B, E36-1, Framida and Tiemarifing. Measurements involved CO2-assimilation (A) and three chlorophyll fluorescence characteristics (electron transport rate through photosystem II [ETR], photochemical [Pq] and non-photochemical quenching [NPq]). Striga infection negatively affected A, ETR and Pq. Based on A and Pq, genotypes with superior levels of tolerance (Tiemarifing) could be discriminated from genotypes with superior level of resistance (Framida). Both A and Pq showed high heritabilities and consequently clear and predictable differences between genotypes. Using discriminative ability, heritability and cost effectiveness as main criteria, photochemical quenching (Pq) was concluded to possess the highest potential to serve as indirect selection measure for host plant tolerance to Striga. Screening should preferably be conducted at relatively high Striga infestation levels, between Striga emergence and host plant flowering.

Trees to control weeds in pearl millet

This study shows that growing trees represents a sustainable and efficient alternative to control weeds in millet. A survey was conducted on farmers’ fields in September 1998 at the end of the rainy season in Maiduguri in the semi-arid zone of Nigeria to assess the potential of the Faidherbia albida tree for the control of the Striga hermonthica weed in millet. A random survey was carried out in four locations on the farmers’ fields. The results show that F. albida trees could completely control S. hermonthica weeds occuring in millet under the tree canopy. Indeed, no single Striga emergence was observed under all trees surveyed, while the adjacent crops outside the tree canopies were seriously infested with S. hermonthica, causing reduction in plant growth of 76.5%, panicle length of 23.8% and millet panicle yield of 171.3%. Furthermore, plant performance under the tree canopies was very high, as plant height was increased by 80.0%, stem diameter by up to 15.9%, panicle length by 34.4% and panicle yield by 298.6% as compared with the Striga-infested plants outside the tree canopy. Plants within the 6–15 m area away from the tree trunk performed better than the 0–5 m area. The implication of these results in relation to Striga control in the semi-arid region of West Africa is discussed.

Agronomic Performance of Striga Resistant Early‐Maturing Maize Varieties and Inbred Lines in the Savannas of West and Central Africa

ABSTRACTThe potential maize (Zea mays L.) yield in the savannas of West and Central Africa (WCA) is constrained by Striga hermonthica (Del.) Benth. parasitism. Field trials at Ferkessedougou, Côte d'Ivoire; Abuja, Nigeria; and Mokwa, Nigeria, in 2002, and at Mokwa and Abuja in 2004, evaluated the performance of 11 early maturing maize varieties under artificial Striga infestation and Striga‐free conditions. Another trial at Mokwa and Abuja in 2004 evaluated 100 early maturing inbred lines under Striga infestation and Striga‐free conditions. The varieties differed significantly in grain yield under both conditions. Acr 94 TZE Comp 5‐W, Acr TZE Comp 5‐Y, and TZE‐W Pop × 1368 STR C1 were identified as promising varieties, based on grain yield, Striga damage ratings, and Striga emergence counts. The most promising variety, Acr 94 TZE Comp. 5‐W in terms of high grain yield, reduced Striga damage and low Striga emergence, outyielded the reference entry by 2% under Striga‐free conditions and 45% under Striga infestation. Ward cluster analysis of the varieties and inbred lines produced four major clusters each, under both Striga‐infested and Striga‐free conditions. In each case, the varieties and inbred lines assigned to each cluster under Striga infestation differed from those in the Striga‐free conditions. Several inbred lines with high grain yield, low Striga emergence and reduced Striga damage were identified as sources of Striga resistance for maize breeding programs in WCA.

Estimates of Genetic Variances in Striga Resistant Extra-Early-Maturing Maize Populations

Breeders should maintain sufficient genetic variability in a population to ensure continuous advance from selection for a target trait. The extra-early-maturing Striga resistant/tolerant maize (Zea mays L.) populations, TZEE-W Pop DT STR (white) and TZEE-Y STR (yellow), have each gone through three cycles of S1 recurrent selection for improvement for grain yield, Striga resistance under artificial Striga infestation, and other desirable agronomic characters. S1 families derived from cycle 3 of each population were evaluated at Mokwa and Abuja in Nigeria in 2003 under artificial Striga hermonthica infestation. The objectives of this study were to determine the genetic variability, herita-bilities, and genetic advance among traits associated with Striga resistance in each population. Results showed that the estimates of genetic variance, a2 were moderate to large for most traits of TZEE-W Pop STR C3 with the exception of husk cover and Striga emergence count at 8 and 10 weeks after planting (WAP). Out of 13 traits of TZEE-Y Pop STR C3, 7 had moderate-to-large genetic variances whereas the others were much lower. Broad-sense heritability estimates of traits in TZEE-Y Pop STR C3 were moderate-to-large for days to silking and anthesis; other traits had generally low heritabilities (<0.50). On the other hand, heritability estimates in TZEE-W Pop STR C3 were moderate-to-large for days to anthesis and plant height, and generally low for the other traits. The percentage predicted gain from selection for the traits of TZEE-Y Pop STR C3 ranged from 2.82 for Striga damage rating at 8 WAP to 22 for grain yield; the percentage gain for grain yield of TZEE-W Pop STR C3 was 25 and 0.37–39.21 for 12 other traits. Highly significant positive or negative phenotypic correlations were detected between grain yield and most other traits for TZEE-W Pop STR C3. The correlations between grain yield and other traits of TZEE-Y Pop STR C3 followed trends similar to those of TZEE-W Pop STR C3. The results suggest that sufficient residual genetic variability still exists in each population to allow good progress from further selection for grain yield and Striga resistance.

Characterization of host tolerance to Striga hermonthica

One of the most promising control options against the parasitic weed Striga hermonthica is the use of crop varieties that combine resistance with high levels of tolerance. The aim of this study was to clarify the relation between Striga infestation level, Striga infection level and relative yield loss of sorghum and to use this insight for exploring the options for a proper screening procedure for tolerance. In three pot experiments, conducted in Mali (2003) and The Netherlands (2003, 2004), four sorghum genotypes were exposed to a range of Striga infestation levels, ranging from 0.0625 to 16 seeds cm?3. Observations included regular Striga emergence counts and sorghum grain yield at maturity. There were significant genotype, infestation and genotype × infestation effects on sorghum yield. The relation between infestation level and infection level was density dependent. Furthermore, the relation between Striga infection level and relative yield loss was non-linear, though for the most resistant genotype Framida only the linear part of the relation was obtained, as even at high infestation levels only moderate infection levels were achieved. The results suggest that for resistant genotypes, tolerance can best be quantified as a reduced relative yield loss per aboveground Striga plant, whereas for less resistant genotypes the maximum relative yield loss can best be used. Whether both expressions of tolerance are interrelated could not be resolved. Complications of screening for tolerance under field conditions are discussed

Seed treatment technology: An attractive delivery system for controlling root parasitic weed Striga with mycoherbicide

Coating sorghum seeds with Fusarium oxysporum (Foxy 2) for control of the root parasitic weed Striga, appears to be an attractive option for minimizing the inoculum amount, establishing the biocontrol agent in the potential infection zone of the host plants, and offering a simple, easy and economical delivery system. Our investigations resulted in the selection of appropriate seed coating materials and a suitable type and form of fungal inoculum. The coating materials tested were arabic gum (AG10%, 20%, 40%), carboxymethylcellulose (CMC1%, 2%) and pectin (LS 440, LM-5 CS) 1%, while the fungal inoculum included microconidia and fresh and dried chlamydospores produced using different substrates. Foxy 2 survived the seed treatment processing and showed excellent viability on seeds for at least 8 months of storage after coating. In general, the performance of 40% arabic gum in combination with dried chlamydospores was the best among the other types of inoculum and coating material tested. Regardless of the type and form of inoculum and coating materials tested, Foxy 2 was able to colonize all roots, even root tips and hairs of the host (sorghum), thereby meeting important criteria of a promising candidate for controlling Striga when applied as a seed treatment. The efficacy of treated sorghum seed with Foxy 2 using different coating materials in reducing S. hermonthica infestation was evaluated in pot and root chamber trials. Foxy 2 markedly reduced Striga emergence and dry weight and increased the percentage of the diseased emerged Striga shoots. However, the efficacy of seed coating varied according to the type and form of fungal inoculum as well as coating material. Coating sorghum seed with dried chlamydospore inoculum homogenized into 20% arabic gum (as adhesive) showed the highest efficacy of 81 and 77% (i.e., percent reduction in healthy emerged Striga shoots compared to the control treatment) against Striga using either sterilized or non-sterilized soil, respectively. In root chamber bioassays, the application of Foxy 2 in combination with AG40% significantly caused disease in 77% of the germinated Striga seeds and in all tubercles after 25 days of sowing. These findings provide an optimized coating protocol as an attractive delivery system for bioherbicides for root parasitic weeds.