Effects of host plant genotype and seedbank density on Striga reproduction

Prevention of seed input to the seedbank of Striga hermonthica‐infested fields is an important objective of Striga management. In three consecutive years of field experimentation in Mali, Striga reproduction was studied for 10 sorghum genotypes at infestation levels ranging from 30 000 to 200 000 seeds m^?2. Host resistance was identified as an important determinant of Striga reproduction, with the most resistant genotypes (N13, IS9830 and SRN39) reducing Striga reproduction by 70–93% compared with the most susceptible genotype (CK60‐B). Seedbank density had a significant effect on Striga seed production. Higher seedbank density resulted in more Striga plants, which led to increased intra‐specific competition and consequently a reduced level of reproduction per plant. For the most susceptible sorghum genotypes, density dependence also occurred in the earlier belowground stages. Striga reproduction continued beyond harvest. At the high infestation level just 8% of the total reproduction was realised after harvest, whereas at the low infestation level 39% was attained after harvest. Even though host‐plant genotype plays a significant role in Striga reproduction, calculations indicated that only at very low infestation levels the use of the most resistant genotype was able to lower the Striga seedbank.     

Presowing Hardening of the Host with Phenolic Acids Reduces Induction of Seed Germination in the Root Parasite Striga asiatica

Abstract

Striga asiatica (L.) Kuntze, a root parasite, causes severe loss of yield in sorghum and several other crops. The seeds of the parasite are induced to germinate by a stimulant in the host root exudate. Presowing hardening of the host with vanillic acid, caffeic acid and ferulic acid (25 ppm) reduces the induction of seed germination in the parasite by the host root exudate. The treatment causes a slight improvement in the dry matter production in the host and in addition, increases the phenolics level in the host root exudate. The latter effect might be responsible for reducing germination in Striga. If the treatment remains effective under field conditions also, it reduces significantly the incidence of Striga in cultivated fields.     

Fenugreek root exudates show species-specific stimulation of Orobanche seed germination

Various Orobanche species are weedy and cause severe reduction in the yields of many important crops. The seeds of these parasitic weeds may remain dormant in the soil for many years until germination is stimulated by the release of a chemical signal from a host plant. In order to determine the effects of fenugreek root exudate on the induction of Orobanche crenata, Orobanche ramosa and Orobanche foetida seed germination, root exudate was collected from hydroponically grown fenugreek seedlings. Fractionation patterns obtained from column and thin layer chromatography of the fenugreek root exudate showed a set of metabolites differing in their polarity with stimulatory activity on Orobanche seed germination. The crude root exudate stimulated both O. ramosa and O. crenata seed germination to the same level caused by the synthetic germination stimulant GR24 at 10 mg L^?1. It also stimulated O. foetida seed germination which did not respond to GR24. Active fractions of root exudate stimulated the germination of Orobanche species differentially.     

Factors affecting the activity of ethephon in stimulating seed germination of Striga hermonthica (Del.) Benth.

Ethephon (2-chloroethylphosphonic acid), which generates ethylene, stimulated the germination of pre-conditioned seeds of Striga hermonthica when it was added to the alkaline Gezira clay soil at concentrations of 2.5 to 30 mg kg^?1. As little as 5 min contact with treated soil was enough to stimulate germination. Ethephon in soil did not cause germination of unconditioned seeds for periods of up to 12 days and also had an adverse effect on seed germination when such seeds were given a second ethephon exposure after a storage period which was adequate, in untreated soil, to give the necessary pre-conditioning. Ethephon activity persisted in air-dry soil but declined over a 14 day period in moist soil. In the field ethephon at 0.6 to 4.8 kg ha^?1 decreased the number of Striga shoots and increased sorghum height and flowering.     

Influence of Root Exudates on Orobanche Seed Germination*

Broomrape (Orobanche ramosa L.) is a common root parasite of solanaceous, leguminous and other crops grown in the semi-arid regions of the world. The seeds germinate when root exudates from host plants are released in their immediate vicinity (Lindley, 1853; Koch, 1887; Chabrolin, 1934). Brown et al. (1951a) reported that non-host plants, such as flax (Linum usitatissimum L.) may stimulate Orobanche seed germination without being parasitized. The stimulating properties of flax exudate were studied by Brown et al. (1951b). They reported that the stimulant was unstable in alkaline solutions, but moderately stable in weakly acidic media, which may indicate the presence of an acidic (lactone) grouping. Nash & Wilhelm (1960) reported that gibberellic acid in agar media stimulated O. ramosa seed germination. Abu- Shakra, Miah & Saghir (1970) found that pre-treatment of 0. ramosa seeds with 100 ppm of gibberellic acid followed by incubation on a flax-root diffusate agar medium gave a high (81·7%) germination. The purpose of this study was to collect root exudates from three species of plants cultured under three experimental systems, namely (a) germ-free, (b) glasshouse (non-sterile), and (c) growth chamber (hydroponic, initially aseptic), and to evaluate their biological activity as germination stimulants for O. ramosa seeds. The plants used were tomato (Lycopersicon esculentum Mill.), sorghum (Sorghum vulgare Pers.) and flax. Exudate from marigold (Tagetes erecta L.) also was collected from germ-free culture.     

Effect of growth medium, Striga seed burial distance and depth on efficacy of Fusarium isolates to control Striga hermonthica in Burkina Faso

Striga hermonthica is a destructive parasite of cereal crops in the semi‐arid tropical zone. Two greenhouse experiments were conducted at Kamboinsé, Burkina Faso, to investigate the effect of inoculum substrate and location of Striga seeds on the ability of 14 indigenous Fusarium isolates to control the parasite. In Expt 1, Fusarium isolates reduced emerged Striga number, Striga vigour and dry biomass. As a result, sorghum dry biomass and grain yield were enhanced. Inoculum substrate did not influence the ability of Fusarium isolates to control Striga. In Expt 2, Fusarium isolates, substrate and their interaction significantly influenced germination of Striga seeds at both 35 and 50 days after sowing. Isolates grown on compost were more effective at reducing germination of Striga seeds than those grown on chopped sorghum straw. The per cent germination of seeds 50 days after sowing, buried at 5 cm depth, was significantly lower than that of seeds buried at 10 cm. At 10 cm depth, Fusarium isolates still reduced Striga seed germination with respect to the control; horizontal planting distance, 5 or 10 cm from sorghum hills, had no effect.     

Nature of resistance to Striga hermonthica (Del.) Benth. parasitism in some Sorghum vulgare (Pers.) cultivars

Ten Sorghum vulgare (Pers.) cultivars varying in tolerance to Striga hermonthica (Del.) Benth. parasitism were grown with or without Striga infection. Endodermal thickening, pericycle lignification and silica crystal deposition were studied microscopically and measured for infected and non-infected sorghum cultivars. Although differences in the root character measurements were statistically significant they were not closely related to the response of the plant to infection. Low stimulant producing cultivars showed low or medium root cell thickening. The cv. Framida had both low stimulant production and high root cell thickening and was the best of the tolerant cultivars. High stimulant producing, tolerant cultivars generally showed heavy or intermediate cell thickening. The high stimulant producing, susceptible cultivar Debaikri also showed intermediate root cell thickening.‘Antibiosis', measured by the content of phenolic compounds in the plant, was then studied. Varietal differences in quality and quantity of phenolic substances in the roots and shoots of sorghum cultivars infected or non-infected with Striga were observed. Infection increased total phenolic contents in both shoot and root extracts. Differences in the total phenolic content in the shoot of non-infected cultivars did not reflect tolerance to Striga infection. The total phenolic acid content of the root extracts was closely related to the response of the host plant to Striga infection, tolerant cultivars having greater total phenolic acid content than susceptible ones.     

Striga seed avoidance by deep planting and no-tillage in sorghum and maize

The possibility of reducing Striga hermonthica (Del.) Benth. parasitism in severely infested fields, by means of deep planting - thereby reducing the root length in the upper layers of the soil where Striga seeds are predominantly found - was tested in field trials with maize and sorghum in western Kenya. Sorghum seeds were planted in Striga-infested fields approximately 2.5 cm deep in the soil or at the bottom of conically-shaped plant holes (15-20 cm deep). Depth of plant holes for maize varied from 0 to 30 cm, in un-tilled soil. Deep planting in un-tilled soil gave higher (up to double) grain yields, compared with standard planting in tilled soil. Parasite emergence was related negatively to planting depth of maize (p< 0.05). Deep planting in tilled soil gave 74% more sorghumgrain yield relative to standard planting. In this treatment Striga seed production was not reduced but in un-tilled fields with deeply planted sorghum Striga seed production was completely suppressed. Therefore, a combination of zero-tillage and deep planting seems to be the most effective treatment. The probable mechanism causing these results is avoidance of Striga seed by the host root system, resulting in a delay in the onset of Striga attachment and the formation of smaller numbers of attachments.     

Vegetative Growth of Sorghum and Striga hermonthica in Response to Nitrogen and the Degree of Host Root Infection

The effects of nitrogen and the extent of sorghum root infection by Striga hermonthica on host-parasite association during vegetative growth were studied using a split root system in a 3 × 3 factorial combination of N (37mg on one, 18.5 or 37mg on both root-halves) and Striga (no, one or both root-half infection). High N increased sorghum shoot weight by 22% more than low N, but did not significantly affect Striga growth 64 days after transplanting sorghum (DAP). Striga reduced sorghum stem height and weight by 22% and 25% at 38 DAP, and by 34% and 36% at 64 DAP, respectively. Leaf weight was not affected. Striga stimulated root growth 38 DAP, but not 64 DAP. In partially infected sorghum, 64 DAP, the parasite shoot number, shoot height and shoot dry weight were 36%, 46% and 35%, respectively and host shoot dry matter was 142% of those in fully infected plants, indicating an inverse relationship between the degree of host root infection and the level of resistance. The results suggest that sorghum released resistance-confering substances to the infection points after sensing infection. When infection points are widely distributed as in fully infected sorghum, less of such substances appear to render the host more vulnerable.     

Induction of Striga seed germination by thidiazuron

Thidiazuron at 0.1 to 10 mg 1^?1 induced concurrent germination and haustorium initiation in Striga asiatica (L.) Kuntze and S. hermonthica (Del.) Benth, but it had no effects on S. gesnerioides (Willd.) Vatke. Both millet and sorghum strains of S. hermonthica were equally responsive. The response of Striga seeds to thidiazuron increased with conditioning. Early applications of the compound induced some germination, but had adverse effects on the conditioning process. Induction de la germination des graines de Striga par le thidiazuron Le thidiazuron de 0,1 à 10 mg 1^?1 a induit de façon conjointe le germination et l'initiation d'haustoria chez Striga asiatica et S. hermonthica, mais n'a pas eu d'effet sur S. gesnerioïdes. Les souches de S. hermonthica liées au millet et au sorgho ont un comportement équivalent. La réponse des graines de Striga au thidiazuron a augmenté par conditionnement préalable. Des applications précoces de produit ont induit quelques germinations, mais ont eu des effets contraires sur Ie processus de conditionnement. Einleitung der Samenkeimung bei Striga-Arten durch Thidiazuron Mittels Thidiazuron-Lösungen von 0,1 bis 10 mg 1^?1 konnten eine gleichzeitige Keimung und Ausbildung der Haustorien bei Striga asiatica (L.) Kuntze und S. hermonthica (Del.) Benth. herbeigeführt werden, nicht jedoch bei S. gesnerioides (Willd.) Vatke. Der Rispenhirsen-und der Sorghum-Typ von S. hermonthica reagierten in gleichem Maße. Konditionierung der Striga-Samen förderte die Wirkung von Thidiazuron. Frühe Anwendung des Wirkstoffs führte zu einer geringen Keimung, beeinträchtigte jedoch die Konditionierung.     

Measuring cysteine biosynthesis activity from serine in extracts from sorghum, corn and grass weeds, and their metolachlor susceptibility

In vitro assay procedures for measuring the activity of cysteine biosynthesis from serine (CBS), which is a coupled reaction catalyzed by serine acetyltransferase and cysteine synthase, were developed using crude extracts from sorghum shoots. Cysteine biosynthesis from serine activity was dependent on acetyl‐CoA concentrations (up to 1.5 mmol L^?1), serine (at least up to 20 mmol L^?1) and sulfide (up to 0.25 mmol L^?1), respectively, and was proportional to the protein concentration in the reaction mixture below 0.4 mg mL^?1. The reaction rate was 6.6 nmol min^?1 per mg of protein during the first 5 min, but increased to 45.6 nmol min^?1 per mg of protein between 30 and 45 min after reaction initiation. Sorghum had the highest CBS total activity (222.4 nmol min^?1 per g of fresh weight), and large crabgrass had the lowest CBS total activity (4.7 nmol min^?1 per g of fresh weight) when CBS activity in shoots was extracted from sorghum, corn, johnsongrass, barnyardgrass, goosegrass, green foxtail and large crabgrass. Similar results were obtained for CBS specific activity (nmol min^?1 per mg of protein). There was no correlation between total CBS activity and susceptibility to metolachlor; however, when corn was excluded, a correlation of R² = 0.690 was found. Flurazole seed treatment (1.25 g per kg of seed) conferred metolachlor resistance by sorghum, and enhanced total CBS activity and non‐protein thiol content by 27 and 61%, respectively. The increase in thiol content presumably contributed to metolachlor tolerance in sorghum. From these results, the difference in CBS activity partially contributes to the selectivity to metolachlor among certain grass species, and to the safening action of flurazole by increasing thiol content.     

A bio-economic model of long-run Striga control with an application to subsistence farming in Mali

A bio-economic model of Striga control is developed and applied to Mali's Mourdiah Zone. Various constraints are added, and optimal production practices identified based on Striga infestation levels, rainfall levels, and economic parameters. Model optimization suggests efforts to suppress Striga with nitrogen applications are both expensive and risky. The efficacy of hand-pulling Striga in reducing the Striga seedbank depends on Striga infestation levels and climatic conditions, as does the profitability of hiring labour to expand cultivated acreage. Under all climatic conditions and infestation levels considered, millet in a pure stand generated greater expected net returns than a millet - groundnut or millet - cowpea association. Under conditions of low rainfall, the model suggests planting millet at a density of 0.5 hills m^?2. With average or higher rainfall, the model suggests planting millet at a density of 3.5 hills m^?2. Estimates of Striga-induced net revenue losses also vary with climatic conditions, ranging from 6% to 85%. Model results are encouraged to be used as a guide in the design and evaluation of research and extension programmes aimed at identifying long-run Striga control strategies and promoting their adoption.     

Genetic diversity of East and West African Striga hermonthica populations and virulence effects on a contrasting set of sorghum cultivars

The root hemiparasite Striga hermonthica causes very significant yield loss in its dryland staple cereal host, Sorghum bicolor. Striga‐resistant sorghum cultivars could be an important part of integrated S. hermonthica control. For effective resistance breeding, knowledge about the diversity of the parasite is essential. This study aimed (i) to determine the genetic diversity within and between seven S. hermonthica populations from East and West Africa using 15 microsatellite markers and (ii) to assess the virulence and host–parasite interactions of these Striga populations grown on 16 diverse sorghum genotypes in a glasshouse trial. Most of the genetic variance (91%) assessed with microsatellite markers occurred within S. hermonthica populations. Only a small portion (8%) occurred between regions of origin of the populations. A positive correlation (R² = 0.14) between pairwise geographic and genetic distances reflected the slightly increasing differentiation of S. hermonthica populations with increasing geographic distance. East African S. hermonthica populations, especially those from Sudan, had significantly greater average infestation success across all sorghum genotypes than West African populations. Some specific host–parasite interaction effects were observed. The high genetic variation among individuals of each S. hermonthica population underlines the high potential adaptability to different hosts and changing environments. This points to the need to manage sorghum resistance alleles in space and time and to employ resistant varieties as part of integrated S. hermonthica control, so as to hinder the parasite overcoming resistance.     

Farmers' perspectives on the biotic constraint of Striga hermonthica and its control in western Kenya

Witchweed, Striga hermonthica (hereafter, referred to as “Striga”), is a major biotic constraint to cereal production in sub‐Saharan Africa. The parasitic plant is a socioeconomic problem that has forced some resource‐poor farmers to abandon their farms due to high infestation. This study was designed in order to elucidate farmers' perceptions of Striga control measures and to determine their potential adoption in two villages in western Kenya. Participatory rural appraisals and individual interviews were conducted in 2009 and 2010 in a sample of 128 and 120 households in Kaura and Kogweno‐Oriang villages in Homabay and Rachuonyo districts, respectively. The results revealed that crop production was the main occupation in most households. The farmers identified Striga as one of the major constraints to maize, sorghum, and finger millet production. According to the farmers, the most popular control measures were hand‐pulling, crop rotation, and intercropping, even though rotational systems might need a longer timeframe to reduce the soil seed bank of Striga. Although the level of Striga infestation and damage were increasing in the farmers' fields, the adoption of the control options was limited. The reason for the low adoption level of the control methods by the farmers is because they are “too risky” as there is no guarantee of a direct pay‐off in increased crop yield. Farmer‐led evaluation and adaptation of the various Striga control technologies in real‐life situations will facilitate the choice of appropriate options and facilitate their uptake.     

Nematodes

Abstract

Studies were carried out on farms to evaluate potential control practices which could be constituted into a package of recommendations for the control of Striga hermonthica in the Gambia. ICSV 1002, a variety of sorghum, was identified as being relatively more tolerant to Striga and to the common insect pests of sorghum. Spot treatment of emerged Striga shoots with 2% solution of product paraquat using a pistol‐grip hand sprayer was found to control Striga without stimulating regrowth, improved yields and was more acceptable and cost‐effective than handpulling of the shoots in early millet and sorghum. Where there might be objections to the use of paraquat because of its toxicity hazards, a mixture of 2,4‐D (1 % soln.) plus glyphosate (1 % soln.) or 2,4‐D (2% soln.) was a useful substitute. A tentative control package consisting of ICSV 1002, spot spraying of Striga shoots with paraquat, and side dressing of urea fertilizer at 30 kg N/ha at 4 w.a.p. was tested at two sites against farmers’ practice on pilot scale. Infestation of Striga was reduced and yields were increased by 119% and 37% by the package at the two sites.     

Evaluation of integrated crop management strategies employed to cope with Striga infestation in permanent land use systems in southern Benin

Striga hermonthica and S. gesnerioides pose serious threats to cereal and cowpea production, endangering peoples' livelihoods on the Abomey plateau, Benin. A 2-year joint experiment was undertaken with farmers in two hamlets to investigate the potential of managing sowing dates of cowpea, sorghum transplanting, and trap cropping as ways of increasing agricultural production and reducing Striga damage. Early sowing of cowpea failed due to dry spells. Late sowing reduced cowpea yield due to water deficiency at the end of the growing season. Transplanting sorghum seedlings raised in fertilised or Striga-free nurseries doubled or tripled cereal yield and substantially reduced S. hermonthica infestation compared to direct early-sown sorghum. Transplanting sorghum from plant hills to fill gaps was unsuccessful. Trap crops such as cowpea and groundnut increased subsequent maize yield. Trap cropping had only a small effect on S. hermonthica infestation. The very poor soils in Somè central were a major constraint upon yield improvement to acceptable levels even after the introduction of the new crop (and Striga) management methods.     

Social science input into IPM

Abstract

Metsulfuron methyl (Ally‐Dupont), a sulphonylurea herbicide, was tested at rates of 0.5–2.0 g a.i./ha for the control of Striga hermonthica (Del.) Benth. in two cultivars of sorghum in pot experiments. There was good to excellent Striga control at 1.0–2.0 g a.i./ha applied either pre‐emergence or post‐emergence to CSH‐1 or N‐13 sorghum. The herbicide was unacceptably toxic to the Striga‐susceptible CSH‐1 cultivar when applied pre‐emergence, thus eroding any benefits of Striga control. In the Striga‐tolerant N‐13 cultivar there were considerable increases in the growth of infected plants. The herbicide was better tolerated from post‐emergence applications by both cultivars, and CSH‐1 plants recovered enough growth from Striga infection to produce grain yields at 1–5 and 2.0 g a.i./ha. Herbicide application at 4 weeks after planting sorghum was less damaging than at 2 weeks. Herbicide safening with 1,8‐naphthalic anhydride did not provide any additional benefits with post‐emergence application. The efficacy of the pre‐emergence herbicide was similar with surface or incorporated application.     

A novel approach to Striga and Orobanche control using synthetic germination stimulants

A series of synthetic germination stimulants for the plant parasites Striga and Orobanche has been prepared. These compounds, analogues of the natural Striga germination stimulant, strigol (I), (Cook et al., 1966, 1972) cause signiticant germination of Striga hermonthica (Del.) Benth. seed at concentrations as low as 10^?9 M, and an even greater activity against species of Orobanche has been demonstrated. Initial outdoor box trials of the compounds against Striga asiatica (L.) O. Ktze. in Hyderabad, India, have shown a reduction of up to 65% of the seed after a single treatment of the soil 6 weeks before the planting of the sorghum host.     

Farmers’ perceptions of sorghum production constraints and Striga control practices in semi-arid areas of Tanzania

The objectives of this study were to investigate constraints affecting sorghum production and farmers' approaches of Striga management in the semi-arid regions of Tanzania. Focus group discussions based on a semi-structured questionnaire and observations following transect walks were used for data collection. Only 35%, 15%, and 10% of the farmers from Igunga, Kishapu, and Meatu districts, respectively, reported growing newly released varieties. The major constraints affecting sorghum production in the study areas included Striga infestation, drought, storage pests, damage by birds, a lack of access to improved varieties, and a lack of access to production inputs, such as fertilizers, insecticides, fungicides and herbicides. Hand weeding, crop rotation, fallowing, intercropping, and organic manure application were the most common practices of farmers for reducing Striga infestations, but most farmers (79.7%) had little knowledge of the best recommended Striga management practices. About 65% of the farmers did not use fertilizers and herbicides for soil fertility improvement and weed management, respectively, creating favourable conditions for Striga infestation. A systematic breeding programme aiming at improving sorghum varieties for Striga resistance, including farmers' preferred traits, should be designed and implemented to increase the adoption of these new varieties by the farmers.     

Integrated management of Striga hermonthica in sorghum using a mycoherbicide and host plant resistance in the Nigerian Sudano‐Sahelian savanna

Striga hermonthica is a major biotic constraint to sorghum production in Nigeria, sometimes causing total yield loss. Recommendations for Striga management often include the use of cultural and agronomic practices, herbicides and host plant resistance when available. The use of biological control has not been commercialized. Fusarium oxysporum (isolate PSM 197)‐based mycoherbicide was used in combination with selected sorghums (the Striga‐resistant cultivar Samsorg 40, and the Striga tolerant landrace Yar'ruruka) as an Integrated Striga Management strategy (ISM) in on‐farm trials in the Sudano‐Sahelian savanna of Nigeria. Crop stands were significantly (P = 0.05) higher in ISM compared with non‐ISM plots on which the mycoherbicide was not applied. Similarly, ISM plots had significantly (P = 0.05) lower Striga counts than non‐ISM plots. Striga emergence was reduced by ISM by around 95%. Sorghum yields were 49.6% higher where integrated management was used. Cost benefit analysis of the ISM package shows that use of the mycoherbicide increased the profitability of sorghum production on Striga‐infested soils. Farmers’ preferences monitored during and after the trials highlighted the need for careful selection and integration of control components into an ISM package.