Parasitic Witchweed Research Articles

Conferring resistance to parasitic witchweed by shifting strigolactone biosynthesis

Strigolactones from the exudates of maize root induce germination of the parasitic witchweed Striga. Recently, Li et al. characterized the biosynthesis pathway of two strigolactones, zealactol and zealactonoic acid, which induce less Striga germination than the major maize strigolactone, zealactone. This study provides a promising strategy for plant protection against parasitic witchweed.

Role of Variety and Fertilizer Practices on Cowpeas (Vigna unguiculata) Yield and Field Incidence of the Parasitic Weed Alectra vogelii (Benth) in Central Malawi

Grain legumes are an important component of the food systems in Malawi. The parasitic legume witchweed species Alectra vogelii (Benth) is among the problem pests with serious infestations in groundnuts (Arachis hypogea), soybeans (Glycine max), cowpeas (Vigna unguiculata) and other legumes. A study was conducted in 2013/14 and ‘14/15 seasons to evaluate the effects of three cowpea varieties (IT82E-16, Sudan 1 and Alectra-resistant Mkanakaufi) and fertilizer practices (no fertilizer applied, 5 t ha-1 cattle manure and 100 kg ha-1 of inorganic 23:21:0+4S on cowpea grain yield, yield components and Alectra emergence at three sites in central Malawi. Cowpea grain yields ranged 400-2400 kg ha-1. There were significant (P < 0.05) variety effects on yield in 4 of 6 site-years, with variety IT82-16 consistently giving the highest yields (range 1200-2400 kg ha-1). There were significant variety effects on A. vogelii emergence with Mkanakaufiti having no Alectra throughout. Application of cattle manure strongly suppressed A. vogelii in 6 site-years all at 60 days after planting, while inorganic fertilizer suppressed Alectra in 3 of the 6 site-years. Cattle significantly but marginally (about 250 kg ha-1) increased yield in 2 of the 6 site-years. The results show potential to suppress A. vogelii with cattle manure application. However, further studies are required to understand the causes of the limited yield response under manure or fertilizer application to make the practices attractive to farmers. More variety improvement studies to produce a range of varieties with better local adaptability and response to fertility amendments are recommended.

Effects of Sodium Carbonate on Striga hermonthica Del. Infestation and Agro-morphological Parameters of Sorghum bicolor (L.) Moench in the Sudano-Sahelian Zone of Cameroun

Sorghum bicolor is the main food widely used to feed millions of people in Central and West Africa. Sorghum production in Sub-Saharan Africa is limited by biotic factors including the parasitic witchweed Striga hermonthica. In Northern Cameroon chemical and organic fertilizers, biological methods and cultivation practices were used by farmers to control Striga hermonthica in field. Some farmers spread sodium carbonate salt (locally named natron or kilbou) in high rates in fields to reduce parasite infestation but in high rate. This salt plays an important role in soil pH buffering. This study was conducted to evaluate effects of different rates of natron on striga emergence and on agro-morphological parameters of sorghum using a susceptible genotype GD-MP04. Field experiments have been conducted from 2017 to 2018 at Touboro, Mayo-Rey Division in the North Region located between longitude 13°34’; 12°07’ East and latitude 7°21’; 15°01’North. The experimental design was a complete randomized bloc with pots (11 m × 11 m) filled with soil naturally infected by striga seeds and different rates of sodium carbonate (0 g, 2 g, 4 g, 6 g, 8 g, 10 g and 12 g per 5 dm3 of soil) were employed. The numbers of emerged striga and sorghum parameters were evaluated. Results showed that striga-infestation and agro-morphological parameters of sorghum varied significantly (p <0.05) with rates of sodium carbonate. The mean number of emerged striga plants per pot varied from 0.08 in pot of 2 g of salt to 5.4 in the control. The number of sorghum leaves per plant, stem diameter, sorghum height, panicle weight and sorghum kernel yield varied significantly (p <0.05) with natron application rate. The application of 2 g of sodium carbonate per pot was the optimal rate to control Striga hermonthica and improve the main agro-morphological parameters of sorghum with 78.27% increase in g of kernel yield and simultaneously inhibited striga infestation by 98.62%. Application of 2 g of sodium carbonate in area naturally infested by striga for improving sorghum yield is required.

Reactive Oxygen Species (ROS) Generation Is Indispensable for Haustorium Formation of the Root Parasitic Plant Striga hermonthica.

The parasitic witchweed Striga hermonthica causes devastating damage to crops in sub-Saharan Africa, yet the mechanism of its parasitism is not well understood. Parasitic plants form a special organ called a haustorium to obtain water and nutrients from host plants. The haustorium is induced by host-derived small molecules, collectively named haustorium-inducing factors (HIFs). The most active HIF known to date is 2,6-dimethoxy-p-benzoquinone (DMBQ), originally isolated from sorghum root extracts. It has been suggested that DMBQ is produced by oxidation of its precursor, syringic acid, and that reactive oxygen species (ROS) and peroxidases are involved in the process. However, the roles of ROS in haustorium formation after HIF recognition remain to be elucidated. Here, we investigated the effects of various inhibitors of ROS and ROS-regulating enzymes on haustorium formation in S. hermonthica. Inhibitors of NADPH oxidases and peroxidases inhibited haustorium formation during treatment with DMBQ, syringic acid, and host root extracts, suggesting that ROS production and/or regulation via NADPH oxidases and peroxidases are essential for haustorium formation. We observed hydrogen peroxide accumulation in the haustorium upon treatment with various HIFs. Our results suggest that ROS and ROS-regulating enzymes are indispensable in downstream signaling of HIFs for haustorium formation.

A Review of a Half Century Hybrid Maize Breeding Experiences with Combined Tolerance to Major Biotic and Abiotic Stresses in China and Other Developing Countries

Genetic vulnerabilities (GV) of maize (Zeamays L.) have been experienced for a half century globally. Research on quantitative gene resistance (here called “co-survival tolerance”) against GV have been studied on Puccinia sorghi common rust, Exserohilum turicum northern corn leaf blight, Bipolaris maydis Southern corn leaf blight, maize streak virus, parasitic witchweed Striga species, Puccinia polysora tropical rust, Peronosclerspora sorghi downy mildew and Ustilago maydis smut. Among the above GE experiences, the most striking experience was parasitic witchweed Striga hermonthica, S. asiatica and S. aspera species in Africa. Without co-survival tolerance, Striga problems in Africa may not be solved in next few decades. The tolerance is controlled by quantitative gene. African origin parasite problems are causing damage of Africa’s food crops in 7 billion US dollars annually. Parasites and hosts must be live together and co-survive in nature. The author’s experiences of GV with maize in China have been two popular hybrids; Jeungdan 958 and Seunok 335. The former hybrid was covered over 10 million ha during the period of 2007-2013. Its genetic vulnerability due to too wide areas of cultivation of a single hybrid was occurred by corn borers and smut. While Seunok 335 and its sister hybrids now cover majority of maize cultivation of North Eastern (NE) region of China. Seunok 335 has severe root lodging problem under storm. The author observed the lodging in the north of Shenyang, Liaoning Province in 2008, followed in the NE provinces. In addition, it was attacked by E. turcicum in the western region of Jilin Province. Both USA and China have a quarter of the world maize cultivation, each 35 million ha and produce average yield of 10.5 and 6.5t/ha, respectively. To increase maize production for feed, food and industries as well as to reduce climate changes, China’s maize cultivation and breeding must be focused on diverse hybrid development and machine harvest. The later returns stalk and leave to soil. Copies of super hybrids e.g. Seunok 335 by other companies must be discouraged. Under climate changes, breeding stable hybrids with tolerance controlled by QTL must be encouraged. Tolerance breeding has clear advantage from GMO because it shall provide a buffering effect minimizing mutation of aggressive genes. Under the climate changes, the tolerance breeding confers buffering effects of host stability. The author has bred hybrids with tolerance against major biotic and abiotic stresses. Target traits are normal field, sticky, sweet maize, bio energy maize with genes of bm3 + high sugar + leafy, and black herb maize.

The path from β-carotene to carlactone, a strigolactone-like plant hormone.

Strigolactones, phytohormones with diverse signaling activities, have a common structure consisting of two lactones connected by an enol-ether bridge. Strigolactones derive from carotenoids via a pathway involving the carotenoid cleavage dioxygenases 7 and 8 (CCD7 and CCD8) and the iron-binding protein D27. We show that D27 is a β-carotene isomerase that converts all-trans-β-carotene into 9-cis-β-carotene, which is cleaved by CCD7 into a 9-cis-configured aldehyde. CCD8 incorporates three oxygens into 9-cis-β-apo-10'-carotenal and performs molecular rearrangement, linking carotenoids with strigolactones and producing carlactone, a compound with strigolactone-like biological activities. Knowledge of the structure of carlactone will be crucial for understanding the biology of strigolactones and may have applications in combating parasitic weeds.

Companion Cropping to Manage Parasitic Plants

Parasitic plants, through a range of infestation strategies, can attack crop plants and thereby require management. Because such problems often occur in resource-poor farming systems, companion cropping to manage parasitic plants is an appropriate approach. Many examples of companion cropping for this purpose have been reported, but the use of cattle forage legumes in the genus Desmodium as intercrops has been shown to be particularly successful in controlling the parasitic witchweeds (Striga spp.) that afflict approximately one quarter of sub-Saharan African cereal production. Through the use of this example, the development of effective companion crops is described, together with developments toward widespread adoption and understanding the underlying mechanisms, both for sustainability and ensuring food security, and also for exploitation beyond the cropping systems described here.

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

Registration of Striga‐Resistant and Drought‐Tolerant Tropical Early Maize Populations TZE‐W Pop DT STR C4 and TZE‐Y Pop DT STR C4

Two maize (Zea mays L.) populations, TZE‐W Pop DT STR C4 (Reg. No. GP‐574, PI 654407), white‐ grained, flint/dent, and TZE‐Y Pop DT STR C4 (Reg. No. GP‐575, PI 654408), yellow‐grained, flint/dent with drought tolerance and moderate levels of resistance to the flowering parasitic witchweed, Striga hermonthica (Del.) Benth, were developed at the International Institute of Tropical Agriculture (IITA). The two breeding populations were released to national maize programs in west and central Africa as source germplasm in 2004 for the development of Striga‐resistant synthetic varieties, parental inbred lines, and hybrids. The populations were released for their superior grain yield under Striga‐infested and noninfested conditions. They also have good levels of resistance to maize streak virus, tropical lowland rust (incited by Puccinia polysora Underw.), and blight [caused by Bipolaris maydis (Nisikado & Miyake) Shoemaker]. In multilocation trials in Benin Republic and Nigeria, 2006 and 2007, TZE‐W Pop STR C4 outyielded the Striga‐susceptible check TZE Comp. 4 by 44% under Striga infestation and 12% when noninfested. Under the same conditions, the increased yield for TZE‐Y Pop DT STR C4 was 42% (Striga infested) and 16% (noninfested). The check variety TZE Comp. 4 suffered the highest Striga damage and supported the highest number of emerged Striga plants when Striga infested.

Global Invasive Potential of 10 Parasitic Witchweeds and Related Orobanchaceae

The plant family Orobanchaceae includes many parasitic weeds that are also impressive invaders and aggressive crop pests with several specialized features (e.g. microscopic seeds, parasitic habits). Although they have provoked several large-scale eradication and control efforts, no global evaluation of their invasive potential is as yet available. We use tools from ecological niche modeling in combination with occurrence records from herbarium specimens to evaluate the global invasive potential of each of 10 species in this assemblage, representing several of the worst global invaders. The invasive potential of these species is considerable, with all tropical and subtropical countries, and most temperate countries, vulnerable to invasions by one or more of them.

Multi-site, multi-season field tests demonstrate that herbicide seed-coating herbicide-resistance maize controls Striga spp. and increases yields in several African countries

Plant parasitic Striga (witchweed) species have not been controlled in susceptible host crops prior to exerting damage. High dose, localized herbicide levels can be applied on or near maize seed bearing acetolactate synthase (ALS) target-site resistance. Such seed coating was cost-effective in preventing damage from parasitic witchweeds Striga hermonthica and S. asiatica in Kenya, Malawi, Tanzania, and Zimbabwe. Imazapyr at 30–45 g ha −1 and pyrithiobac at 11–21 g ha −1 were used at 3 experiment stations and in 93 farmers’ fields over six seasons to further evaluate the effectiveness of this technology. Seed coating with imazapyr and pyrithiobac gave season-long Striga control in most cases resulting in a 3–4-fold increased maize yield when Striga density was high. Once herbicide resistant maize has been produced using locally adapted varieties, this technology should greatly benefit small-scale farmers in sub-Sahara Africa.

Control of witchweed Striga hermonthica by intercropping with Desmodium spp., and the mechanism defined as allelopathic.

During investigations into the control of insect damage to maize crops in subsistence farming in Kenya, which involved intercropping with repellent plants, the fodder legumes silverleaf (Desmodium uncinatum) and greenleaf (D. intortum) were also found to reduce dramatically the infestation of maize by parasitic witchweeds such as Striga hermonthica. This effect was confirmed by further field testing and shown to be significantly greater than that observed with other legumes, e.g., cowpea, as were the concomitant yield increases. The mechanism was investigated, and although soil shading and addition of nitrogen fertilizer showed some benefits against S. hermonthica infestation, a putative allelopathic mechanism for D. uncinatum was observed. In screenhouse studies, a highly significant reduction in S. hermonthica infestation was obtained when an aqueous solution, eluting from pots in which D. uncinatum plants were growing, was used to irrigate pots of maize planted in soil seeded with high levels of S. hermonthica. Growth of the parasitic weed was almost completely suppressed. whereas extensive infestation occurred with the control eluate. Laboratory investigations into the allelopathic effect of D. uncinatum, using samples of water-soluble chemical components exuded from cleaned roots, demonstrated that this involved a germination stimulant for S. hermonthica and also an inhibitor for haustorial development.

Exploiting chemical ecology and species diversity: stem borer and striga control for maize and sorghum in Africa

Stem borers, comprising the larvae of a group of lepidopterous insects, and parasitic witchweeds, particularly Striga hermonthica and S asiatica, cause major yield losses in subsistence cereal production throughout sub-Saharan Africa. Studies are described that have led to the development of a ‘push-pull’ strategy for minimising stem borer damage to maize and sorghum. This involved the selection of plant species that could be employed as trap crops to attract colonisation away from the cereal plants, or as intercrops to repel the pests. The two most successful trap crop plants were Napier grass, Pennisetum purpureum, and Sudan grass, Sorghum sudanensis. The intercrop giving maximum repellent effect was molasses grass, Melinis minutiflora, but two legume species, silverleaf, Desmodium uncinatum, and greenleaf, D intortum, gave good results and had the added advantage of suppressing development of S hermonthica. In terms of stem borer control, the plant chemistry responsible involves release of attractant semiochemicals from the trap plants and repellent semiochemicals from the intercrops. With M minutiflora, parasitism of stem borers was also increased by certain chemicals repellent to ovipositing adults. The mechanism of striga control has not been fully elucidated, but allelopathic effects from the Desmodium species have been shown to involve stimulation of germination and interference with haustorial development. Significant beneficial effects have been obtained with the individual components of these push-pull strategies. However, the most robust crop-protection package is obtained when these components are combined. The trap crop and intercrop plants also provide valuable forage for cattle, often reared in association with subsistence cereal production. There has been considerable take-up of the system within the communities where farmer-managed trials have been carried out, particularly in the Trans Nzoia and Suba districts of Kenya, and the programme is set to expand throughout and beyond Kenya. © 2000 Society of Chemical Industry

Control of parasitic witchweeds (Striga spp.) on corn (Zea mays) resistant to acetolactate synthase inhibitors

Parasitic witchweeds inflict most of their damage while still underground and attached to crop roots. Most selective translocated herbicides are detoxified by crops such as corn and thus cannot reach the attached parasites. Corn with target site resistance to acetolactate synthase (ALS)-inhibiting herbicides was tested to ascertain whether these herbicides could control witchweeds, assuming that witchweeds do not obtain amino acids from the crop. Postemergence directed sprays of 27 g ae ha−1 imazapyr 54 d after planting (DAP) delayed Striga asiatica emergence on corn in South Carolina from 3 wk (control) to 7 wk and to 11 wk when mixed with 45 g ae ha−1 AC 263 222. Treatments with up to 71 g ae ha−1 imazamox, and up to 71 g ae ha−1 AC 263 222 only delayed Striga emergence by 1 wk, and 71 g ae ha−1 imazethapyr was ineffective. ALS-inhibiting herbicides were far more effective when applied in 1-ml drenches above the seed at planting. Chlorsulfuron (10 g ai ha−1) and sulfometuron (50 g ai ha−1) were somewhat phytotoxic to Pioneer 3245IR. Rimsulfuron (30 g ai ha−1), metsulfuron (10 g ai ha−1), halosulfuron (120 g ai ha−1), and imazethapyr (140 g ae ha−1) were marginally active in Kenya, with some mature Striga hermonthica seed-bearing capsules appearing at harvest (12 wk). Imazapyr at 15 g ae ha−1 gave 70 to 95% suppression of capsule formation, whereas no capsules appeared at 30 g ae ha−1. The use of imazapyr in Kenya increased the harvest index by 17% when corn plants in Striga-infested soils were kept insect and disease free by using insecticides and fungicides. Thus, complete control can be achieved at affordable cost by farmers in subsistence conditions.

Development of synthetic maize populations for resistance to Striga hermonthica

ABSTRACTThe parasitic witchweed, Striga hermonthica (Del.) Benth, is the greatest biological constraint for cereal crop production by resource‐poor farmers in sub‐Saharan Africa. Maize, Zea mays L., is a widely grown cereal crop in this region (22 × 106 ha). Striga‐resistant maize populations were produced and tested as half‐sib families in West and Central Africa. Three populations with white (W), yellow (Y), or mixed (Y/W) grain colour were formed by: (1) intercrossing Striga resistant maize inbred lines followed by two generations of random mating; (2) testing far under artificially induced S. hermonthica infestations in Nigeria, Cameroon, and Ivory Coast and selection of resistant families; (3) two generations of random mating; and (4) two years of testing for resistance. Striga‐resistant synthetic W, Y and Y/W populations were produced by compositing resistant half‐sib families. Outstanding performance in grain yields and host plant resistance was observed. Maize damage ratings and number of harvested ears were highly correlated with grain yield. High variation was observed for Striga emergence counts. The populations have combined resistance to Striga, maize streak virus (MSV), and other major biotic constraints for maize cultivation in Africa, thus providing the opportunity for improved sustainable maize production under stress environments. Breeder's seed of these synthetic varieties are being multiplied for distribution to national programmes.

An Automated System for Large-Scale Recovery of Germination Stimulants and Other Root Exudates

A large-scale continuous system for recovery of root exudates from millet and cowpea was developed and tested extensively. Large quantities of germination stimulants were exuded into the medium flowing through the plant trays, which could be demonstrated by germination of seeds of three species of angiospermous parasitic plants,Alectra vogelii, Striga gesneriodes, and witchweed. Recovery of root exudates was affected by adsorption on the macroreticular polymer resin XAD-4 followed by desorption with methanol. Furthermore, indications were found that germination inhibitors are released into the medium. Subsequent testing confirmed the quantitative release of the germination stimulants as well as a large number of other compounds.

Witchweed Management by Sorghum‐Sudangrass Seed Size and Stage of Harvest

AbstractCareful cultural management of field crops has become imperative in countries where chemical measures are not economically feasible. The occurrence of the endemic parasitic witchweed [Striga hermonthica (Del.) Benth] in production fields of sorghum‐sundangrass hybrid [Sorghum bicolor (L.) Moench ✕ S. sudanense Stapf. cv. Piper], Pioneer brand hybrid 988 (coded P988), is one example that requires such procedures. Study objectives were to relate: (i) P988 seed size (large and small) to witchweed germination, P988 root and shoot length, and weight and (ii) P988 seed size and harvest growth stage to P988 greenchop yield and sucrose content at three initial and subsequent P988 harvests at boot stage, late bloom stage, and maturity in witchweed‐infested fields. Seedlings of P988 grown from small seed had significantly shorter shoots but longer roots compared to those from large seed. Witchweed germination was not signficiantly affected by seed size of P988 but was postively correlated to P988 root length and root dry weight. Seed size of P988 had no significant effect on forage greenchop yield and forage sucrose of P988 at all harvests. Delaying initial harvest of P988 increased witchweed shoot dry weight with time and equated total yields, but reduced the quality of greenchop at late bloom and maturity. Delaying second and third harvests of P988 to maturity significantly increased greenchop yield of P988. Forage sucrose at maturity stage of second and third harvests exceeded forage sucrose at maturity of first harvest by 106 and 60%, respectively. Witchweed shoot dry weight was positively correlated with forage sucrose of P988. Growing small seeded P988 reduced witchweed incidence and sustained P988 yields when combined with harvesting at the boot stage on first harvest and at maturity of later harvests on witchweed‐infested soils.