Striga Control Research Articles

Striga control—technologies and their dissemination

Striga spp. are endemic in Sub-Saharan Africa and pose one of the most severe biological constraints to cereal production in low-potential areas. Although the level of Striga infestation and damage is increasing, farmers rarely adopt Striga control methods. The paper discusses various Striga control methods for their effectiveness and adoption potential by small-scale farmers. It further addresses the specific constraints to the adoption of improved farming techniques in low-potential areas. A Striga control program in western Kenya is presented, which tackled various aspects of the development of a Striga control strategy, including adapted control methods, training materials and concepts for farmers and extension agents, the implementation of technology diffusion approaches and the implementation of stakeholder fora to raise awareness.

Striga Resistance in the Wild Relatives of Sorghum

Witchweeds (Striga spp.) are noxious parasitic weeds that cause considerable crop damage in the semiarid tropics. Genetic control of striga is effective, although sources of resistance are limited in most crops. Useful resistance sources have been obtained in sorghum [Sorghum bicolor (L.) Moench], an important host crop that has coevolved with the parasite. Fifty-five wild accessions within the primary gene pool of sorghum and 20 sorghum cultivars were screened for resistance to Striga asiatica L. Kuntze in the laboratory. Wild sorghums assayed included S. almum Parodi, S. bicolor subsp. drummondii (Steud.) De Wet, race drummondii and race hewisonni, S. bicolor subsp. verticilliflorum (Steud.) Piper with races aethiopicum, arundinaceum, verticilliflorum, and virgatum; S. halepense (L.) Pers.; S. miliaceum; S. rhizomatores; S. sorghastrum; and S. usamberense Wild sorghum accessions varied in their effects on S. asiatica at the preattachment level of association. Potential striga-resistance mechanisms of low germination stimulant production, germination inhibition, and low haustorial initiation activity were observed in this collection of sorghums. Some of these potential striga-resistance mechanisms, reported here for the first time, appear to be unique to wild sorghums. The results described in this study offer the possibility of introgressing valuable resistance genes from wild to cultivated sorghum.

Possibilities for control of Striga hermonthica in Sorghum (Sorghum bicolor) using neem (Azadiractha indica) and parkia (Parkia biglobosa)-based products

Striga hermonthica is a major threat to cereal production in Nigeria. Management approaches for control of Striga are generally based on cultural control strategies and host plant resistance. The use of plant products for the control of S. hermonthica is limited. Seed and leaf powders of neem and fruit and fruit peel powders of parkia trees were evaluated under screenhouse and field conditions in the Nigerian Savanna. Screenhouse evaluation of the plant materials showed that neem seed powder was the most effective, with only 16.5% of Striga emergence. This was followed by parkia fruit powder and parkia fruit peel powder, with 29.1% and 38.8% Striga emergence respectively. In the field, all plant materials significantly reduced Striga emergence. The lowest number of emerged Striga plants was observed on plots treated with neem seed powder (with 1.7 emerged Striga per 3m2 while the control plots had 30.3 emerged Striga plants per 3m2). Crop plants treated with neem seed powder did not show any reaction to Striga infestation, while parkia fruit powder and parkia fruit peel powder had reaction scores of 1.7 and 2.0 respectively when compared to neem leaf powder with 3.0 and the control with 4.7. Treatment by all the plant products however, significantly increased grain yield, which was highest in the treatment with neem seed powder. Our results highlight the possibilities of using these plant products for the control of S. hermonthica.

AN ASSESSMENT OF INTEGRATED STRIGA HERMONTHICA CONTROL AND EARLY ADOPTION BY FARMERS IN NORTHERN NIGERIA

Two sets of on-farm trials, each covering two years, were conducted in the northern Guinea savannah of Nigeria over the period 1999–2001, the objective being to compare integrated Striga hermonthica control measures (soybean or cowpea trap crops followed by maize resistant to Striga) with farmers' traditional cereal-based cropping systems. In both sets of trials, this proved to be highly effective in increasing productivity over the two year period, especially where soybean was used as a trap crop. Resistant maize after a trap crop increased the net benefit over the two cropping seasons in both trials by over 100% over farmer practice. However, in the second set of trials there was no significant increase in productivity between a trap crop followed by Striga resistant maize, and a trap crop followed by local maize especially where legume intercropping and fertilizer had been applied in the farmer practice. There was also no increase in productivity between two years' traditional cereal cropping and one year's local maize followed by Striga resistant maize. This indicates the importance of a legume trap crop in the first year in order to ensure high productivity in the second year, regardless of variety. Up to 20% of farmers obtained higher productivity from their own practices, notably intercropping of cereals with legumes and use of inorganic fertilizers. Leguminous trap crops and Striga resistant maize, together with two key management practices (increased soybean planting density and hand-roguing) were seen to be spreading both within and beyond the research villages, indicating that farmers see the economic benefits of controlling Striga. Survey findings show that explaining the reasons why control practices work can greatly increase the adoption of these practices. Wider adoption of Striga control will therefore require an extension approach that provides this training as well as encouraging farmers to experiment and adapt Striga control options for their local farming systems.

FARMERS' PERCEPTION OF THE STRIGA PROBLEM AND ITS CONTROL IN NORTHERN NIGERIA

The parasitic angiosperms, Striga hermonthica and S. gesnerioides, obligate root parasites endemic in sub-Saharan Africa, constitute severe constraints to cereal and legume production in West and Central Africa. Over the years, a range of effective component technologies has been identified for Striga control in Africa. The potential of these technologies has been demonstrated under researcher-managed conditions. To promote farmer testing of the technologies, community workshops were conducted in 42 rural communities in Kaduna State, northern Nigeria. These revealed that agriculture was the main source of livelihood for most households. The three most important crops, maize, sorghum and pearl millet are attacked by S. hermonthica, regarded as the major constraint to crop production, often causing 70–100% crop loss. Farmers recognised two types of Striga damage (underground and aboveground), with greater damage being caused by underground Striga. Farmers attributed increasing incidence and severity of Striga damage to lack of capital, poor soil fertility, infestation of previously uninfested land by Striga seeds, and continuous cropping of host crops. The most widely used among the 15 existing Striga control techniques identified by the farmers were hoe weeding and hand pulling, application of inorganic fertilizer and manure, crop rotations, fallowing, and early planting. In assessing possible control measures farmers considered increased crop yield, reduced Striga reproduction and Striga emergence, greater crop vigour, and increased soil fertility as positive attributes. Negative attributes comprised increased labour requirement, higher costs, increased risk of crop damage or yield reduction, and lower quantity and quality of produce. Overall, a legume-cereal rotation was the most highly rated control option for S. hermonthica management evaluated by the farmers. The implications of these results are examined with respect to farmers' adoption and adaptation of Striga control options beyond the experimental plots.

Potential for sesame to contribute to integrated control of Striga hermonthica in the West African Sahel

Striga hermonthica is an important constraint to the production of pearl millet, a staple cereal in many parts of sub-Saharan Africa. Sesame is an important oilseed crop well adapted to the sandy soils of the West African Sahel. Intercropping of sesame and pearl millet has been reported to reduce emerged striga numbers, but formal research into the potential for sesame to contribute to control of the parasite is lacking. Field trials were undertaken to evaluate the potential of sesame grown in rotation with pearl millet to reduce striga infestation of the cereal. Emerged striga numbers and striga fruiting were strongly reduced on pearl millet following sesame compared to sole millet. To maximize cereal yield, soil fertility enhancement and water conservation are indispensable elements of integrated striga control. The results can guide future research at a time where sesame is being promoted to diversify agricultural production in the Sahel.

Technical and economic evaluation of some Striga control packages in maize in the Nigerian Guinea Savanna

Earlier reports have confirmed the technical feasibility and socio-economic acceptability of some integrated Striga control packages (ISCOPA) at farmers’ level. However, the performance of ISCOPA based on the synthetic varieties of maize have not been consistent. With the development of new open pollinated low management requiring Striga-tolerant maize varieties, further evaluation of the new packages has become necessary. In 1995, farmer-managed trials were, therefore, conducted at two locations Dakachi and Kaya in the Northern Guinea Savanna (NGZ) of Nigeria to compare the new ISCOPAS with the promising one reported earlier and relate crop yield to Striga/weed complex parameters. The ISCOPA tested had tolerant maize varieties, fertilizer application, weed control (pre-emergence herbicide application or hoe-weeding) and additional Striga control (post-emergence herbicide or hoe-weeding) as components. The ISCOPAs that had post-emergence herbicide components resulted in significantly higher maize grain yields than the farmers’ practice. Striga infestation was also significantly lower with the ISCOPAs containing post-emergence acifluorfen compared with the farmers’ practice. When three hoe-weedings replaced herbicide applications, only ISCOPAs of maize varieties 9022-13, Oba Super II and Acr 92 TZE produced higher yield than the farmers practice. Among the varieties, Oba Super 1 and 2 supported lower Striga shoot emergence than the farmers’ practice. However, all the improved varieties exhibited lower Striga syndrome reaction compared with farmers variety. The full ISCOPAs effectively reduced Striga shoot count and weed cover score and resulted in higher maize grain yield than the farmers’ practice. Maximum gross and net revenue were recorded from ISCOPAs containing codal (Metolachlor+Prometryne 1.0+1.0 kg a.i./ha) followed by post-emergence acifluorfen. The treatment also gave the highest return per naira invested in the control of Striga/weed complex.

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.

Arbuscular mycorrhizal fungi-parasite-host interaction for the control of Striga hermonthica (Del.) Benth. in sorghum [Sorghum bicolor (L.) Moench

Five Glomus species (G. intraradices, G. albidum, G. mosseae, G. fasciculatum, and G. etunicatum) were compared against a check [without arbuscular mycorrhizal (AM) fungi, plus Striga] and control (without AM fungi or Striga) treatments for the control of Striga in a tolerant sorghum variety (War-wara bashi) in an experiment carried out in 12-cm-diameter clay pots. The experiment was carried out in a controlled growth chamber. G. mosseae significantly reduced the number of Striga emerging per plant, increased plant growth, shoot and total dry matter yield of sorghum, did not affect the root dry matter compared with the other AM fungi species, but had a comparable effect to the control treatment. All the AM fungi except G. mosseae, and also the Striga-infested treatment, increased the root:shoot ratio compared to the control treatment. The percent reduction (62%) of Striga emergence after G. mosseae inoculation resulted in about a 30% increase in total dry matter yield of sorghum over the control, while the total loss in dry matter yield of sorghum due to Striga infestation was 36%. Root colonization of sorghum by AM fungi was highest for G. mosseae (44%) followed by G. intraradices (24%) and G. albidum (23%) then G. fasciculatum (18%), with the lowest recorded for G. etunicatum (14%). No colonization of Striga roots was observed. The potential of AM fungi to reduce or to compensate for Striga infestation could be important for soil management, especially in the tropics, and for the reduction of Striga-resistant varieties of sorghum which are mycorrhiza-responsive.

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.

On-farm evaluation of improved maize varieties intercropped with some legumes in the control of Striga in the Northern Guinea savanna of Nigeria

Witchweed ( Striga spp.) is one of the major threats to cereal production in the West African savanna. Two on-farm trials were conducted in 1999 and 2000 at Layin Taki and Kayawa to evaluate the use of tolerant maize varieties intercropped with some legumes in the management of the parasitic weed. Striga incidence and maize reaction score were significantly ( P=0.05) reduced by the variety Acr.97 TZL Comp. 1 intercropped with soyabean (var. SAMSOY II) and groundnut (var. RMP-91) in both years and by Oba Super 1 intercropped with the two legumes in 2000. Oba Super 1+RMP-91 increased cob numbers and weight and grain yield over the local cultivar in the two years. Acr.97 TZL Comp. 1+RMP-91 increased cob number and weight in both the years and grain yield in 1999. Grain yield and cob weight had significant ( P=0.05) negative correlations with Striga incidence and crop reaction score. Intercropping the two improved varieties with either soyabean or groundnut was more profitable than the local cultivar grown alone.

Legume seed flavonoids and nitrogenous metabolites as signals and protectants in early seedling development.

Flavonoids and nitrogenous metabolites such as alkaloids, terpenoids, peptides and amino acids are major components of plant seeds. Conjugated forms of these compounds are soluble in water, and therefore, are easily released as chemical signals following imbibition. Once in the soil, these metabolites are first in line to serve as eco-sensing signals for suitable rhizobia and arbuscular mycorrhizal (AM) fungal partners required for the establishment of symbiotic mutualisms. They may also serve as defence molecules against pathogens and insect pests, as well as playing a role in the control of parasitic members of the family Scrophulariaceae, especially Striga, a major plant pest of cereal crops in Africa. Seed metabolites such as flavonoids, alkaloids, terpenoids, peptides and amino acids define seedling growth and, ultimately, crop yields. Thus, an improvement in our understanding of seed chemistry would permit manipulation of these molecules for effective control of pathogens, insect pests, Striga and destructive weeds, as well as for enhanced acquisition of N and P via symbioses with soil rhizobia and AM fungi.

Intercropping controls Striga in maize based farming systems

Intercropping maize was evaluated as a means to reduce Striga emergence and survival and improve overall farm productivity. Field experiments were conducted at two sites in western Kenya from 1996 to 1999 to study the effect of eight different intercrops on Striga populations and crop yield. Factors studied included relay or simultaneous planting of maize or intercrops, ridge and flat planting systems and in-row or between-row planting of intercrops. Mode of planting maize and intercrops (ridge/flat or in/between row planting) did not affect maize grain yield. However, ridge planting improved intercrop yields and reduced Striga densities. Peanut, bean, yellow gram, bambara nut and soybean intercrops produced considerable yields and increased the overall productivity of the cropping systems. The increase in output of farm produce ranged between 40% and 120% depending on the local agro-ecological conditions. Of the most productive intercrops, yellow gram provided the most stable Striga control over years and locations comparable only to cowpea. Peanut, bambara nut, bean and soybean were more variable in terms of Striga suppression in maize depending on the specific agro-ecological conditions. Shading, higher humidity and lower temperatures under the intercrop canopies were most likely the mechanisms which caused the decline of Striga numbers in intercropping systems. Intercropping can be an effective component of an integrated Striga control program compatible with the limited resources of small-scale subsistence farmers. In the long term, however, Striga populations can only diminish if intercropping is combined with handweeding of mature Striga plants to avoid the replenishment of the Striga seed bank in the soil.

Response of maize varieties to transplanting in Striga-infested fields

Striga hermonthica reduces cereal yields in large areas of sub-Saharan Africa. Recent research has shown that transplanting maize seedlings that are more than 15 d old reduced Striga emergence and improved crop yields. Field experiments were conducted in 1998 and 1999 to determine whether maize varieties with different maturity periods and susceptibility to Striga parasitism respond similarly to transplanting. There was a considerable difference in Striga emergence between varieties in direct-seeded maize, but transplanting clearly reduced Striga emergence for all varieties. Transplanting of maize in plastic tubes gave the best Striga control until 8 wk after transplanting, whereas transplants from nurseries provided better season-long control. Transplanting improved grain yields 50 to 100% compared with direct seeding for three of the four varieties tested. Only the early-maturing variety ‘Morogoro’ had lower yields with transplanting than with direct seeding, indicating that transplanting caused more stress on the plants than was alleviated by the lower Striga infestation. The two varieties (‘Pioneer 3251’ and ‘H622’) most susceptible to Striga parasitism profited the most from transplanting, and the concomitant reduction in Striga induced stresses. Increases in productivity because of transplanting were associated with increases in biomass or harvest index. Transplanting of different maize varieties under rain-fed conditions has proven to be a biologically efficient method to improve maize yield and reduce Striga infestation within one season.

Integrating long-run biological and economic considerations into Striga management programs

A framework for integrating biological and economic considerations into long-run Striga management decisions is developed. At the center of this approach is a dynamic optimization model comprised of two components: a biological component modeling Striga population dynamics, and an economic component representing the production opportunity set, resource constraints, and price parameters farmers face. The model's potential to inform economic and policy decisions related to Striga control at household and regional levels is discussed.

Habitat management strategies for the control of cereal stemborers and striga in maize in Kenya

Maize is the principal food and cash crop for millions of people in the predominantly mixed crop-livestock farming systems in Kenya. Stemborers and striga (Striga hermonthica) are major constraints to increased maize production in eastern Africa. An intercropping and trap crop system has been developed, using a ’push-pull’ strategy, for the control of stemborers in small scale maize farming systems. The ’push-pull’ strategy involves trapping stemborers on highly susceptible trap plants (pull) and driving them away from the crop using repellent intercrops (push). Napier grass (Pennisetum purpureum Schumach) and Sudan grass (Sorghum vulgare sudanense Stapf.) are used as trap plants, whereas molasses grass (Melinis minutiflora Beauv.) and two species of desmodium (Desmodium uncinatum Jacq. and Desmodium intortum Urb.) repel ovipositing stemborers. The integrated ’push-pull’ strategies were shown to increase parasitism of stemborers through attraction of parasitoids to one of the intercrops, molasses grass. The leguminous intercrop, silver leaf desmodium, drastically reduced damage to maize by the parasitic weed, striga. This aspect was further investigated and developed for integration **********w;th stemborer control. On-farm trials with farmers in Kenya have shown significant yield increases in maize farming.

Imazapyr seed dressings for Striga control on acetolactate synthase target-site resistant maize

High herbicide levels can be localized on or near seed of acetolactate synthase (ALS) resistant maize ( Zea mays). The magnesium salt of imazapyr was optimal for seed dressings (drenching, priming, and coating), for preventing field damage from parasitic Striga hermonthica (witchweed) in three seasons. Striga emerged on untreated maize from 6 to 12 weeks after planting. There was almost no Striga emergence for 3 months on imazapyr-dressed homozygous ALS-R 3245IR maize seeds. Occasional Striga that emerged and flowered formed no seed by harvest. Dust or polyvinylpyrrolidone adhesive coatings were safer in maize and as effective in Striga control as priming. Seed dressing coupled with pulling Striga escapes reduces infestation and can be used to deplete the Striga seed bank until genetic crop resistance becomes available.

Striga asiatica seed conditioning and 1‐aminocyclopropane‐1‐carboxylate oxidase activity

Conditioned seeds of Striga asiatica (L.) Kuntze release ethylene, which elicits germination. We investigated the activity of 1‐aminocyclopropane‐1‐carboxylate (ACC) oxidase and respiration during conditioning. Seeds incubated in vivo with ACC, the substrate for ACC oxidase, produced negligible ethylene at the beginning of conditioning or if they were dormant (i.e. would not germinate after conditioning and treatment with stimulant). Non‐dormant seeds produced 3000 ηL of ethylene/600 seeds/24 h after 12 days of conditioning. In vitro ACC oxidase activity at day 0 of conditioning produced 500 ηL of ethylene/μg protein/h and 8000 ηL of ethylene/μg protein/h after 12 days of conditioning. Incubation of seeds in strigol before protein extraction did not enhance enzyme activity. Seeds released 4000 μL/L CO2 in the first 24 h of conditioning, with the rate increasing to 15 000 μL/L/24 h on day 4 and then remaining roughly unchanged. Maximum in vitro activity of ACC oxidase required ACC, catalase, O2, Fe2+, ascorbate and CO2. In vivo activity of ACC oxidase required ACC and/or germination stimulant(s), suggesting that stimulants may be involved in providing substrates for the ACC oxidase. No difference was observed in the separation of extracted proteins, which suggests that ACC oxidase is activated during conditioning, perhaps as a result of changes in co‐factor concentration. Application of these findings to Striga control is discussed.

Striga control and improved farm productivity using crop rotation

Crop rotations with crops that are non-hosts of Striga were evaluated to reduce the Striga seed bank in the soil and increase farm productivity. Field experiments were conducted at two sites in western Kenya from 1996 to 1998 to study the effect of eight different crop rotations on Striga populations, Striga seed bank in the soil, maize yields and overall productivity of these cropping systems under low-input rainfed field conditions. A variety of crops, such as peanut, soybean, sunflower, pigeon pea showed greater economic potential than maize. The productivity of the best rotation under low soil fertility conditions was five times greater than maize mono-cropping. All crop rotation treatments reduced Striga seed numbers in the soil. Less Striga emerged if maize was planted after a two- season rotation that included pigeon pea. Crop rotation is probably one of the most effective ways to reduce Striga infestations and increase maize yields and income considering the limited resource base of small-scale subsistence farmers in sub-Saharan Africa.

A review of spatio-temporal modelling of quadrat count data with application to striga occurrence in a pearl millet field

This paper describes how spatial statistical techniques may be used to analyse weed occurrence in tropical fields. Quadrat counts of weed numbers are available over a series of years, as well as data on explanatory variables, and the aim is to smooth the data and assess spatial and temporal trends. We review a range of models for correlated count data. As an illustration, we consider data on striga infestation of a 60 × 24 m2 millet field in Niger collected from 1985 until 1991, modelled by independent Poisson counts and a prior auto regression term enforcing spatial coherence. The smoothed fields show the presence of a seed bank, the estimated model parameters indicate a decay in the striga numbers over time, as well as a clear correlation with the amount of rainfall in 15 consecutive days following the sowing date. Such results could contribute to precision agriculture as a guide to more cost-effective striga control strategies.