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African Crop Science Journal
African Crop Science Society
ISSN: 1021-9730 EISSN: 2072-6589
Vol. 4, Num. 2, 1996, pp. 263-273
African Crop Science Journal
Vol.5. No.2, pp. 263-273 1997

FORUM

INTEGRATED MANAGEMENT FOR STRIGA CONTROL IN MALAWI

H. R. MLOZA-BANDA and V. H. KABAMBE^1

University of Malawi, Bunda College of Agriculture, P.O.Box 219, Lilongwe, Malawi
^1 Department of Agricultural Research, Chitedze Research Station, P.O.Box 158, Lilongwe, Malawi

(Received 25 March, 1996; accepted 30 June, 1996)


Code Number: CS96065
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ABSTRACT

In Malawi, Striga asiatica (L.) Kuntze infestation presents a serious constraint to small-scale farmers who must grow cereals, in particular, maize, for food. The successful introduction of highly productive and acceptable flint maize hybrids is under threat from Striga. Little is known of the economic importance of Striga in other host crops. When the control of this weed is reviewed, it appears that information applied on the management of Striga asiatica is largely based on extensive work on Striga species infesting sorghum. It is, however, generally agreed that the best solution in the control of Striga is an integrated approach that includes a combination of methods that are within reach of the farmer and considered worthwhile by the farmer. This paper attempts to mould a topical inventory of work on S. asiatica in the region into a research perspective for the small-scale farmer in maize-based cropping systems in Malawi.

Key Words: Maize, Malawi, small-scale farmer, Striga

RESUME

Au Malawi, l'infestation des cereales par le Striga asiatica (L.) Kuntze, en particulier sur le mais, produit alimentaire de base, constitue une contrainte importante pour les petits producteurs. La reussite des efforts destines e introduire des hybrides de mais acceptables et a haut rendements est ainsi compromise. Il n'existe pas beacoup d'information sur l'importance economique de Striga sur d'autres cultures h™tes. Lorsque l'on evalue les methodes de contr™le de cette herbe, on constate que l'information appliquee a sa gestion est largement basee sur le travail extensif mene sur les especes de Striga infestant le sorgho. Il est, toutefois, generalement accepte que la meilleure solution pour son contr™le s'est trouve dans une approche integree qui comprend une combinaison des methodes abordables par le petit paysan et considerees valorisantes par celui-ci. Ce document, tente de constituer en inventaire des themes de recherches qui peuvent etre menees sur le S. asiatica dans la region, pour les petits producteurs au Malawi, dans des systemes de cultures incorporant le mais.

Mots Cles: Mais, Malawi, petit producteur, Striga

INTRODUCTION

Small farming families occupy 68.7% of total cultivated land in Malawi (ICLARM and GTZ, 1991). They rely on maize and grain legume cropping systems to provide their staple food. Extensive damage and yield losses are inflicted upon these major food crops grown on small scale farms by insects, weeds, and diseases. The problem of pests and diseases and how to control them remains a challenge. There are indications that one of the most important and intractable pest problem in maize-based production systems in Malawi is Striga.

Striga infestation is apparently wide-spread in maize which is the predominant staple cereal crop in Malawi. The detrimental effects of this weed is well known to farmers who associate increasingly wide-spread infestation with declining soil fertility (Riches et al., 1993). This is in agreement with research findings from trials in Malawi (Johnson, 1971; Ngwira and Nhlane, 1986) and other locations in Africa (Hassan et al., 1994). Although there has been some evidence of increased adoption of hybrid maize varieties and inorganic fertilizer in recent years in Malawi (Conroy, 1992), a number of socio-economic factors have forced farmers into using lower rates of fertilizers, maintaining the production of local open pollinated varieties with low yield potential, and weed late and ineffectively (Mkamanga, 1986; Ngwira et al., 1989). As a result of increasing population pressure, crop rotation has been displaced by continuous cropping of maize leading to decline in soil fertility and maize productivity with a concomitant emergence of the Striga problem.

Control of Striga has been attempted through the integration of various measures into simple control packages deemed appropriate to local farming systems (Ogborn, 1987). However, when research on the control of this parasitic weed is reviewed, the most important unsolved Striga spp. control problem in Southern Africa is the attack of Striga asiatica (L.) Kuntze on maize grown by smallholder farmers. It appears that information applied to Striga asiatica is largely based on extensive work on Striga hermonthica infesting sorghum in East and West Africa. There is a persuasive case, however, for increased understanding of these parasitic weeds, and application of a remedy for the small-scale farmer, particularly at a time when increased crop production must be weighed against a deteriorating natural environment exemplified by loss of soil productivity and weather anomalies in the region.

OCCURRENCE OF STRIGA IN MALAWI

Banda and Morris (1985) described Striga as a common and widely distributed weed in Malawi in cultivation as well as Brachystegia woodlands below about 1350 m. Binns (1968) provided the first listing of various genera of the Scrophulariaceae family identified in Malawi based on a floristic survey. The following Striga species were identified in the wild or on cultivated land: Striga asiatica (L) Kuntze (= S. lutea Lour, red witchweed), Striga aspera (Willd.) Benth; Striga gesnerioides (Willd.) Vatke (tobacco witchweed) and Striga orobanchoides Benth. Moriarty (1975) noted the existence of S. pubiflora Klotzsch. while Riches et al. (1987), described the presence of S. euphrasioides in Malawi. The distribution and extent of crop infestation by these species in Malawi is largely unknown except for S. asiatica which is noted to be the most widespread of all witchweeds.

Striga forbesii is described as a pink-flowered giant witchweed with toothed leaves and restricted to moister sites and generally of less common occurrence in the region (Terry, 1988; Musselman, 1994). In Tanzania, it has been reported as a parasite of Setaria sphacelata (Schumach.), rice and sorghum, while in Zimbabwe it has been reported in maize (Terry, 1988). Striga euphrasioides has been reported to be an occasional parasite of rice, maize, sorghum, sugarcane, and wild grasses in both Tanzania and Malawi (Riches et al., 1987). Striga gesnerioides lacks developed leaves and uses scales and stems of the plant for photosynthesis (Musselman, 1994). Further, infestation is restricted to broadleaf crops, mainly cowpea, but also tobacco and sweet potato.

Striga asiatica is prevalent throughout Malawi and the East and Central Africa region where it parasitises upland rice, maize, sorghum, the millets and sugarcane (Terry, 1988). It is known to be the most variable in morphology and taxa. In Malawi, although the red-flowered type predominates field infestations, small proportions of the yellow-flowered types have been noted (Parker and Riches, 1993). Physiological variation within this species has also been reported between populations that form distinct ecotypes differing in morphology, vigour, leaf size, and even host range. Bharathalakshmi et al. (1990) reported work in India which showed S. asiatica populations that specifically attacked sorghum, pearl millet, or Paspalum spp. They suggested that host-preference was based upon different germination stimulant requirements. Parker and Riches (1993) argued that, because of the autogamous nature of S. asiatica, this species adapts less readily from its wild hosts to crop, or from crop to crop; rather, it displays an overlap of different morphotypes in different areas.

The prevalence of Striga has also been ascribed to changes in seasonal weather, such as drought or locations with unreliable rainfall (Ayensu et al., 1984; Ogborn, 1984). Ogborn (1984) gave an example of the savannas of southern Guinea where, in particular areas, successive seasons of maize were completely free from Striga attack, while in another season the maize was widely attacked to the point of total crop failure. The seasonality of the severity of Striga attack has also been observed in Malawi. Banda and Morris (1985) describe Striga as less common in high rainfall areas. This demonstrates the need for a fundamental study of the interaction between climate and incidence of Striga attacks on all susceptible crops. In Malawi, maize is grown in a very wide range of ecologies including areas marginal for its production. If Striga control is to be achieved by the integration of various methods into simple control packages (Ogborn, 1987), then all relevant fundamental information about the ecological range of the parasite and its host species must be included.

ECONOMIC IMPORTANCE OF STRIGA IN MALAWI

The current maize yield levels in Malawi range from 2000 to 3000 kg ha^-1 for hybrids, 1400 to 2400 kg ha^-1 for composites, and 880 to 1300 kg ha^-1 for unimproved (local) maize. It is perceived that with proper production practices, including timely and effective weed control, smallholder farmers' yield can be increased to more than 8000, 5000, and 3000 kg ha^-1 for hybrid, composite, and local maize, respectively (Anonymous, 1994).

Striga species were already clearly recognised as parasitic weed problems in Malawi four decades ago (Anonymous, 1959, 1963) and experimental work on control methods appear to have begun in the early 1960s (Anonymous, 1965). However, data on yield losses caused by Striga on maize in Malawi is very limited. A recent survey showed that 63% of the 295 fields surveyed in Malawi were infested with Striga, with 25% of the plants per field being infested. This represented an average total yield loss of 2.5% for maize production due to S. asiatica occurrence (Kroschel et al., 1996). In the Southern African region, visual estimates for grain yield loss due to Striga in farmers' fields has been put at between 15% (in tolerant cultivars) and 95% (in susceptible cereals) (Obilana, 1988). Estimating yield loss due to Striga within the agronomic weed complex has been reported to be very difficult. Parker and Riches (1993) cited the difficulty of creating Striga-free plots, the serious non-uniformity of natural infestations, both within and between fields, and the confounding micro-variability of soil fertility which interacts with the level of damage caused to host plants. Nevertheless, economic justification for working on the problem arises from farmer perception of the problem (Riches et al., 1993). In heavily infested areas of the Southern Region of Malawi, farmers frequently identified Striga as the most important weed and have described having to abandon fields when infestation levels become severe. Thus, Striga also causes indirect production losses when farmers change production strategies to respond to heavy infestations.

EFFICACY OF AVAILABLE CONTROL METHODS

Resistant varieties. Development of resistant varieties of susceptible plants has been attempted in a number of crops including sorghum, maize, cowpeas, rice, and millet (Pieterse and Pesch, 1983). Resistance in sorghum materials has been confirmed for S. asiatica and S. forbesii in Southern Africa and has been incorporated into testing and breeding programmes (Riches et al., 1987; Obilana et al., 1991). One of the major problems though, is that resistance to witchweed has been associated with low grain yield and poor grain quality (Parker and Riches, 1993).

Considerable variability in resistance or tolerance has been reported in maize for S. asiatica (Ransom et al., 1990). At IITA, some maize lines with partial tolerance and/or resistance to both S. hermonthica and S. asiatica (in U.S.A.) were identified and hybrid lines developed (Kim et al., 1985; Kim and Winslow, 1991). However, there are no maize cultivars specifically developed with resistance or tolerance commercially available in East and Southern Africa (Ransom and Odhiambo, 1995). Development of resistant maize genotypes is further complicated by the existence of biotypes and the presence of three different and economically important Striga species in Africa that infest maize, and the potential build up of the parasite where tolerant maize lines are used (Efron et al., 1989; Parker and Riches, 1993). Ogborn (1987) suggested that sources of tolerance may exist among locally adapted African maize varieties because of the longer exposure to Striga attack. Farmers in the lake region in Kenya reported that their local yellow maize variety showed some tolerance to Striga (Hassan et al., 1994).

It is suggested that combined with other methods of control that prevent the seeding of Striga, resistant varieties of crops are compatible with the low-cost input requirements of the small-scale farmer. Additional research is needed to confirm the role of some genotypic traits of the crop along with other host-parasite interactions and their contribution to Striga resistance (Ogborn, 1987; Ejeta and Butler, 1993; Ransom and Odhiambo, 1995).

Manual methods. Hoe weeding remains the most common method of weed control in Malawi. Farmers will normally weed twice, the second time is through the banking operation where soil is pulled-up the ridge. Inconsistent results have been obtained in Malawi in the effectiveness of hoe-weeding for Striga control. District witchweed control trials were conducted to examine the effectiveness of simple cultural techniques in the control of Striga and in rehabilitating badly infested fields (Anonymous, 1976). Ordinary hoe weeding (twice) followed by spot spraying of Striga with gramoxone every ten to fourteen days showed higher yields than ordinary hoe weeding (twice) alone or hoe weeding coupled with hand pulling. However, earlier results (Anonymous, 1965) had shown that normal hoe weeding gave best average yields contrasted with spraying 2,4-D. It appears that success of hoe weeding in controlling Striga infestation depends largely on the timing of the operation itself which is often done following appearance of weed plants. For Striga, this would mean after the parasite has already attached to the host crop. Hoe weeding seems to be of limited application for Striga control except where any general late weeding is practised and incidental Striga control resulting from it.

Handpulling of witchweed at fourth-nightly intervals till harvest has been recommended for controlling light infestations of the parasite in Malawi (Anonymous, 1974). Several investigators have reported possible benefits from handpulling (Ramaiah, 1985). However, this has been with the larger Striga species, namely, S. hermonthica, S. forbesii and S. aspera. In trials conducted in Malawi on S. asiatica, Kabambe (1991) reported that handpulling did not improve grain yield or reduce Striga incidence. Parker and Riches (1993) suggested that handpulling of S. asiatica plants may be more difficult and expensive because plants are individually small, less conspicuous, and may be much more numerous. The high labour input demanded of handpulling would always lead to control being protracted and therefore ineffective in reducing crop losses (Carson, 1988). Handpulling of S. asiatica, may however, be used as part of a long-term strategy to prevent seeding of residual populations of the weed plants after other control measures have been applied.

Cropping systems. Another means by which the number of witchweed seeds in the soil can be reduced is crop rotation or intercropping with trap or catch crops (Wild, 1948; Robinson and Dowler, 1966; Ogborn, 1987; Parkison et al., 1989; Bebawi and Michael, 1991). Kroschel and Sauerborn (1996) reported a case study which showed that crop rotation with trap crops was thought to be a feasible Striga control strategy by farmers from three districts (out of twenty-four) surveyed in Malawi. The feasibility of using trap crops in rotation is limited to very few areas in Malawi owing to population pressure. The fact that at least three years of rotation are likely to be needed emphasizes the practical limitation of the rotation option (Parker and Riches, 1993). Both trap crops and catch crops do not bring about complete eradication and it may take several years before an effect becomes apparent. However, for heavily infested fields, trap crops can accelerate the depletion of the reservoir of Striga seed in the soil (Robinson and Dowler, 1966).

In Malawi, 21% of the maize crop is grown in association with other crops mainly pulses and oilseeds (ICLARM and GTZ,1991). Inter-cropping maize with these potential trap crops represents a ready made strategy owing to current crop diversification extension messages. Parker and Riches (1993) observed that even though Saunders (1933) showed a considerable reduction in S. asiatica from intercropping maize with cowpea as early as 1933, intercropping has received less research emphasis as a viable method for reducing the weed despite the importance of the intercropping system in many Striga-infested regions. Work on S. hermonthica has shown that intercropping significantly reduced Striga (Carson, 1989; Hassan et al., 1994; Odhiambo and Ransom, 1995). In these studies, intercropping was effective where the intercrop was within the cereal crop row. It is not clear, however, how mixed cropping results in reduced Striga incidence (Parker and Riches, 1993). Further, it is suspected that multiple infestation by the parasites and their wide host range may preclude effective control through the use of crop mixtures or even rotation with trap crops. It also appears there is no clear distinction between trap crops efficacious for Striga asiatica and those for other witchweed species.

Soil amendments. Striga infestation on cereal crops has often been reported to be most severe on soils of low fertility and that nitrogen can reverse this effect (Farina et al., 1985; Ogborn, 1987). However, the results of field trials with basal applications of nitrogen fertilizer over many years in many countries have not been consistent in terms of crop yields or Striga numbers (Parker and Riches, 1993). Only where ÒhighÓ levels of nitrogen have been applied has there been a more distinct and consistent beneficial effect in reducing parasite numbers (Agbobli, 1991). It is also not clear how high levels of nitrogen suppress Striga (Ogborn, 1984; Bebawi, 1987; Parker and Riches, 1993). The reduction in stimulant exudation by the host and the concomitant reduction in numbers of emerged and maturing Striga is reported to be the best documented nitrogen effect (Parker and Riches, 1993).

In Malawi, Ngwira and Nhlane (1986) recommended an application rate of 11,000 kg per hectare of farm yard manure and 100 kg N and 10 kg P ha^-1 from inorganic fertilizers to improve the nutrient status of the soil and to allow the maize crop to compete better against the parasite. Kabambe (1991) reported 50-75% reduction of S. asiatica in maize using 112 kg N ha^-1 as ammonium sulphate. However, even in traditional cattle raising areas of Malawi, there is never enough farmyard manure to protect more than a small fraction of the crop from Striga. In terms of fertilizer, such rates appear uneconomic for the smallholder farmer whose most immediate reason for investing in nitrogen fertilizer is the prospect for increased yield with marginal costs. Data from two long term experiments in Kenya suggest that inputs of fertilizer and organic materials are essential in the control of Striga even while other methods of control to stop Striga reproduction are practised (Odhiambo and Ransom, 1995). There is a clear need now, in Malawi, to integrate recent recommendations on optimum time, rate, and method of fertilizer application in maize into a witchweed control package. The potential role of soil fertility improvement by alley cropping being recommended for, and practised in, different areas in Malawi needs to be explored to restore the fertility of Striga infested soils.

Chemical control methods. Various chemicals, such as fumigants, germination stimulants, antitranspirants, or host-crop seed hardening with organic acids, have been employed to destroy either the quiescent Striga seed in the soil, the germinating seed, or the emerged weed (Lagoke et al., 1991). Invariably, however, no method has yet been devised for making practical use of those natural or synthetic substances under field conditions for the control of Striga (Parker and Riches, 1993).

Most farmers will not for the time being be able to take advantage of herbicides for general weed control owing to the cost of the chemicals, skills required for their safe application, and because they carry a greater risk of crop damage which preclude their use in most mixed crop situations (Parker and Riches, 1993). However, herbicides may be one single solution where it is feasible for them to be introduced, in particular, as an aid to prevent seeding and as an alternative to handpulling for destruction of the emerged parasite.

Herbicides, such as trifuralin and pendimethalin, have been effective against S. asiatica when incorporated shallowly in a layer above the cereal seed by inhibiting shoot growth of the parasite. Translocated herbicides, such as 2,4-D or Dicamba, or soil-acting herbicides, such as 2,4-D itself, MCPA, or benazolin, have shown unsatisfactory results in the control of Striga before or immediately after attachment. The success of soil-acting herbicides largely depends on irrigation or rainfall washing the chemical into the soil at the critical time of Striga germination. The ineffectiveness of translocated herbicides is ascribed to the absence of a phloem bridge in the cereal/parasite haustorium which has been shown to be present in other parasitic relationships such as that between cowpea and S. gesnerioides (Parker and Riches, 1993).

A number of alternative herbicides safe enough for mixed cropping and control of Striga on grass weeds in rotational crops have been evaluated (Ogborn, 1987; Langston and English, 1990). It is particularly significant that, in USA, herbicides are used intensively for control of grass weeds on which S. asiatica might grow and seed (Parker and Riches, 1993). Ogborn (1984) observed that in Africa and Asia, where Striga spp. are endemic, reinfestation from wild hosts may make it very difficult to eradicate the weed.

Biological control. Various insects which feed on witchweed have been described and include: the weevil, Smicronyx spp., the moth, Eulocastra argentisparsa, and the stem mining fly, Ophiomya strigalis (Pieterse and Pesch, 1983). The species of Smicronyx are seed, pod, and root gallers of S. hermonthica and have been observed to cause severe damage reducing seed production by the weed by 80% or more. A butterfly Gunonia orithya, which occurs in Africa and Asia, feeds voraciously on witchweed leaves while O. strigalis only causes superficial damage (Pieterse and Pesch, 1983). Microbial antagonists mainly of the genus Fusarium, have been shown to penetrate and parasitise Striga seeds and attacking germ tubes and Striga shoots thereby reducing seed germination (Kroshel et al., 1996b). A suitable product has not been developed for field testing. The possibility for deliberate use of insects or bioherbicides for classical biological control remains attractive but may not be of immediate importance in Malawi.

STRIGA CONTROL RESEARCH IN MALAWI

A review of Striga control methods at present available to the small-scale farmer shows that there is no single method against Striga that is both effective and economically feasible (Ogborn, 1984). Parker and Riches (1993) argue that there is a feeling that the available solutions are deemed inadequate and that the only hope is some dramatic research breakthrough such as resistant varieties, genetic engineering, or a miracle herbicide. Instead, control measures must involve a long-term, integrated approach. With adequate knowledge about Striga asiatica and its relationship to crops in different agroecological zones in Malawi, it is feasible to support an approach in which Striga prevention and control have companion roles. There are a number of research and extension activities which can generate and deliver information from which farmers in Malawi can be helped towards solution of the Striga problem. These include synthesis and communicating optimum decisions on rotation and/or intercropping maize with trap crops, soil amendment practices to restore fertility, and weed control methods that are relevant to the prevailing farming/cropping systems for Striga control.

Ecogeographical study of Striga in Malawi. First, the occurrence and distribution of Striga asiatica in Malawi is believed to be widespread and severe in some areas while that of other species is largely unknown despite two recent surveys (Kroschel et al., 1996a; Munthali et al., 1992). Little is known about the genetic diversity of Striga in Malawi nor is there descriptive information. Specimen collections have been limited to those at the National Herbarium being preserved for floristic purposes. A comprehensive documentation of species occurrence, distribution, and severity of Striga in Malawi is the basic tenet for research. Second, the general belief that Striga is a problem associated with low rainfall seasons and low fertility soils needs to be validated. Quantified information on the influence of various environmental factors on Striga will be useful for simulating these factors in research methodologies and/or avoiding the occurrence of some factors while formulating cultural practices to reduce Striga attack.

Evaluation of trap crops for Striga control. Kroschel and Sauerborn (1996) argue that in regions with possibilities for marketing Striga trap crops such as cotton or soybeans, farmers have an advantage in their Striga control efforts. However, rotating the cereal and other crops as sole crops in succeeding years rather than as mixed crops every year is viewed as a better option. There exists variations in cropping systems in Malawi for which the adoption of different trap-crop control methods can be expected.

Agroforestry remains a critical component in an integrated approach to soil fertility improvement in maize-based cropping systems in Malawi (Wendt et al., 1991). There is need to investigate the efficacy of multipurpose trees being employed in agroforestry systems in the control of Striga. For instance, Thijssen (1995) reported that trees of the species Croton megalocarpus have been shown to be an effective trap crop, inducing germination of Striga seeds.

Weeding practices. Farmers weeding practices in Malawi seem to be aimed at preventing crop losses from current weed infestation without regard to subsequent infestations. Striga plants may escape the first two weedings in absence of any late weeding or may mature and shed seed where weeded Striga and other weeds are left between crop rows as practised in Malawi (Kroschel and Sauerborn, 1996). Most smallholder farmers only have a hand hoe and family labour at their disposal. There is often conflict in the amount and timing of labour allocated to crop establishment practices such as land preparation, planting and weeding whose labour requirement and timing coincide at the beginning of the season. The notion that more than two weedings, or handpulling, may be required to stop Striga reproduction is not just a biological issue but a socio-economic one as well, as it will entail a shift in the amount and timing of labour requirement for Striga control. Farmers will have to be aware of the biology of Striga, the limitation of present weed control practices, and the long-term practices to be followed to control Striga effectively given the resources at their disposal.

Striga control package for small-scale farmers in Malawi. Several packages for the control of Striga have been developed and tested based on integrated control practices (Carson, 1988; Mboob, 1989; Matthews, 1989; Parker and Riches, 1993; Oryokot, 1994; Odhiambo and Ransom, 1995; Kachelriess et al., 1996). Ogborn (1984) argued that the ability to measure Striga seed populations from the soil seed bank is the only way in which success of integrated control of Striga in smallholder systems can unambiguously be evaluated.

In Malawi, for the small-scale farmer, an effective Striga control programme would include four aspects: (1) elimination or reduction of Striga seed numbers from heavily infested soils; (2) prevention of flowering and seed shed of Striga; (3) restoration of soil fertility for direct benefit of the host crop; and (4) communication and training for Striga control. Lack of relevant information remains a constraint to adoption of perceived Striga control methods. Shaxson et al. (1993) and Debrah (1994) argue that providing information to farmers on why a control method works rather than merely recommending which ones to use is essential for a successful crop protection programme. This is because farmers' knowledge about the parasite is generally very limited. With the present understanding of Striga and potential control strategies, a dissemination package and a training package can be developed for farmers through an interactive and creative process with them, researchers, and extension agents (Kachelriess et al., 1996).

ACKNOWLEDGEMENTS

Thanks are due to the Rockefeller Foundation Forum for Agricultural Resource Husbandry for a preparatory grant that was used in a literature survey and creation of a Striga database at Bunda College of Agriculture, Lilongwe. Comments of Dr. G.K.C. Nyirenda, Dr. S. Snapp, and Dr. S. Waddington during preparation of an earlier manuscript are acknowledged. Thanks are also due to Dr. C. Riches for an earlier synopsis on Striga research in Malawi, to Dr. M.A.R.Phiri for translation of the abstract and to Mr. A.M.Z. Chamango for work on the database.

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Copyright 1996 The African Crop Science Society

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