Cowpea (Vigna unguiculata L. Walp.)

ELSEVIER

Field Crops Research 53 (1997) 187-204

Field Crops Research

Cowpea ( Vigna unguiculata L. Walp.) J.D. Ehlers *, A.E. Hall Department of Botany and PlantSciences, Universityof California, Riverside, CA 92521-0124, USA

Abstract Cowpea (Vigna unguiculata L. Walp.) is a widely adapted, stress tolerant grain legume, vegetable, and fodder crop grown on about 7 million ha in warm to hot regions of Africa, Asia, and the Americas. This review focuses on major breeding achievements, current objectives, and future opportunities for cowpea improvement. Early maturing cultivars have been developed with regionally acceptable grain quality and resistance to some important diseases and pests including bacterial blight (Xanthomonas campestris), cowpea aphid-borne mosaic virus (CABMV), cowpea aphid (Aphis craccivora), cowpea curculio (Chalcodermus aeneus), root-knot nematodes (Meloidogyne incognita and M. javanica), cowpea weevil (Callosobruchus maculatus) and the parasitic weeds Striga gesnerioides and Alectra vogelii. Earliness is important in Africa and other regions because early cultivars can escape drought and some insect infestations, can provide the first food and marketable product available from the current growing season, and can be grown in a diverse array of cropping systems. New early maturing cultivars with indeterminate growth habits have been very effective in the extremely dry and hot environment of the Sahel. Heat tolerant breeding lines have been developed which have markedly higher pod set than most cultivars under high night temperature conditions. Development of cultivars with multiple resistances to biotic and abiotic stresses is an important current breeding objective. Earliness, delayed leaf senescence, and indeterminate growth habit are characteristics which are being combined to improve drought adaptation. In the future, high levels of resistance to very important insect pests such as flower thrips (Megalurothrips sjostedti), maruca pod borer (Maruca testulalis), lygus (Lygus hesperus), and pod bugs (Clavigralla tomentosicollis and others) need to be identified. Genes from wild cowpeas or related Vigna species or genetic engineering may be necessary to develop cultivars with high levels of resistance to several of the major insect pests. © 1997 Elsevier Science B.V.

Keywords: Vigna unguiculata L. Walp.; Cowpea, southernpea; Blackeye bean; Grain legume; Plant breeding

I. Introduction C o w p e a (Vigna unguiculata L. Walp.) is one o f the most widely adapted, versatile, and nutritious grain legumes. The crop is grown on about 7 million ha in warm to hot regions of the world (Rachie, 1985). About two-thirds of the production and more

* Corresponding author. Tel.: + 1-909-787-4401; fax: + 1-909787-4437; e-mail: [email protected].

than three-fourths of the area of production is spread over the vast Sudan Savanna and Sahelian zones of sub-Saharan Africa from Senegal going east through Nigeria and Niger to the Sudan, in Kenya and Tanzania, and from A n g o l a across Botswana to Mozambique. Substantial quantities of cowpea are also produced in South A m e r i c a (largely in semiarid northeastern Brazil), Asia, and the southeastern and southwestern regions o f North America. Cowpeas have been grown in parts of southern Europe (Italy) from before Roman times and attempts are being made to

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introduce the crop to parts of southeastern Europe. Commercial production in the US extends as far north as 40 ° latitude and experimental plantings have been successful as far north as Minnesota (45°N latitude) (Davis et al., 1986). Cowpeas are typically grown in hot low elevation equatorial and subtropical areas, often being replaced by common bean (Phaseolus vulgaris L.) at altitudes above 1300 m, although cowpeas are grown at altitudes up to 1600 m in Kenya and at high elevations in Cameroon. Because of its superior nutritional attributes, versatility, adaptability, and productivity, cowpea was chosen by the US National Aeronautical and Space Administration as one of few crops worthy of study for cultivation in space stations. Cowpea has considerable adaptation to high temperatures and drought compared to other crop species. As much as 1000 kg ha -j of dry grain has been produced in a Sahelian environment with only 181 mm of rainfall and high evaporative demand (Hall and Patel, 1985). Presently available cultivars of other crop species cannot produce significant quantities of grain under these conditions. The crop is tolerant of low fertility, due to its high rates of nitrogen fixation (Elowad and Hall, 1987), effective symbiosis with mycorrhizae (Kwapata and Hall, 1985), and its ability to withstand both acid and alkaline soil conditions (Fery, 1990). Dry grain yields above 7000 kg ha -1 have been achieved in large field plots with guard rows in the southern San Joaquin Valley of California (Sanden, 1993) and growers in this region often obtain yields above 4000 kg ha -1. Clearly, cowpea is both responsive to favorable growing conditions, and well adapted to drought, high temperatures, and other abiotic stress. Dry grain for human consumption is the principal product of the cowpea plant, but leaves (many parts of eastern Africa), fresh peas (the southeastern US and Senegal) and fresh green pods (humid regions of Asia and the Caribbean) are consumed. The crop is also used for green manure (southeastern US and Australia) and fodder (parts of the Sahel). Most cowpea grown in Africa is intercropped with sorghum (Sorghum bicolor L. Moench), pearl millet (Pennisetum glaucum L. R. Br.), maize (Zea mays L.), cassava (Manihot esculenta Crantz) or cotton (Gossypium barbadense L.) (Blade et al., 1997). Sole-crops are becoming important as cowpea

production is commercialized to meet the demands of a rapidly increasing urban population. In Senegal, most cowpea is sole-cropped (Thiaw et al., 1993), in part due to the light sandy soils and availability of easily modified horse-drawn peanut seeders. Animal-draft cultivation is also used to control weeds. In Asia and Brazil, both sole-cropping and intercropping are practised (Pandey and Ngarm, 1985; Watt et al., 1985), while in the US sole-crops predominate, although cowpeas are sometimes planted in orchards between rows of young fruit or nut trees. Cowpea can be a valuable component of crop rotations due to the ability of resistant cultivars to suppress root-knot nematode (Meloidogyne spp) reproduction. The nutritional profile of cowpea grain (Bressani, 1985) is similar to common bean but cowpeas have higher levels of folic acid and lower levels of antinutritional and flatulence producing factors. Dry cowpea grain can be cooked quickly, an important consideration in developing countries with shortages of fuel. Extensive genetic resources are available to researchers through the International Institute of Tropical Agriculture (IITA), Ibadan, Nigeria (about 15,200 cultivated and 1646 wild accessions), the USDA Plant Introduction Station, Griffin, GA, (about 7400 accessions), and the University of California, Riverside (5275 cultivated and 50 wild accessions). Passport and characterization data for the USDA accessions is available from the Germplasm Resources Information Network (GRIN) of the USDA on diskette or through the world wide web (address: http//www.ars-grin.gov). Requests for small quantities of seed may be made to all of these institutions. Cowpea was among the first species to be investigated by geneticists (Fery, 1985), with the accumulated information on the inheritance of traits recently reviewed by Fery and Singh (1997). Cowpea breeding has been reviewed recently (Hall et al., 1997a). This paper will emphasize aspects not covered in these reviews such as major breeding achievements and the current goals of breeding programs, and will provide a practical perspective.

2. Origin and general botany West Africa is a major center of diversity of domesticated cowpea (Ng and Padulosi, 1988). India

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appears to be a secondary center of diversity since significant genetic variability occurs on the subcontinent and it is likely that the crop was first introduced to India during the Neolithic period (Pant et al., 1982). The center of diversity of wild Vigna species is southeastern Africa (Ng and Padulosi, 1988; Padulosi et al., 1997). Vigna unguiculata ssp dekindtiana is thought to be the immediate progenitor of cultivated cowpea as members of this group can be hybridized with cultivated cowpea. Natural hybrids between cultivated and wild (ssp dekindtiana) cowpeas occur and form 'weedy' populations in some parts of West Africa (Rawal, 1976). Despite these introgression events, and the extensive variation in morphological and phenological traits among cultivated cowpea accessions, genetic variability in the cultivated gene pool appears to be limited. In several recent studies assessing genetic variability based on isozymes (Panella and Gepts, 1992; Vaillancourt et al., 1993), seed storage protein diversity (Panella et al., 1993), and chloroplast DNA (Vaillancourt and Weeden, 1992), the cultivated cowpea has been shown to have a narrow genetic base suggesting the crop went through a 'genetic bottleneck' during domestication. Four cultigroups of cowpea are recognized (Baudoin and Marrchal, 1985): (1) unguiculata, which is the common form; (2) biflora or catjang, which is characterized by small erect pods and found mostly in Asia; (3) sesquipedalis, or yard-long bean, also mostly found in Asia and characterized by its very long pods which are consumed as a green snap 'bean'; and (4) textilis, found in West Africa and which was used for fibers which were obtained from its long peduncles.

3. Reproductive biology Floral buds of cowpea are born on peduncles arising in the axils of the mainstem or branches. As the floral buds develop, elongation of the peduncle occurs rapidly, typically reaching 5 to 10 cm in length at anthesis. Floral buds complete their development in one to two weeks. Typically each floral raceme produces 2 to 4 flowers sequentially over several days. After two to four pods are set, further development of other floral buds on the apex of the

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peduncle (raceme) is arrested until the first set pods become mature. The length of peduncles typically doubles after anthesis. Cowpea is highly self-pollinated in most environments, the result of a cleistogamous flower structure and simultaneous pollen shed and stigma receptivity. Flowers are large, about 2 cm in length and breadth (when open), and typically purple or white. The style and stigma are surrounded by anthers tightly enclosed in a straight keel. Anther dehiscence and pollination of a particular flower normally occurs in the early morning on the day that the flower opens. Flowers open only once, and remain open for several hours. Stigmas become receptive about 12 hours before anther dehiscence, which is useful in making artificial hybrids. Occasionally, significant but low levels of outcrossing occurs in breeding nurseries and seed production fields, which may be due to visitation by large bees.

3.1. Artificial hybridization Artificial hybridization is relatively easy due to the large size of the keel, anthers and style, and the fact that the keel is straight, unlike many other grain legumes which have small flowers and curved or coiled keels and stigmas. The keel of cowpea is easily opened with tweezers revealing nine readily accessible anthers. The first author has had greater than 90% hybrid pod set using the following method in a greenhouse. In the late afternoon on the day prior to pollination, floral buds that are destined to open the following morning are identified and emasculated. Emasculation is performed by opening the sepals enclosing the keel, opening the keel and removing anthers. All operations are performed with tweezers. Care must be taken that anthers are not broken, which could result in self-pollination since the stigma is receptive by this time. Emasculated flowers are left uncovered and tagged for easy identification the following morning. In early to midmorning, pollination is achieved by taking open flowers of the desired male parent and brushing pollen from the burst anthers and style of the male parent onto the emasculated flower. Male flowers should be chosen that are shedding copious pollen. Pollinations performed after 10 am are less successful than pollinations performed between 7 - 1 0 am.

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An alternative method is to collect open flowers of the male parent in the early morning, refrigerate them at 4 to 10°C until late afternoon. At this time, mature but unopened floral buds are chosen, emasculated, and then pollinated with the male flowers. Night temperatures greater than 20°C impair microsporogenisis leading to formation of indehiscent anthers and pollen with low viability (Warrag and Hall, 1983; Ahmed et al., 1992). Thus hybridization efforts are most effective in environments with night temperatures cooler than 20°C. Greenhouses with night temperatures between 13 to 18°C have provided effective environments for artificial hybridization of cowpea. 3.2. Cytogenetics

Cowpea is diploid with 2 n = 2 x = 2 2 chromosomes (Faris, 1964) that are small and difficult to manipulate (Saccardo et al., 1992). Advanced cytogenetic techniques such as fluorescent staining of chromosomes, silver staining of nucleolar organizing regions, and in situ hybridization are beginning to be employed and promise to be useful to plant breeding programs in the future (Galasso et al., 1997). A linkage map for cultivated cowpea has been constructed that spans 916 cM over 12 linkage groups and includes 133 RAPDs, 19 RFLPs, 25 AFLPs, and 3 morphological markers (Menrndez et al., 1997).

4. Major cowpea breeding achievements The development of productive early maturing cultivars with an array of regionally preferred seed types and resistance to at least some critical diseases and pests has been one of the major recent achievements in cowpea breeding (Singh and Ntare, 1985). Such early cultivars can produce grain yields of 2000 k g / h a in as few as 60 to 70 days in many of the cowpea production zones of Africa. This compares with a continent-wide average grain yield of less than 300 kg/ha. In many parts of Asia where intensive year-round cultivation of crops is practiced, such early maturing cultivars are grown in rotation with staple cereal crops (Pandey and Ngarm, 1985). Early maturity is an important characteristic for reasons that differ with the region. Early maturing

cultivars can escape the end-of-season drought that often occurs in semiarid zones such as the Sahel. This has helped increase or at least stabilize cowpea production in the face of a continuing long-term drought in this region (Cisse et al., 1995, 1996). Since 1968, the onset of rainfall in this region has been late by historical standards with traditional varieties beginning to flower far too late. Early cultivars also are valuable in the Sahel because they produce the first food available in the current growing season, thus breaking the hungry period when grain stores from the previous years harvest are likely to be low. Such early crops can also command premium prices both as fresh peas and dry grain. In other locations, especially areas in East Africa which have 'long' and 'short' seasons of rainfall, early cultivars are used in areas where the 'short rains' are too short for most other crops. In some regions, early cowpea cultivars are interplanted into cereal crops and mature before the cereal crop. Early cultivars also are grown on residual moisture following rice cultivation or in flood plains after water recedes (Ashraf, 1985; Pandey and Ngarm, 1985; Ntare and Williams, 1993). In some environments, when early flowering cultivars are planted early, they escape damage from insect pests such as flower thrips and pod bugs. The International Institute of Tropical Agriculture (IITA) and the Institut Srnrgalais de Recherches Agricoles (ISRA) have been at the forefront in developing early maturing high yielding and pest resistant cultivars. Important examples of early maturing cultivars developed by IITA and ISRA are described that were released in countries in Africa, Asia, and Latin America. The breeding line IT84S-2246, which was developed at IITA, has large seed with brown color and a rough testa that is desired in many parts of West Africa, matures in about 70 days, and has a combination of resistances to cowpea yellow mosaic virus (CYMV), anthracnose (Colletotrichum lindemuthianum (Sacc. and Magnus) Lams.-Scrib.), cercospora leaf spot (Cercospora canescens Ellis and G. Martin), web blight fungus (Rhizoctonia solani Kuhn), bacterial pustule (Xanthomonas campestris pv vignaeunguiculatae Patel and Jindal), cowpea aphid (Aphis craccivora Koch), and the cowpea storage weevil (Callosobruchus maculatus Fabricius). In addition, it has moderate resistance to sev-

J.D. Ehlers,A.E. Hall~Field CropsResearch53 (1997) 187-204 eral other foliar diseases and flower thrips (Megalurothrips sjostedti Trybom) and high yield potential across a wide range of production zones. This line also exhibits a high level of resistance to two important root-knot nematodes (Meloidogyne incognita Kofoid and White and M. javanica (Treub) Chitwood) in California (Roberts et al., 1997). The cultivar 'Bambey 21' was developed by ISRA and released in Senegal in 1975 (S~ne and N'Diaye, 1974) and was perhaps the first early cowpea cultivar to be adopted by farmers. Two early semi-erect indeterminate cultivars, 'Mouride' (Cisse et al., 1995) and 'Melakh' (Cisse et al., 1996) were released recently in Senegal and have produced 1000 kg ha-1 in dry Sahelian environments where traditional land races mainly produce hay. Mouride has resistance to the parasitic weed striga (Striga gesneroioides (Wild.) Vatke), cowpea weevil, bacterial blight (Xanthomonas campestris pv vignicola (Burkholder) Dye), and cowpea aphid-borne mosaic virus (CABMV), while Melakh, which is several days earlier than Mouride, has resistance to cowpea aphid, bacterial blight, and CABMV. These new cultivars complement each other and could have substantial impacts on cowpea production when grown together in the Sahel. Adoption of early cultivars in Africa has not been rapid. Reasons include ineffective extension efforts, and other social and economic factors. One problem is that high density, sole-crop planting of these cultivars is required to achieve high grain yield and changes in production practices have not occurred on a large scale. Some early-erect cultivars were not adopted because they were sensitive to midseason stresses (Thiaw et al., 1993). For some farming systems with low planting density, traditional late-maturing spreading cultivars can be more effective than early cultivars. IITA has developed a range of advanced breeding lines for traditional low density intercropping systems. These lines have strong resistance to cowpea aphid, diseases, the parasitic weeds striga and alectra (Alectra vogelii Benth.), moderate levels of resistance to cowpea weevil and flower thrips, and low levels of resistance to maruca pod borer (Maruca testulalis Geyer), and pod sucking bugs (e.g. Clavigralla tomentosicollis Stahl., Nezara viridula L.) (Singh et al., 1997).

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Several other national and international institutions in Africa have made important contributions to the improvement of cowpea. In Kenya the National Program has released several popular varieties, including 'Machakos 66' and 'Katumani 80'. These varieties are well adapted to elevations between 1000 to 1600 m and are drought resistant. The International Centre of Insect Physiology and Ecology (ICIPE), based in Nairobi, Kenya has released a set of lines selected for insect resistance from East African landraces, including two lines with strong resistance to the cowpea aphid (Pathak and Olela, 1986). The line ICV1, developed by ICIPE, is an early spreading cultivar that escapes both drought and insect damage. In the southeastern US, high yielding cultivars with specific grain quality characteristics have been developed for the canning and freezing industries. These cultivars have resistance to root-knot nematodes and several major bacterial, fungal, and viral diseases. (Fery, 1990). The variety 'Bettergro Blackeye', developed by the US Vegetable Laboratory in South Carolina (Fery and Dukes, 1993), has a high level of resistance to cowpea curculio (Chalcodermus aeneus Boheman), as well as high yield, erect plant type, synchronous flowering and pod maturity, and excellent seed quality. The curculio resistance of this cultivar is due to a pod factor that inhibits pod wall penetration by the adult insect resulting in 75% to 98% fewer larvae than susceptible cultivars (Cuthbert et al., 1974). Higher levels of resistance to this pest are needed if insecticides are not going to be used because consumers demand a virtually insectfree product. Two traits that are important for their consumer appeal are the green seed coat (Chambliss, 1974) and green cotyledon (Fery et al., 1993). Both traits are controlled by single recessive independent genes (Chambliss, 1974; Fery and Dukes, 1994) and give dry imbibed cowpeas the look of fresh harvested 'peas'. This allows growers to harvest the crop as dry grain, which is less costly and results in less seed damage and waste than harvesting fresh mature-green peas. 'Freezegreen' (Chambliss, 1979) and 'Genegreen' (Chambliss and Hunter, 1992) have the green seedcoat trait, while 'Bettergreen' (Fery et al., 1993) possesses the green cotyledon trait. Currently, no cultivars are available that have both y,~tits though

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advanced lines with both characteristics have been developed (R.L. Fery, personal communication, 1996). These advanced lines have deeper green color than lines having either trait individually. In the southwestern US two blackeye cultivars, 'CB46' and 'CB88' (Helms et al., 1991a; Helms et al., 1991b) have been developed with resistance to Fusarium wilt (Fusarium oxysporium Schlechtend.: Fr. f. sp. tracheiphilum (E.F. Smith) W.C. Snyder and Hansen), and the root-knot nematode Meloidogyne incognita, and which also have greater yield potential than the standard cultivar 'CB5' (Hall and Frate, 1996). Viral diseases of cowpea have been an important problem in Brazil (Watt et al., 1985). The new cowpea cultivar 'BR17 Gurgu~ia' has resistance to Cowpea Severe Mosaic Virus, Cowpea Golden Mosaic Virus and several potyvirnses, as well as desirable grain quality, high yield potential and yield stability (F.R. Freire-Filho, personal communication, 1995). In India, breeding programs have developed an array of early erect cowpea cultivars for diverse environments and uses (Mishra et al., 1985). Fresh green pod production is important in India. 'Pusa Komal' is a bush cultivar of this type with resistance to bacterial blight (Singh and Patel, 1990). The bush type vegetable-pod cultivar No. 750, developed by the Indian Agricultural Research Institute, New Delhi, and registered as UCR 193 (Patel and Hall, 1986), yielded 84% more than the standard US cultivar 'Snapea' producing an average of 19 t ha-1 of fresh pods over two seasons in high density, irrigated plantings in California (Kwapata and Hall, 1990). Strong resistance to Asian and African biotypes of the cowpea aphid has been identified and incorporated into breeding lines at IITA (Bata et al., 1987). This resistance is inherited as a single dominant gene (Bata et al., 1987; Ombakho et al., 1987; Pathak, 1988), and seedling screening methods for aphid resistance are highly developed and effective (Hall et al., 1997a). Aphid resistance is particularly important if insecticide usage is to be reduced. Aphids commonly attack early in the season, necessitating insecticide application before other pests have appeared in damaging numbers. This can reduce or eliminate pest predator populations increasing the severity of later

attacks by other pest species and necessitating further use of insecticides. The California biotype of the cowpea aphid and newly identified but apparently not widely distributed African biotypes overcome existing sources of resistance. Screening at the University of California, Riverside has revealed that cowpea accessions TVu 8016 and TVu 8381 have resistance to the California biotype of the cowpea aphid (unpublished data of A.N. Eckard, 1996). A moderate level of resistance to flower thrips identified in the accession TVu 1509 (Singh and Jackal, 1985) has been extensively used by the cowpea breeding program at IITA. This resistance has not been effective where flower thrip pressure has been high, and higher levels of resistance to this pest are needed to prevent significant yield losses. Resistance to the cowpea storage weevil (Singh and Jackal, 1985) has been incorporated into a number of breeding lines and cultivars by IITA (Singh, 1993) and ISRA (Cisse et al., 1995). The resistance provides effective protection from weevil damage for about two months, but after 6 months of infestation levels of damage in resistant cultivars approach that found in susceptible cultivars. This resistance is useful in many developing countries but alone provides insufficient protection to ensure the high quality of grain required in some markets.

4.1. Germplasm evaluation and enhancement Breeders have made relatively little use of the numerous accessions of cowpea maintained in collections. Lack of information about these accessions, regional requirements for specific seed quality and agronomic characteristics, and the low frequency of desirable progeny that often result from crosses between such accessions and adapted cultivars or breeding lines has discouraged the use of exotic parents in breeding programs. Valuable genetic variability exists in these collections and research has been conducted on how to effectively incorporate exotic cowpea germplasm into breeding programs (Ehlers, 1984). Three-way crosses of the type (adapted X exotic) X adapted can result in populations with both high average performance and high genetic variability (Ehlers and Foster, 1993). The USDA-ARS Vigna Germplasm Committee is promoting cowpea germplasm evaluation and en-

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hancement activities. A diverse core collection of about 1,000 accessions was recently chosen to represent the diversity available in cowpea and it is planned that this core collection will be evaluated for important traits over the next several years. Due to the evolutionary 'bottleneck' mentioned previously, insufficient genetic variability may exist in the species for pest resistance and to support long term crop improvement efforts. Wild cowpeas possess extensive genetic variability and valuable traits that are not available in cultivated germplasm (Ng and Padulosi, 1988). The incorporation of wild cowpea germplasm is producing uniquely useful genotypes. Increased pubescence, has been transferred to cowpea breeding lines from V. unguiculata ssp dekindtiana var. pubescens at the University of California, Riverside. Other breeding lines developed from three-way crosses of the type mentioned above flower profusely with a unique raceme structure that appears to result in greater pod numbers per peduncle through more sustained flower production at the floral apex. Some lines developed from wild X cultivated crosses have also been discovered to be parthenocarpic (J. Ehlers, unpublished data, 1996). Emasculated, unpollinated flowers on these lines do not abscise but produce seedless pods. Mature parthenocarpic pods are of normal size but contain only small shriveled and poorly developed 'seed'. The 'seeds' within a single pod typically differ in size and apparent stage of development and are strongly attached to the pod. This suggests that partial development of some ovules occurs. These lines could be useful as embryo 'nurse' lines in attempts to cross V. oexillata or other Vigna species which can not be successfully hybridized at this time due to abortion of the hybrid embryo at the globular stage (Barone and Ng, 1988), or in screening for apomixis since any seeds developing in such unfertilized pods could be the result of apomictic seed formation. Apomixis could make possible the development of hybrid cowpeas (Hall et al., 1997a).

5. Current goals of breeding programs 5.1. Insect resistance

Insect damage is the number one constraint for cowpea grain production in most cowpea producing

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regions (Singh and Van Emden, 1979; Daoust et al., 1985) and insect resistance is a key objective of many breeding programs. In most locations in Africa, two well-timed applications of appropriate insecticides will at least double grain yields, illustrating the extent of insect damage to the cowpea crop. Unfortunately, insecticides or the capital to purchase them or application equipment are simply not available to the vast majority of farmers in Africa, and increased use of insecticides could cause major environmental and safety problems. Breeding insect resistant cowpeas would have a revolutionary impact on food availability and nutritional status in many regions. This goal will not be easy, however, as a number of different pests can attack the crop and attacks by any of the major pests can be devastating. For example, if cultivars were developed with high levels of resistance to flower thrips, the flowers and pods which then developed would likely be destroyed by pod bugs and borers. However, resistance to individual pests can reduce the number of sprays needed to obtain optimal yields, and would generally increase yields without insect protection in regions where pest pressure is moderate, as in the case of the Sahel. Insect resistance is an increasingly important consideration in the US because of restrictions or anticipated restrictions on the use of some popular insecticides and the lack of new alternative insect control products registered for use on cowpea. The increased cost of registering new insecticides for use on individual crops has discouraged chemical companies from pursuing such registrations for relatively minor crops such as cowpea. Screening methods have been developed for flower thrips and pod-sucking bugs (Jackai, 1984; Jackai and Singh, 1988), maruca pod borer (Jackai, 1982; Oghiake et al., 1992), lygus bug (Lygus hesperus Knight) (Moshy et al., 1983), and cowpea curculio and stink bugs (Chambliss and Hunter, 1997). Despite the evaluation of hundreds to thousands of cowpea accessions high levels of resistance have not been identified. Recurrent selection is being used to combine low and moderate levels of resistance to flower thrips, pod bugs, and maruca pod borer identified in several genotypes (Singh, 1993), but higher levels of resistance than can be achieved by this approach may be needed. High levels of resistance to flower thrips, pod

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sucking bugs, and maruca pod borer may not exist in the primary genepool of the species, but has been detected in wild cowpeas (Vigna unguiculata ssp dekindtiana, var. pubescens, and ssp tenuis), and other cowpea relatives including V. vexillata L. (Ng, 1990), despite the fact that relatively few accessions have been evaluated. Attempts to hybridize cultivated cowpea with V. vexillata have not been successful to date despite a large scale effort at IITA and the employment of techniques such as embryo rescue. Genetic engineering offers hope for the development of insect resistant cowpeas. Candidate genes for transformation include the alpha-amylase inhibitor gene for cowpea weevil resistance and Bt genes for maruca pod borer resistance (Monti et al., 1997; Murdock, 1992). Progress is being made in the procedures necessary for transfer of insect resistance traits, especially transformation (Kononowicz et al., 1997) and regeneration and the successful development of genetically engineered cowpeas is expected in the near future. Resistance-breaking biotypes of some cowpea pests have already been observed. Resistance to the cowpea aphid is biotype specific with the strong resistance identified in Africa completely ineffective in California (Martyn, 1991). Resistance-breaking biotypes of cowpea weevil have been found in nature, and selection experiments have shown that weevil biotypes can be selected such that they are able to feed as well on a resistant cultivar as on a susceptible cultivar (R.E. Shade, unpublished data, 1995). Virulent strains of root-knot nematode (M. incognita) have been found in farmers fields in California which overcome the resistance in commercial cultivars (Roberts et al., 1995), and it is possible to select similar virulent strains of this nematode in controlled experiments (P.A. Roberts, personal communication, 1996). Thus, there will be a continued need to search for and utilize new sources of resistance. Pyramiding and selective deployment of resistance genes should also be considered in some situations. However, resistance to highly mobile polyphagus pests such as lygus and pod sucking bugs may be durable since cowpea crops usually are dispersed in large areas of other crop species and represent only a small part of the food supply of these pests.

5.2. Resistance to viral, fungal, pathogens

and bacterial

Resistance to viruses is an important objective of cowpea breeding programs in Brazil, Africa, and parts of the southeastern US. Important viruses include cowpea aphid-borne mosaic virus (CABMV), blackeye cowpea mosaic virus (B1CMV), cucumber mosaic virus (CMV), cowpea mosaic virus (CPMV), cowpea severe mosaic virus (CSMV), southern bean mosaic virus (SBMV), cowpea mottle virus (CPMoV), cowpea golden mosaic virus (CGMV) and cowpea chlorotic mottle virus (CCMV). The seed-borne viruses (CABMV, B1CMV, CMV, CPMV, CSMV, SBMV, and CPMoV) are particularly damaging since infection can result in early season spread of the virus to neighboring plants. Sources of resistance have been identified to all of these viruses (Hampton et al., 1997) and cultivars with resistance to several viruses have been developed at IITA (Singh, 1993). The presence of different strains of CABMV in Senegal and Nigeria that overcome resistance complicates breeding efforts, but breeding lines have been developed that have resistance to multiple strains of this virus as well at to other viruses. IT86D-880, developed at IITA, is exceptional in that it has resistance to four strains of CABMV, cowpea yellow mosaic virus, CYMV, and cowpea mild mottle virus, CMMV. Cowpea stunt disease, caused by a dual infection with B1CMV and CMV, is particularly severe in its effects (Kuhn, 1990). Several fungal and bacterial diseases are important in Africa (Emechebe and Florini, 1997), and their incidence and severity varies with the amount and distribution of rainfall (Williams, 1975; Singh, 1993). Septoria leaf spot (Septoria vignae P. Henn and S. vignicola Vasat Rao, S. Kozopolzanii Nikolajeva), scab (Sphaceloma sp.), brown blotch (Colletotrichum capsici (Syd.) Butler and Bisby and C. truncatum (Schwein.) Andrus and W.D. Moore), and cercospora leaf spot are important in the Guinea Savanna; cercospora leaf spot, bacterial blight and ashy stem blight caused by Macrophomina phaseolina (Tassi.) Goid., are important in the Sudan Savanna; and ashy stem blight is important in the Sahel. Bacterial blight, scab, cercospora leaf spot, powdery mildew (Erysiphe polygoni DC), Fusarium

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wilt, and ashy stem blight are important diseases in Brazil (Lin and Rios, 1985). Sources of resistance to diseases have been identified and screening techniques developed for: anthracnose (Adebitan et al., 1992), cercospora leaf spot (Fery and Dukes, 1977), septoria leaf spot, rust (Uromyces appendiculatus (Pers.: Pers.) Unger), and brown blotch (Abadassi et al., 1987; Adebitan et al., 1992). Ashy stem blight is important in Africa and India but to-date no genetic resistance to this disease has been identified. Bacterial blight is an important disease in the southeastern US, and sources of resistance are known, but little progress has been made in incorporating resistance to this disease into commercial cultivars (Patel, 1985).

gene does not provide protection from virulent populations of M. incognita recently identified in California (Roberts et al., 1997), or from M. javanica isolates from this state. The IITA breeding lines IT84S-2049 and IT84S-2246 have strong resistance that is effective against these virulent isolates of M. incognita and M. javanica (Roberts et al., 1997). The strong resistance in IT84S-2049 is conferred by a dominant allele of the Rk locus, or by another gene very tightly linked to Rk within 0.17 map units (Roberts et al., 1996). Thus Rk may be a complex nematode resistance locus, analogous to those reported for other plant pathogen-host combinations (Roberts et al., 1996). This resistance is being incorporated into adapted breeding lines in California.

5.3. Resistance to parasitic weeds

5.5. Multiple pest and disease resistance

The parasitic weeds Striga gesnerioides (Wild.) Vatke and Alectra vogelii Benth. cause considerable damage to cowpea in Africa, especially in the Sudan Savanna and Sahelian regions, and in the Guinea Savanna, respectively. Several sources of resistance to these parasites have been identified and the resistance has been incorporated into advanced breeding lines by IITA (Singh and Emechebe, 1997) and by ISRA into a cultivar for Senegal (Cisse et al., 1995). Striga exhibits strain variation such that cultivars that are resistant in one location may be susceptible in another (Lane et al., 1994). Genetic studies have shown that three dominant, nonallelic genes confer resistance to different striga biotypes but that the mechanisms differ (Singh, 1993). A different set of three dominant, nonallelic genes have been identified for resistance to alectra (Singh, 1993). Accession B301 is an important source of resistance to both striga and alectra, but has extremely poor agronomic traits.

Until recently, the focus of many cowpea improvement programs was the transfer of individual disease or pest resistance factors from an exotic to an adapted genetic background. Breeders lacked an array of well adapted pest and disease resistant parental lines that could be recombined to generate cultivars with multiple resistances. Such elite lines now exist in many programs and the challenge is to incorporate all of these desirable traits into individual cultivars with acceptable grain quality and adaptation to an array of farming systems and environments. In California, the immediate challenge is to develop cultivars that combine resistance to several races of two species of root-knot nematodes, two races of fusarium wilt, cowpea aphid, a disease that causes early senescence (Erwin et al., 1991), and tolerance to heat. These traits now exist individually, or in some cases doubly in elite backgrounds, but no single elite line has all of the desired traits. A similar situation exists in Senegal where cultivars and advanced breeding lines have been developed that collectively have resistance to aphids, cowpea weevil, striga, CABMV, and bacterial blight but no one line has all of these resistance traits.

5.4. Resistance to nematodes Root-knot nematode resistance in cowpea was described as early as 1902 and a number of resistant cultivars such as 'California Blackeye No. 5' (CB5), 'Mississippi Silver', and 'Colossus' have been developed. Virtually all these cultivars rely on the Rk gene, which provides protection against a broad spectrum of Meloidogyne species. However, this

5.6. Seed characteristics Cowpea exhibits a range of individual seed weight from slightly less than 100 mg seed -1 up to nearly 300 mg seed -1, although cowpea cultivars used

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primarily for their dry grain typically weigh between 130 and 250 mg seed -1 . Cowpea grain consumers in most regions prefer relatively large grain ( > 180 mg seed-l), although, it is difficult to breed high yielding, well adapted cultivars with such large grain because most progeny from crosses have smaller seed. Smooth seed coats are preferred in East Africa while rough or wrinkled seed coats are preferred in most areas of West Africa because the seedcoat is easily removed following brief soaking in water. Removal of the seedcoat followed by wet milling is done for making 'akara' and some other popular foods in West Africa. Cowpea used primarily for forage typically have small seed (100 to 130 mg seed- 1). Cowpea cultivars differ in nutritional composition and cooking characteristics. Seed protein of 100 advanced breeding lines developed at IITA ranged from 23 to 33% on a dry weight basis (Nielsen et al., 1993). For these same breeding lines, fat content ranged from 1.4 to 2.7%, and 50% cooking time ranged from 21 to 62 minutes. These results suggest that there is sufficient genetic variability for breeders to develop cultivars with high protein content which cook quickly. Production of cowpea with grain types that suit specific local preferences can limit domestic trade and export opportunities if the preferred type is not desirable in other regions. Development of cultivars with widely accepted seed characteristics could be important for regions that produce large surpluses in some years or that wish to export cowpea internationally on a regular basis. 'Blackeyes' are an important market class of cowpea traded internationally that have large white grain with a black pigmented area (the eye) around the hilum. Large, all-white cowpea seed with a rough seed coat is widely accepted for direct human consumption, and would be acceptable to industry if all-white seed produces a more useful flour after grinding (McWatters, 1985). Pigmented areas of the seedcoat of colored seeds produce dark-colored flecks in flour. Cowpea is used in the production of value-added foods such as cowpea flour for traditional dishes such as akara, moinmoin, weaning foods, extruded snack items, cowpea 'rice' and cowpea 'couscous'. Large-seeded, all-white breeding lines with rough seed coat have been developed at the University of

California, Riverside with a limited backcrossing program using the medium-sized all-white Senegal cultivar 'Bambey 21' as the nonrecurrent parent and several large-seeded California cultivars and breeding lines as recurrent parents. Transfer of the all-white trait is relatively easy because this trait segregates as a single recessive factor in crosses with blackeye seeded lines, and can be scored before flowering because plants exhibiting all-white seed lack the normal red anthocyanin pigmentation at plant nodes (J. Ehlers, unpublished data, 1994).

5. 7. Breeding for plant morphological and phenological characteristics Cowpea exhibits a wide range of plant habits, flowering times, and maturities. Plant architectural traits such as determinacy, branch angle, and internode length interact with flowering time to determine the basic plant size and shape (Ehlers, 1984). Planting density, extent of fruit set, insect damage, temperature, availability of moisture, and other environmental factors also influence plant development, and affect the shape and size of the plant and canopy. Furthermore, plant architecture varies widely by location and planting date if photoperiod sensitivity is present. Cultivars that are early and erect in short-day environments may be significantly delayed in flowering in long-day environments and exhibit a prostrate growth habit. Cowpea cultivars are generally described by their appearance at maturity as being either erect, semierect, semi-spreading, or spreading (prostrate), and as either extra-early, early, medium, or late. These categories are generally useful to breeders and agronomists, but only are valid for specific climatic zones and are imprecise in that they do not denote determinacy, extent of branching and internode elongation, and angle of branching. It would be useful to develop and adopt a set of standardized 'plant types' as has been done for common bean in which ten basic plant types are defined based on type of stem termination and branching pattern. As a general rule, early-erect determinate culfivars may not be effective in production zones where midseason stresses, such as droughts, are common. Medium-cycle, spreading types may be more effective in these zones. Spreading cultivars are well suited to low-density intercrop-

J.D. Ehlers, A.E. Hall~Field Crops Research 53 (1997) 187-204

ping systems (Ntare, 1989) because they can more rapidly achieve complete ground cover, intercept more light, and shade weeds. In higher density intercropping systems, early-erect breeding lines have performed well (Singh et al., 1983; Ehlers, 1994), although the optimum plant type for such systems needs further research. Erect types are needed for mechanized sole-crop production to enable movement of farm machinery down the rows while cultivating, spraying, and harvesting. Synchronous flowering and maturity are other attributes that are useful in intensive mechanized cowpea production because simultaneous pod maturity facilitates efficient 'once-over' harvesting (Fery, 1990). In contrast, sequential flowering and fruiting is useful in developing countries because fresh pods are available over a long period of time and such cultivars can have greater tolerance of midseason stresses than synchronous flowering cultivars. Cowpea is a short day plant and many cowpea accessions are photoperiod sensitive (Ehlers and Hall, 1996; Craufurd et al., 1997). For some genotypes, the degree of sensitivity to photoperiod (extent of delay in flowering) is modified by temperature (Wein and Summerfield, 1980; Ehlers and Hall, 1996). In West Africa, selection for differing degrees of photosensitivity or differences in juvenility has occurred in different climatic zones such that pod ripening coincides with the end of the rainy season in a given locale, regardless of the planting date, which must be varied with the onset of the rainy seasons (Steele and Mehra, 1980). This attribute allows pods to escape damage from excessive moisture and from pathogens. Photoperiod sensitivity when appropriately deployed in a breeding program can be valuable to ensure crop maturity after wet seasons or before drought or cold weather limits crop growth. However, it may constrain the direct usefulness of an otherwise desirable cultivar to a small area of adaptation or even to a specific season within this restricted area. Because of the diversity of photothermal and hydrologic environments and the limited resources for cowpea breeding programs, it will be difficult to breed multiple-pest-resistant cowpea varieties which possess specific photoperiod adaptation for all major production zones. Day-neutral cowpeas have distinct advantages in irrigated production schemes, or in systems with

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predictable season length such as those relying on residual moisture from receding floods or after cultivation of lowland rice (Oryza sativa L.). Development of day-neutral cultivars that are widely adapted has precedence in the development of CB5, which was the dominant cultivar in all areas of California for almost 50 years. The available growing season for cowpea is 120 to 150 days in most production areas of California, yet commercial cultivars produce their first flush of pods in 80 to 90 days. A significant number of growers continue to irrigate their crops for an additional 30 to 60 days and can obtain very high yields if the crop does not senescence after the first flush of pods is produced (Hall and Frate, 1996). This early senescence is caused by a complex of soil pathogens aggravated by plant stress (Erwin et al., 1991). The delayed leaf senescence (DLS) trait prevents early senescence so that a substantial second flush of pods can occur (Gwathmey et al., 1992). This trait has been incorporated into advanced lines which look promising in replicated yield tests. Development of later flowering cultivars would be another approach to better utilize the available growing season. Genotypic differences in onset of reproductive development (juvenility) of about 14 days have been observed among California breeding lines and other accessions evaluated for this trait (Ehlers and Hall, 1996) and presumably this lateness could be transferred to adapted genotypes.

5.8. Drought adaptation Physiological aspects of breeding for adaptation to drought in cowpea have been reviewed recently (Hall et al., 1997a). Selection for early flowering and empirical yield testing of breeding lines under dry production conditions have been successfully used to develop cowpea cultivars adapted to low rainfall areas of the Sahel (Hall and Patel, 1985; Cisse et al., 1995, 1996) and have been recommended for other grain legume crops as well (Subbarao et al., 1995). One trait which may contribute to drought adaptation is the delayed leaf senescence (DLS) trait (Gwathmey et al., 1992). The main feature of this trait is enhanced plant survival after a mid-season drought damages the first flush of pods, which enables a substantial second flush of pods to be pro-

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duced (Gwathmey and Hall, 1992). Cultivars with DLS also have enhanced production of forage because their leaves remain green and attached to the plant until harvest. This trait can be readily manipulated by breeding and is expressed in both California and Senegal (Hall et al., 1997b). Cultivars that flower early, possess the DLS trait, and have an indeterminate growth habit should exhibit drought adaptation and yield stability in many environments. Early flowering is useful in years when the rainy season is short, allowing the production of some food when other crops will fail, while the DLS trait allows the crop to stay alive through midseason drought and recover when rainfall resumes. An indeterminate growth habit makes possible a longer reproductive period that contributes to drought adaptation because in most years periods of water stress do not occur during the entire reproductive period, and such cultivars can resume vegetative and reproductive growth more quickly once moisture stress is alleviated. 5.9. Heat tolerance

Breeding for heat tolerance was recently reviewed (Hall, 1992) with many specific examples from cowpea. Cowpea suffers damage to reproductive processes when night temperatures exceed 20°C (Nielsen and Hall, 1985). Genotypic variation for two of the primary processes disrupted by heat, development of floral buds and pollination, has been described (Warrag and Hall, 1983; Patel and Hall, 1990). Heat-tolerant large-seeded blackeye germplasm has been developed (Hall, 1993) that is effective in both long-day and short-day conditions (Ehlers and Hall, 1997). Tolerance to heat-induced suppression of floral buds appears to be conferred by a major recessive gene (Hall, 1992; Hall, 1993), whereas the ability to set pods under hot conditions is controlled by a single dominant gene (Marfo and Hall, 1992). Heat tolerant breeding lines developed at the University of California, Riverside have produced more than twice the yield of commercial cultivars in two years of field trials conducted in the Coachella Valley of California during the summer season, a very hot environment in the low-elevation desert (Ismail and Hall, 1997). The heat tolerant breeding lines were bred by growing segregating populations in very hot field

environments in the Imperial and Coachella Valleys of California and selecting for flowering and podding intensity (Hall, 1993). While heat tolerant lines, exhibit markedly improved pod set they also suffer significant reductions in number of seeds per pod under high temperature conditions (Ehlers and Hall, 1997; Ismail and Hall, 1997). Preliminary screening of 56 genotypes revealed two accessions, TN88-63 from Niger, and Vita 1 from Nigeria, which had no reductions in numbers of seeds per pod under very hot temperatures. 5.10. Chilling tolerance

Cowpeas are sensitive to chilling temperature with rate of germination and final plant emergence reduced at soil temperature less than 19°C (El-kholy and Hall, 1997; Ismail et al., 1997). Increased chilling tolerance at emergence would be valuable in subtropical zones where it could allow earlier planting in the spring and provide a longer growing season. Also, in subtropical California, cowpeas that are sown early and exhibit adequate emergence usually have greater grain yields than cowpeas that are sown late. Chilling tolerance could also be useful in high elevation (1300 to 1600 m) equatorial locations, such as in the highlands of Kenya, Uganda, and Cameroon, and in cool-season production of cowpea at low elevation tropical areas such as along the river Niger, in the Senegal river basin, and near Lake Chad (Ntare and Williams, 1993). 5.11. Harvesting efficiency

Harvesting costs represent perhaps 10% of the total production cost of cowpeas that are harvested mechanically (Fery, 1990). In California, cowpeas are presently harvested in a three-step process involving: (1) cutting the crop at ground level with a tractor mounted cutter, (2) windrowing, and (3) threshing the crop with a tractor pulled thresher after drying in the field for about two weeks (Hall and Frate, 1996). Direct-combining with grain combines has been attempted but present cultivars are not well suited as their thick stems resist drying and require vigorous threshing, they have variable pod position in the canopy, and the seed is fragile. In addition to reducing costs, direct-combining would also reduce

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production risks because standing crops are much less susceptible to wind and rain damage compared to when the crop is lying in windrows (Hall and Frate, 1996). Genetic variability exists in cowpea to create direct-combine type cowpeas. Our design for a cultivar and management methods effective for directcombining is: (1) an erect plant type which resists lodging, and has pods held uniformly high but within the canopy; (2) a seed that resists damage at threshing due to its round shape and small air-gap between the cotyledons and (3) high density sowing in narrow rows to promote the development of thin stems. The small air-gap trait can be selected by choosing seed with high density, a highly heritable trait (Robertson, 1985), which can be readily measured (WesselBeaver et al., 1984).

6. Design of the complete breeding program Standard breeding methods applicable to self-pollinated crops for the development of pure-line cultivars are commonly used in cowpea improvement and their use in cowpea breeding has been recently reviewed (Hall et al., 1997a). The diversity of biotic and abiotic environments and cropping systems in which cowpea is grown, together with the particular uses made of the crop and the amount and type of resources available to the program will dictate the optimal design of the complete breeding program serving a particular region. Most breeding programs should use multiple methods for different breeding objectives each of which in turn will have a different expected time-frame for development of an improved cultivar. For example, a newly organized cowpea breeding program might have three components to meet short, medium, and long term goals. Breeding methods applicable to short term goals would include selection within locally adapted landraces, backcrossing (especially for transfer of simply inherited traits), and possibly mutation breeding (if the desired character is easy to screen). These methods require relatively little time to obtain fixed lines and because these new lines will closely resemble one of parental cultivars for most agronomic traits, only a limited number of years of field testing would generally be required prior to release. The

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pedigree method is particularly useful for medium term objectives such as the development of cultivars that possess desirable traits presently existing separately in two different parents. Population improvement methods such as recurrent selection are useful for longer term objectives such as incorporating multiple desirable traits into lines, in incorporating exotic germplasm into programs, and to improve insect or disease resistance in cases where only low to moderate levels of insect resistance has been identified. An effective form of recurrent selection for combining multiple desirable traits consists of developing sets of early generation (F 3 or F4) inbred lines from different biparental crosses, screening these to identify individuals (or lines) with the desired new combination of traits, and crossing these selected individuals with selected individuals from a set of lines developed from a different biparental cross made to combine a different set of desirable traits and conducting a further round of selection to incorporate desirable features from all four parental lines. Practical yet creative designs and approaches are required to maximize the usefulness of the limited resources devoted to cowpea improvement. Programs in Africa are particularly limited in infrastructure and monetary resources but may have relatively abundant human labor available and multiple generations can be grown each year in the field if irrigation is available. This type of program can be effective if it has a large well-managed empirical field evaluation program, and progress can be rapid if three or four generations are grown per year. Specialized field nurseries can be implemented at low cost to screen for specific traits. For example, a screening nursery can be planted late without insect protection to subject breeding lines or accessions to uniformly high populations of insect pests. Early planting of diseaseor insect-susceptible 'spreader' rows around blocks of nursery rows can be used to uniformly buildup pest populations. Locations with regularly occurring and high pest pressure can be identified and used for pest resistance screening. Yield evaluations must be conducted in environments and cropping systems that are representative of the region being served by the program (Ehlers, 1994). Cowpea researchers in Africa have often conducted yield evaluations under insecticide protected, sole-crop conditions, but most farmers grow unpro-

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tected intercropped cowpeas. Such testing regimes are unlikely~ to identify superior genotypes for the farmers' cropping system, and grain yield evaluations should be conducted under the predominant systems being served by the breeding program although unprotected sole-crops may be used in early generation breeding nurseries (Ntare et al., 1984; Ehlers, 1994). Exchange of germplasm and information among breeders is a valuable activity, although precautions should be taken to ensure than seed-borne viruses are not spread through these activities. Each year, seeds of promising new breeding lines developed at IITA are formulated into several types of pre-packaged yield trials in the form of 'Cowpea International Trials' and made available to cowpea researchers in 50 countries worldwide (Singh, 1993). The lines in these trials and others made available from IITA are a valuable resource both as potential cultivars and as improved parents for crossing.

7. Future prospects Breeders and agronomists must be familiar with existing local farming practices, systems constraints, and preferences for grain, pod, leaf, or forage quality. They must also look forward and predict likely changes in these factors and initiate long term breeding efforts to accommodate these projected changes. In the face of mounting population pressure, fanning must become more productive in sub-Saharan Africa if this region is to avoid increasing poverty and dependence on food imports and further environmental degradation. This implies that cowpea cultivars will need to be developed which are adapted to more intensive cultivation. Improved hay and dualpurpose cultivars are needed for some regions where livestock are important (Tarawali et al., 1997). As an efficient nitrogen fixing legume, cowpea could have an increasingly important role in maintenance of soil fertility in this region. Rapidly growing urban populations of cowpea consumers in sub-Saharan Africa and other countries of the world should provide strong markets for this crop in the future. In many developing countries, local production excesses can occur which when combined with the lack of effective on-farm storage

practices and the farmers immediate need for cash at harvest time, typically result in dramatic decreases in farmer prices at harvest. This can discourage farmers from allocating more land to cowpea production or from intensifying their cowpea production system in future years. An increased level of cowpea weevil resistance in cultivars and extension of improved storage practices (Murdock et al., 1997) is needed together with the development of more effective systems for marketing cowpea both within and among countries. In the US, total production of cowpea for dry grain has remained relatively constant over the last several decades, even with large increases in productivity (Hall and Frate, 1996). Demand for cowpea appears to be relatively constant and a moderate increase in production depresses prices (Hall and Frate, 1996). The demand for cowpea could be enhanced in the US by marketing efforts to popularize this nutritious grain legume and by research to further develop processed cowpea food products that are attractive to consumers. Insect resistant cowpeas would dramatically increase cowpea productivity in many developing countries and reduce costs, safety hazards, and environmental risks in these and in other countries. High levels of resistance to the major insect pests, especially flower thrips and pod sucking bugs, have not been identified in the cultivated cowpea genepool, yet effective sources of resistance to these pests probably exist in wild cowpeas and in related species such as V. vexillata. Introgression of insect resistance traits from wild cowpeas and efforts to hybridize cowpea with related Vigna species should be pursued vigorously. Biotechnology promises to provide cowpea breeders with effective insect resistance genes in the near future. The transformed genotypes are likely to be adapted to only a few regions, but will be useful parents for breeding programs. Therefore, cowpea breeders need to stay abreast of recent developments in this fast moving area. They should obtain newly transformed insect-resistant genotypes as soon as possible so they can begin to evaluate these genotypes for effectiveness under their conditions. If the insect resistance is effective, they should incorporate it into cultivars suitable for their region of responsibility.

.I.D. Ehlers, A.E. Hall/Field Crops Research 53 (1997) 187-204 This is an e x c i t i n g t i m e to be i n v o l v e d with c o w p e a i m p r o v e m e n t . C o w p e a is a relatively unexploited c r o p c o m p a r e d to other popular grain l e g u m e s and large increases in productivity can be made. R e c e n t contributions f r o m d e d i c a t e d basic and applied scientists in m a n y parts o f the w o r l d h a v e greatly i n c r e a s e d understanding o f c o w p e a , particularly in the areas o f plant stress p h y s i o l o g y , host plant resistance to diseases, insects, parasitic w e e d s and n e m a t o d e s , and genetic resources. B r e e d e r s h a v e d e v e l o p e d an array o f elite cultivars and b r e e d i n g lines that p r o v i d e a strong basis for future crop i m p r o v e m e n t efforts. T h e i n c r e a s e d understanding o f the crop, the avaiiability o f i m p r o v e d g e r m p l a s m , and the potential contributions f r o m b i o t e c h n o l o g y p r o v i d e an opportunity for c o w p e a breeders to dev e l o p cultivars that c o u l d stimulate a ' g r e e n revolution' for this crop in Africa.

Acknowledgements R e s e a r c h that contributed to this r e v i e w was partially supported by the B e a n / C o w p e a C o l l a b o r a t i v e R e s e a r c h Support P r o g r a m o f the U n i t e d States A g e n c y for International D e v e l o p m e n t Grant No. D A N - 1 3 1 0 - S S - 6 0 0 8 - 0 0 . The opinions and r e c o m m e n d a t i o n s are those o f the authors and not necessarily those o f U S A I D .

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