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Striga (Witchweeds) in and Millet: Knowledge and Future Research Needs

A. T. Obilana 1 and K.V. Ramaiah 2

Abstract

Striga spp (witchweeds), are notorious root hemiparasites on cereal and legume crops grown in the semi-arid tropical and subtropical regions of , the southern Arabian Peninsula, India, and parts of the eastern USA. These weed-parasites cause between 5 to 90% losses in yield; total crop- loss data have been reported. Immunity in hosts has not been found. Past research activities and control methods for Striga are reviewed, with emphasis on the socioeconomic significance of the species. Striga research involving biosystematics, physiological biochemistry, cultural and chemical control methods, and host resistance are considered. We tried to itemize research needs of priority and look into the future of Striga research and control In light of existing information, some control strategies which particularly suit subsistence and emerging farmers' farming systems with some minor adjustments are proposed. The authors believe that a good crop husbandry is the key to solving the Striga problem.

Introduction 60%), susceptibility (in about 30%), and resis- tance (in about 10%). On the other hand, in Striga species (witchweeds) are parasitic weeds , susceptibility has been the common reac- growing on the roots of cereal and legume crops tion as resistant varieties are still being identi- in dry, semi-arid, and harsh environments of fied and confirmed. The reaction of millet is tropical and subtropical Africa, Arabian Penin- complex, with ecological zone implications. Re- sula, India, and a small part of USA. In some sistance to Striga has not yet been found in mil- parts of Africa the profusion of witchweeds let, even though millet coexists with sorghum in have serious impact on the socioeconomic life of some environments. Reaction of to Striga is farmers. Heavily infested farms are abandoned not well known, but indications are that suscep- and occasional migrations of farming commu- tibility to Striga parasitism is normal in rice. nities because of Striga has been reported. The The cowpea, Vigna unguiculata, is the legume statement that Striga is a new threat to Africa's most affected by Striga in dry areas of Africa, food crops is not so. It is endemic in Africa's where it is a common food . cereal and legume food crop production. Being an obligate hemiparasite, Striga causes The weed is parasitic to cereals and legumes, tremendous damage to the host before but there is significant variation in the reaction Striga emerges from the soil. Persistent drought of different crop species. Because Striga has worsens the situation. Little attention is paid to evolved, parallel with sorghum, over the centu- it or its control. As a result the Striga problem is ries, the indigenous crop has developed the one of the most serious production problems on whole spectrum of tolerance (on average about cereals and cowpeas south of the Sahara.

1. Principal Sorghum Breeder, SADCC/ICRISAT Regional Sorghum and Millets Improvement Program, PO Box 776, Bulawayo, Zimbabwe. 2. Principal Cereals Breeder, ICRISAT West African Sorghum Improvement Program, B.E 320, Bamako, Mali. Obilana, A.T., and Ramaiah, K.V. 1992. Striga (witchweeds) in sorghum and millet: knowledge and future research needs. Pages 187-201 in Sorghum and millets diseases: a second world review, (de Milliano, W.A.J., Frederiksen, R.A., and Bengston, G.D., eds). Patancheru, A.P. 502 324, India: International Crops Research Institute for the Semi-Arid Tropics. (CP 741).

187 African farmers on impoverished soils in the 500 000 for S. asiatica (several authors) and marginal areas are subsistence-oriented and less 25 000 for S. forbesii (Obilana et al. 1988, pp. 342- resourceful, possibly for survival reasons. They 364). Seeds, small in size, are efficiently dis- recognize the Striga problem, but have no simple persed by man (in use, transport by machinery, control measures to solve it. Costly control and seed movement), by animals (in droppings), methods are available, and are used by farmers and by water (in field erosion). Extensive long- in countries like USA. In developing countries, evity, together with ability to form "biotypes," progressive farmers who can afford to apply fer- "ecotypes," and "crop-specific'' types and its tilizers and follow good agronomic practices ease of dispersal has made necessary serious have fewer problems with Striga. Even in this and significant farming systems changes; sev- situation, use of excess fertilizers in very dry eral small African tribes and family groups have weather may be impracticable. Therefore, the migrated from location to location because of task ahead, to improve the overall productivity Striga. Several cases of farm abandonment or of the farm land and transform subsistence and change in cropping patterns have been reported emerging farmers into entrepreneurs, is tremen- in southern Africa (Obilana et al. 1988, pp. 342- dous. We believe a good crop husbandry is the 364). Some 30 to 40% of total farmlands have key to solving the Striga problem. been abandoned to sorghum or maize cultiva- We review historical developments of formal tion in some countries in western and southern and informal work on Striga reemphasize its so- Africa. cioeconomic implications in terms of its contin- In terms of crop yields, Striga damage has uous spread and its cost in crop loss, and discuss been most significant. In the eastern African re- the latest developments in Striga research. The gion, grain yield loss for susceptible sorghum research needs of priority are itemized and varieties was estimated to be 59% (Doggett the future of Striga research and control is con- 1965). For western Africa, Obilana (1983b) and sidered. Ramaiah (1984) recorded actual yield losses in Finally, control strategies particularly suited sorghum due to Striga damage. Obilana re- to the subsistence and emerging farmers' farm- ported 5% loss of potential yield in resistant cul- ing systems with some minor adjustments are tivars, 95% in susceptible varieties; and 45-63% proposed. in tolerant . Ramaiah (1984) reported yield-loss estimates of 10-35% in experimental plots. Where Striga infestation is intense and Socioeconomic Implications varieties are susceptible, 100% crop losses in farmers' fields are common. However, in most Why research on Striga? The answer to this farmers' plots with mostly tolerant sorghums question lies in the damage done by Striga in and millets, these crops coexist with Striga. In terms of the farming systems, crop losses, and terms of actual monetary values, Striga-caused farm losses (abandonment) in regions of semi- losses between U.S. $28 million and $87 million arid Africa and India where the parasitic weed is annually, are suffered by western African endemic. In these Striga-infested regions, the farmers (Obilana 1983a). Although actual values environment is so harsh and marginal for crop have not been reported for southern Africa, esti- productivity that only a few drought-resistant mated grain-yield losses due to Striga could staple crops are grown—sorghum, millets, cow- reach between 15 and 95% in sorghum, millet, peas, and some maize—and they are extremely and maize varieties and hybrids. parasitized. Food shortages are routine. Sorghum, millets, and maize are principal The socioeconomic impact of Striga infesta- staple foods in most countries of Africa. Consid- tion on cereal crops in western Africa was as- ering that cultivation of these three cereals occu- sessed by Obilana (1983a) and Ramaiah (1984). pies 54.6 million ha and produces 54.3 million t The success of Striga as a parasitic weed is of grain the approximate value of these crops (in due to several of its characteristics, related U.S. dollars) is about $12.438 billion (Ramaiah somehow with the farming systems in semi-arid 1984). As these three crops are major hosts of areas where its hosts are grown, Striga seeds sur- Striga in Africa, and it is known that Striga can vive in arid soils for 15 years. The number of cause yield losses of 100%, this figure could rep- seeds produced per plant ranges from 40 000 to resent the cost of Striga infestation in all African

188 maize, sorghum, and millets fields. Recently, agronomic and cultural practices, e.g., fertilizer Vasudeva Rao et al. (In Press) estimated mean use and crop rotations, were reported by Pear- grain yield loss in sorghum in India to range son (1913). The earlier works concluded that from 14.7 to 32.0% in the rainy season and 21.9 to crop rotations, catch cropping using sorghum 84.5% in the postrainy season. They write that and sudangrass (Timson 1931), and nitrate fertil- potential loss could be total for the crop, with izer were valuable in controlling Striga. average of 19.7% grain yield loss in hybrid sor- Saunders (1933) classified the biology of ghums due to S. asiatica. Assuming only 10% of Striga asiatica, studied the use of catch crops (in- the hybrid sorghum crop is affected, losses of cluding the trap cropping), and pioneered the about 75 000 tons, valued at about U.S. $7.5 mil- selection of resistant varieties in sorghum. This lion occur annually. last activity identified several "resistant" vari- It is possible that Striga could spread to addi- eties, including "Radar" (Saunders 1942). The re- tional areas, especially with the persistent occur- sistance in "Radar" was complex, as it was rence of drought. The parasitic weed is not yet reported to break down, and then again Striga- present everywhere. Unless effective integrated free in recent studies (Riches et al. 1987, pp. 358- control measures are taken as soon as possible, 372). Striga could become a serious threat to the culti- The use of chemicals to control witchweeds vation and productivity of all rainfed cereals in was an early objective. Inorganic herbicides, like the semi-arid tropics (SAT) of Africa. sodium chlorate, were found to selectively kill Similarly alarming is the prospect for le- S. asiatica in maize (Timson 1934). gumes—especially cowpeas. The threat posed to cowpea by Striga is most serious in the semi-arid savannas of western Africa and the dry arid lands of southern Africa. Although exact values Present research of economic losses due to Striga in cowpeas have not been recorded, guestimates and visual ob- Current research into Striga has sought using servations indicate 50-100% yield loss in severe improved techniques to update and confirm ear- infestations (Obilana 1987). Spread of the cow- lier findings; come up with the biosystematics pea Striga into wetter grassland and veld areas and classification of all types of Striga in Africa; has been rapid, confounding the situation, and continue surveying spread of known types and causing much concern. Economic implications possible new types; and continue studies seek- of Striga damage on cowpeas becomes more sig- ing environment-specific integrated Striga-man- nificant in view of the role of the crop in mixed- agement procedures. cropping systems used in the semi-arid and arid Following the findings of earlier workers, af- savanna and veld regions of Africa. ter a lull of about two decades, Visser and Botha (1974), in their chromatographic investigations on Striga seed germination, found that crop-root Striga Research and Control exudates stimulate germination of Striga seeds, and that the stimulant substances can be sepa- Early research rated readily by high-pressure chromatography. Their work illustrated the involvement of sev- Several species of Striga were recognized as se- eral stimulant substances, which need to be rious parasitic weeds as early as 1900 in India, identified and characterized. Africa, and parts of USA. Parker (1983) traced the research history of this parasitic weed, and summarized part of the problem. Latest Achievements As early as 1905, Burtt-Davy in southern Af- rica described witchweeds in the Botanical Musselman's recent book on Striga (1987) pro- Notes of the Transvaal Agricultural Journal, rec- vides "state of art" knowledge on various as- ognizing the species as parasitic. Within a de- pects of the parasitic weed. The attempt here is cade, experiments with Striga were conducted in only to highlight the latest developments, rather the region. Work on control methods—including than to undertake an extensive review.

189 and biosystematics 23 Striga species, suggesting that the African re- gion may be its center of diversity. Lesser cen- In spite of the economic importance of this ge- ters are the southern Arabian Peninsula and nus, relatively little is known about its taxonomy India. In a recent summary of herbarium docu- and biosystematics. The number of species is not ments, Riches et al. (1987, pp. 358-372) and Ob- known. According to Raynal-Roques (1987) ilana et al. (1988, pp. 342-364) found reports of there are about 36 species. Musselman (1987) de- nine of the African Striga species to be occurring scribed 30 species, of which 24 are found in in southern Africa. The distribution and possible western and central Africa alone (Raynal-Ro- hosts of Striga spp found in Africa are listed in ques 1987). Earlier Musselman (1987) indicated Table 1.

Table 1. Distribution and possible hosts of African species of Striga. Species Distribution Host Remarks S. hermonthica (Del.) Western Africa (especially Sorghum, pearl Benth. in Nigeria, Ghana, millet, maize, and Burkina Faso, Niger, wild grasses Chad, Mali, Senegal, and Mauritania); Eastern Africa (especially in Sudan, Ethiopia, Yemen, , and ); and the SADCC (especially in Angola, , and Mozambique) S. asiatica (L.) Kuntze Eastern Africa (Ethiopia, Sorghum, pearl Red-flowered Kenya), more countries in millet, finger types mainly, the SADCC region. Very millet, maize, with occasional limited occurrence in upland rice, orange and western Africa, Burkina , and yellow forms Faso (yellow type) wild grasses S. gesnerioides Western Africa (Nigeria, Cowpea, tobacco, (Willd Vatke = Niger, Burkina Faso, convolvulaceae, (S. orobanchioides) Mali, and Senegal); and fabaceae SADCC region (Botswana, Malawi, Swaziland, Tanzania, Zimbabwe, and Angola?) S. aspera (Willd) Benth Malawi, Tanzania, Rice, wild grasses, Mozambique, Burkina and occasionally Faso, and Nigeria maize, sorghum, and sugarcane S. euphrasioides Benth Malawi, Tanzania, Sorghum, maize, =(S. angustifolia Zimbabwe, Zambia, and sugarcane, (DON) Saldanha) Mozambique upland rice, and wild grasses S. forbesii Benth Tanzania, Botswana, Soxghum, maize, Zimbabwe, Swaziland, rice, and a few and possibly in Angola, wild grasses Zambia, and Mozambique Continued

190 Table 1. Continued Species Distribution Host Remarks

S. bilabiata (Thunb.) In the eastern African Mostly on wild Kuntze lakes region and across grasses western Africa in the Sahel and Sudanian zones; Zimbabwe S. elegans Benth Tanzania, Kenya, Wild grasses Botswana, South Africa, Zimbabwe S. macrantha Benth Sudan, Zimbabwe and Wild grasses Sudanian zone of western Africa S. aequinoctialis Chev. Burkina Faso, Mali, Wild grasses ex Hutch, and Dalz Nigeria, Niger, and Chad (western Africa) S. klingii (Skann) Burkina Faso Sorghum, millet, With small Engler = (S. Dalzielii and wild grasses bluish flowers Hutch.) S. junodkii Schinz Southern Africa, including Mozambique S. hallaei A. Raynal Gabon, Zaire S. fulgens Hepper Tanzania S. elegans Benth Tanzania, Kenya, Cereals and wild Botswana, and South grasses Africa S. chrysantha A. Raynal Zaire and Central African Republic S. brachychalyx Skan Sahelian and Sudanian Cereals and wild zones of Africa grasses S. baumanii Engler Zaire, Kenya, and western Africa S. latericea Vatke 3 Kenya, Tanzania, Ethiopia, Sugarcane and With vegetative and Somalia cereals propagation S. ledermannii Pilger Cameroon S. linearifolia Hepper Western Africa (Schumach et Thona.) =(S. strictissima Skan) = (S. canescens Engl) S. primuloides Chev Cote d'lvoire S. pubiflora Klotzsch Ethiopia and Somalia Status not (=S. agnustoifolia?) confirmed (=S. zanzibarensis Vatke?)

191 Information about this genus is not sufficient bright long-lasting corollas, can be considered to provide comprehensive keys or descriptions, evidence of outcrossing (allogamy). but Raynal-Roques (1987) has made an excellent attempt for the species found in western and central Africa. A Striga identification booklet, Physiology describing the species of agronomic importance, was earlier published (Ramaiah et al. 1983), fol- Many aspects of Striga biology are not fully un- lowed by a recent pamphlet (Obilana et al. 1987). derstood. They include the physiological pro- There is considerable confusion in distinguish- cesses of photosynthesis, respiration, transpira- ing closely related species complexes such as tion, water relations, cause of heavy crop-yield Striga hermonthica - S. aspera - S. curviflora; and reductions, morphology, and analoging of the Striga euphrasioides - S. angustifolia - S. pubiflor - haustorium in relation to its function. The over- S. zanzibarensis. Fairly detailed work and discus- all physiology of resistance or tolerance to Striga sions on taxonomic and biosystematic issues of in cereal species is included. Striga species in southern Africa have been pre- Some understanding of these aspects is pro- sented by Ralston et al. (1986), Riches et al. (1987, vided by the excellent work of Stewart's group pp. 358-372) and Obilana et al. (1988, pp. 342- in University College, London (Press and Stew- 364). art, In Press; Shah et al. 1987; Press et al. 1987a,b). The chromosome number of Striga species is The rates of photosynthesis of Striga species not well studied. Interspecific relationships and were found to be very low in contrast to the the origin of different species are yet to be un- respiration and transpiration rates of higher derstood. Electrophoretic studies, chromosome plants. Stomata of drought-stressed Striga plants studies, and interspecific hybridization should were found to be open in darkness and their provide useful information on these aspects control is poor. This high rate of transpiration is (Musselman, personal communication). The interpreted as a means of maximizing solute preliminary results on crosses between S. her- transfer from host to parasite. Such reliance on monthica and S. aspera indicate that each is cross- high rates of transpiration has suggested use of compatible in either direction. The sympatric na- antitranspirants in Striga control. ture of S. asiatica (red-flowered) and S. forbesii Field experiments in Sudan, using Synchemi- observed in the southern African region needs cals' antitranspirant wilt proof S600 spray, were study. very encouraging. Aerial shoots of treated Striga The floral biology of these Striga species plants turned black after a few days and col- needs more study. There are two distinct re- lapsed. On newly emerged shoots, the spray productive systems—autogamy and al- treatment caused blackening and collapse with- logamy—in this genus (Musselman and in 24 h. The straw and grain yields of sorghum Parker 1983). Autogamy appears to be wide- increased by 25% and 18%, respectively. Striga spread compared to allogamy. Autogamy yields were markedly reduced by 40% in 1985 characterized by a mass of pollen persisting and 20% in 1986. These results, if confirmed, will on the stigma was observed in S. asiatica, S. be very valuable; antitranspirants could effec- gesnerioides, S. bilabiata, S. angustifolia, S. for- tively replace herbicides, and they are less haz- besii, S. passargeii and S. densiflora (Musselman ardous to the environment. 1987). Allogamy appears to be restricted to Press and his colleagues demonstrated that S. hermonthica and S. aspera, the two closely sorghum varieties infected with Striga show related species. In recent field surveys in Bur- massive reduction in photosynthesis and that kina Faso, we observed several pollen vec- Striga plants derive more than 35% of their car- tors—butterflies, bee flies, and moths—on bon from the photosynthetic activity of the host these two species. They are now being identi- plant. The loss of a substantial portion of the fied at the Commonwealth Institute of Ento- host plant's photosynthetic production and mology. Though we (Obilana et al. 1988, pp. drought stress account for the reduction in grain 342-364) confirmed autogamy in S. forbesii by and stover yield attributed to Striga. The reduc- field-cage studies, the exerted stigmas ob- tion in photosynthetic activity of the host in the served in few plants seemed to be persistent. presence of Striga may be useful as a screening This characteristic, in addition to 5. forbesii's indicator of susceptibility.

192 Control Approaches like methyl bromide fumigation, but less toxic. It has been effective on other soilborne pathogens, Cultural including the pigeonpea fusarium wilt disease in India (ICRISAT 1987, pp. 177-178). Limita- Several cultural methods were reported to be tions include availability of water and of the efficient in controlling Striga species (Ramaiah large polyethylene sheets needed to cover the 1987; Bebawi 1987; Ogborn 1987). Some include soil for about 40 days. It appears to have poten- use of costly inputs such as fertilizers, some are tial in countries like Sudan where Striga is a se- labor-intensive, like hand weeding, and some in- rious problem in the irrigated areas, and in the volve minor adjustments in the cropping sys- wetter savanna areas of Nigeria and Tanzania; tems-such as crop rotations. Among these, hand rainfall here may be up to 1200 mm and the weeding is most often recommended to Third sorghum fields become carpeted with Striga. World farmers and is also the one often rejected, mainly because of its low cost-benefit ratio. Ex- tension workers emphasize the importance of Chemical hand weeding as soon as the Striga plants emerge, but most of the damage to the host has Excellent pre- and postemergence Striga her- occurred while the Striga is still in the soil. Hand bicides are now available (Eplee and Norris removal of Striga helps reduce population build- 1987a). Goal ® and 2,4-D are pre- and post- up, and thus is helpful in the long run, but emergence applications, respectively. In addi- farmers seldom think beyond 2 or 3 years ahead. tion, fumigants like methyl bromide and germi- In most nations of western and southern Africa, nation stimulants like ethylene gas effectively land ownership is nonexistent. Farmers seldom reduce Striga seed reserves in infested soils. Fu- undertake long-term strategies to control Striga migants, however, are most expensive for use by or improve the soil by anti-erosion measures, the small-scale farmer. crop rotation, or trap cropping. Though herbicides are available to control Recent experiments on farms in northern Striga as it emerges or later to prevent seed pro- Burkina Faso have produced encouraging re- duction, using them does not prevent the dam- sults on a single hand-weeding of Striga in millet age inflicted by preemergent parasitic attach- fields just before harvest (Ramaiah 1985; ICRI- ment. This limitation is now overcome by the SAT1986). The dramatic reduction of Striga pop- discovery of systemic herbicides (Eplee and ulations in the following 2 years provided Norris 1987b). In USA, Dicamba ® is used on sor- evidence that a majority of the seeds still on the ghum and maize to kill Striga before crop yields Striga plants were destroyed when the plants are reduced. Dicamba ® sprayed at 0.5 kg a.i. ha -1 were pulled and burned. Significant increases in on sorghum or maize no taller than 1 m gave millet yields occurred in the two cropping sea- excellent control of Striga without reduction in sons that followed. Verification is expected in yield. experiments now underway in Mali, Cameroon, and Ethiopia; the practice needs testing in Tanzania, Botswana, Malawi, Swaziland, and Host resistance Zimbabwe. This method along with resistant va- rieties, fertilizers, and other cultural practices Resistant varieties offer an excellent approach to known to reduce Striga populations could be avoiding yield losses to Striga in subsistence used very effectively as a supplementary control farmers' fields. Breeding efforts at ICRISAT in method. Hand weeding may then become more Burkina Faso have developed resistant varieties practical, and perhaps a common practice. acceptable to farmers. We now have identified a In Jordan, Syria, and Israel solar energy was few varieties with very high levels of resistance used successfully for the control of Orobanche, a to S. hermonthica. They include IS 6961, IS 7777, closely related parasitic species. The process is IS 7739, IS 14825, and IS 14928. Grain yields are called soil solarization: it killed Orobanche seeds very low in these varieties, and efforts are un- effectively when the soil was covered with poly- derway to transfer their resistance into high- ethylene for about 40 days following summer yielding backgrounds. Screening for host resis- irrigation. This was found to be very effective, tance in sorghum at SADCC/ICRISAT Sorghum

193 and Millets Improvement Program Bulawayo, sites. As weeds, they appear late in the season, found three cvs—SAR19, SAR 29, and SAR 33— and escape the normal early weeding opera- to be resistant to S. forbesii. These were earlier tions. As parasites, they have the advantage of found resistant to S. asiatica (white-flowered) in remaining out of sight (and frequently out of India. SAR 19 and two others, SAR 16 and SAR mind) while feeding on the host. The three major 35, were resistant to S. asiatica (red-flowered) in principles of Striga control are: (1) reduction of Botswana. seed numbers in the soil; (2) prevention of new Ramaiah (1987) presented a detailed review seed production; and (3) prevention of move- of host resistance to Striga. Except for finding ment of seeds from infested to noninfested these new sources of resistance, there is no ma- areas. Any control strategy should include an jor breakthrough in understanding resistance integration of at least one method from each of mechanisms, genetics of resistance, physiologi- these three major principles (Table 2). cal variability within Striga species in relation to stability of host resistance, screening/infestation techniques, and evaluation for resistance. Com- parison of root-distribution patterns of resistant With minor adjustments in traditional and of susceptible sorghums (Housely et al. farming 1987) is of interest. Resistant varieties, like Fra- mida and P 967083, have deeper root systems Mixed cropping and rotation. Mixed crop- with significantly less total root length in the ping of host and nonhost crops is a very com- upper 30 cm of the soil core than the susceptible mon practice in western Africa; cereal crops variety, Dabar. Babikar et al. (1987) observed that (mostly sorghum) are grown mixed with trap Striga seeds buried deeper than 24 cm undergo crops such as cowpea, groundnut, cotton, soy- dormancy not easily broken by normal precon- beans, field peas, and sesame. Our recent labora- ditioning. Striga seeds 5 to 10 cm below the soil tory experiments with these trap crops show surface are the most active. Thus host varieties that most of these do indeed act as trap crops. with shallow root systems will be more exposed These crops are mixed and sown at random, in- to active Striga seeds than those like P 96083 stead of seeding and cultivating them as sole which have less of the root system in the Striga- crops on areas of variable length and width in active soil zone. Breeding for deep-rootedness in keeping with their importance in the mixture. sorghum varieties may have an advantage for However, sole cropping would permit applica- resistance to Striga and perhaps to drought tion of Striga control methods to the cereal row stress as well. area only, thus reducing the labor required and More intensive efforts to develop high-yield- reducing the possibility of herbicide damage to ing stable and broad-spectrum resistant varieties the associated crops. It will also allow rotation of are desirable. This is possible only if we have these strips, which is useful in two ways; trap reliable screening techniques to identify resis- crops, such as groundnut, will encourage abor- tant plants and we know more about the bio- tive germination of Striga seeds in the soil. Sec- chemistry of the resistance mechanism, and ondly soil fertility is improved through fixation about other factors conditioning resistance. of atmospheric nitrogen. Other beneficial effects Meanwhile, we have observed indications of of trap crops in reducing Striga and improving multispecies resistance in some ICRISAT sor- the yields of cereals can be listed. ghum materials, especially SAR 19 with resis- Subsistence farmers need their cereal staple tance to S. asiatica (white- and red-flowered) and each year. Crop rotation is an ideal practice; the 5. forbesii (Obilana et al. 1988). cereal is grown on a different piece of land each year. This recommendation should meet with least resistance in central and western Africa. Control Strategies A similar approach, together with crop rota- tion, could be used for Striga control in commu- It is now well recognized that no single method nal cereal farms in southern Africa where of control can effectively solve the Striga prob- "monocropping" or "double cropping" is more lem. Striga species are weeds and they are para- common.

194 Table 2. Striga control methods described under three principles. I II III Reducing number of Striga Preventing production Preventing spread from seeds in soil of new seeds infested to noninfested soils 1. Cultural 1. Cultural 1. Cultural Trap crops Resistant varieties Antisoil erosion Catch crops Hand weeding Deep plowing Irrigation/flooding 2. Phytosanitary practices Soil solarization Date of sowing Fertilizers/manures Clean seed Density of sowing/shading

2. Chemical 2. Chemical 3. Feeding only Striga-free fodder to livestock Methyl-bromide fumigation Herbicides Germination stimulants (Ethylene gas and strigol analogs) 3. Biological

Crop substitution, hand weeding, cattle ma- Farmers must be careful not to allow Striga nure. Sorghum and millet, the most important on these introduced crops to set seed, as it is cereals in the semi-arid tropics, occupy different common to find that Striga populations build up ecological zones in the sub-Saharan Africa. Sor- to intolerable levels rapidly Crop substitution ghum and maize are dominant in the Sudanian should, therefore, be accompanied by hand and northern Guinean zones and the dry veld weeding. As fewer Striga plants emerge, the task where rainfall is relatively high (700-1200 mm) should not be extensive. and the soils are heavy and relatively more fer- If supplementary control measures are ig- tile. Millet is the most important crop on sandy nored, the substituted crops can succumb to soils of the Sahel and the dry veld. Accordingly, Striga. This has occurred in western Africa (Por- Striga has developed extreme crop-specificity in ters 1952) and in the Sudan (Bebawi 1987). these zones of western Africa (Ramaiah 1984). We recommend supplementary hand weed- Therefore, introduction of early-maturing and ing to prevent Striga buildup and the parking of drought-resistant sorghum varieties adapted to cattle, sheep, and goats in continuously cropped the sandy soils of the Sahel or dry veld will fields in dry season. The manure will help sus- greatly solve the Striga problem there. Con- tain productivity of the soil. versely, cultivation of millets in the Sudanian zone will be advantageous. This practice need Varietal substitution, hand weeding, and cattle not be followed throughout the region, but will manure. Local varieties of sorghum and millet be very useful in Striga-endemic areas. It may in Africa have some resistance to Striga, though also have limited use in very dry and hot envi- they suffer greatly under heavy infestations. Re- ronments, as in Botswana, where the crop-speci- sistant varieties of sorghum have been known ficity of Striga has not been confirmed. since the 1930s (Saunders 1933), but their use by In the Sudan a similar system is followed by the farmers was limited because of certain unde- the farmers of southern Darfur (Ogborn 1987), sirable agronomic traits. where farmers have started to utilize their free- Nevertheless, varieties like Dobbs, Framida, dom from the Striga menace. Radar, and Mugd are grown by farmers in east-

195 ern and southern Africa. Framida was intro- Date of sowing, land preparation, supplemen- duced into western African countries and is do- tary hand weeding. Farmers in Africa sow ing extremely well in Togo, Ghana, Benin, and their local varieties following the first heavy rain Burkina Faso. Under heavy infestations, this va- very early in the season. Early sowing on land riety is known to produce several times more not well-tilled, before normal rainfalls begin, ex- than the local susceptible varieties. In addition poses seedlings to drought stress. This is known to its resistance to Striga, Framida is excellent to favor early emergence of Striga in western in seedling establishment and resistance to Africa and Ethiopia (Ramaiah 1987) and in the drought, has higher and stable yields, possesses Sudan (Andrews 1945). partial photoperiod sensitivity, resists bird dam- .In a delayed-sowing operation, it is advised age (due to brown grains), and good food qual- to prepare the land with first rains and sow im- ity when properly processed. Its brown-grain proved varieties. The late local varieties suffer characteristic limits its cultivation to red-sor- significant yield losses if there is an early cessa- ghum growing areas, and to production for tion of rainfall. Early varieties sown early should brewing opaque beer in southern Africa. possess resistance to mold and grain weather- The use of recent resistant varieties and im- ing. Early varieties (such as the CK 60 B sor- proved derivatives from older cvs, in place of ghum), support the emergence of many Striga susceptible cvs, plus hand weeding and cattle plants, but because grain in these varieties fills manure can reduce significantly the Striga men- and matures quite early, yields do not seem to be ace and increase cereal productivity. Two of the reduced significantly. most promising varieties are ICSV 1002 BF and In the northern Guinean zone of western Af- ICSV 1007 BE ICSV 1002 BF combines the addi- rica, the rainfall is greater and the season is rela- tional good characteristics of white grains, lodg- tively longer; relay cropping is traditional. This ing resistance (nonsensescence character), and system could be modified to suit our Striga con- resistance to important leaf diseases. This vari- trol strategy. In northern Nigeria we have ob- ety is in the prerelease stage in Burkina Faso and served a relay-cropping system of three crops: Gambia. photosensitive sorghum, millet, and groundnut. The second selection, ICSV 1007 BF, proved Millet is harvested at the end of September; sor- to be the most resistant variety throughout the ghum not until the end of November. In Septem- Striga belts of all continents, and in on-farm tests ber, hardly any Striga is seen on sorghum, but by in heavily infested irrigated areas of the Sudan the end of October, it is in full flower and has (Nour, personal communication). In the south- plenty of time to set seed before sorghum har- ern African region, we have found the promis- vest in November. For this situation, we recom- ing white-grained SARs 16, 19, and 33 to be mend sowing an early- to medium-maturing resistant to species of Striga. Their productivity sorghum that can be harvested by mid-October. is being evaluated in variety trials in the nine Striga by this time is only starting to flower, thus countries of the SADCC. preventing to some extent enormous seed build- Another local variety, IS 9830 from the Sudan, up. These early varieties have a three-fold ad- has good levels of resistance, is early, and yields vantage—escape damage, prevent Striga seed well under a range of growing conditions. buildup, and act as catch crops. This practice has Recently a derivative of Framida, SAR1, was been recommended in Kenya where the sor- released in India for cultivation in Striga-en- ghum cv "Dobbs" would be ready for harvest demic areas (ICRISAT 1986). before most of the Striga matures. Varietal substitution should always be ac- One practical possibility is to move the pho- companied by hand weeding and farm animal tosensitive varieties (flowering in mid-Septem- manure to sustain long-term productivity of the ber) from the Sudanian zone into the north soil. Guinean zone, as replacements for the local pho- Preventing buildup of new strains of Striga to tosensitive varieties that flower in mid-October. levels that overcome the resistance of a new va- But somehow this did not happen in the tradi- riety is an absolute requirement. Breakdowns in tional system, perhaps because of different local- resistance to Striga were reported from the adaptation requirements. A second possibility is Sudan (Ogbom 1984) and southern Africa to introduce improved high-yielding varieties (Riches et al. 1987). for sowing late in the relay-cropping system.

196 This has produced excellent results, in terms of Research Needs improved overall productivity in Mali (ICRISAT 1986). We recommend improved resistant vari- The foregoing shows that Striga is not a new eties, wherever available, for relay sowing on problem on cereals and legumes in Africa; some land well-prepared and fertilized, supported by early work was done and much is now being a late hand weeding. done to understand Striga and its control. How- ever, new Striga problems are being found, and they need to be researched. With major adjustments in traditional systems We recognize three levels of research leading to understanding and control of Striga: (1) adap- There are very effective Striga-control methods tive, (2) applied, and (3) basic. Adaptive re- for farmers who can afford them—costly soil fu- search includes verification of research recom- migations, herbicides, fertilizers, phytosanitary mendations through on-farm testing. These rec- regulations, and antitranspirants. Some of the ommendations, resulting from applied research control strategies using these methods are listed: are practices that will be suggested to, and hope- fully adopted, by farmers. Applied research con- Fertilizers and improved varieties, late-season sists of experiments to determine how the hand weeding. We recommend the use of fer- knowledge gained in basic research can be used tilizers with input-responsive resistant varieties in practical ways to help solve the Striga prob- such as Framida, ICSV 1002 BF, and SAR 19 lem — in other words, to become useful in de- backed with late hand weeding. In Gambia, Car- vising a new control practice. Basic research son (1986) has recommended a similar package seeks new knowledge of the Striga plant and its for Striga-endemic areas. hosts. Conducting research into more biological aspects using the latest research techniques are Ethylene gas, herbicides, phytosanitary regula- employed. In our view the research needs are as tions. This method is being successfully used follows. to eradicate Striga in USA. It represents the best from each of the three major principles of Striga control described earlier: (1) reduction of seeds Host resistance in infested soils (ethylene gas), (2) preventing new seed production (herbicides), and (3) pre- Adaptive research. (1) Demonstration of host vention of spread of Striga from infested to non- resistance in sorghum to farmers and extension infested fields (phytosanitary regulations). Each agents through multilocational on-farm testing. demands a lot of resources and infrastructure to make it effective. A comprehensive review of Applied research. (1) Breeding stable high- these methods was recently published by Eplee yielding acceptable (to farmers) resistant vari- and Norris (1987b). eties of sorghum and millet; (2) screening of Recent research on Striga physiology has pro- wild relatives of to find resistance; vided clues for Striga control through use of an- (3) development of reliable infestation and titranspirants [Press et al. (In press)]. Striga screening techniques; (4) physiological vari- plants were reported to have higher transpira- ability in relation to stability of host resistance; tion rates, and any action that interferes with its (5) field designs and appropriate statistical transpiration function restricts cooling, raises analyses. internal temperatures, and ultimately causes death of the plant. Antitranspirants are less haz- Basic research. (1) Understanding Striga resis- ardous than herbicides to humans and the envi- tance mechanisms active in sorghum and mil- ronment, in general, and therefore should be an lets; (2) inducting resistance by changing host attractive alternative to herbicides. physiology, increasing lignification in root tis- It is our hope that extension services in sue, root exudation, defense chemicals, seed Striga-affected areas will demonstrate these con- hardening, etc.; (3) rapid screening techniques, trol methods, explaining their potential so that based on "chemical markers" produced in farmers become motivated to use them. Striga-infected plants; (4) application of molecu-

197 lar genetics to isolate resistance genes useful as Crop specificity probes to screen germplasm and varieties. Adaptive research. (1) Crop substitution (mil- let in sorghum zones and sorghum in millet Agronomic practices zones); (2) crop rotations using nonhost (trap) crops; (3) study socioeconomic implications of Adaptive research. (1) Demonstrate value of these in the respective ecologies. late hand weeding; (2) demonstrate value of fer- tilizers, wherever available, particularly with Basic research. (1) Determine basis for immu- maize; (3) demonstrate value of rotating mod- nity in nonhost (trap) crops. ified mixed and host and nonhost crops.

Applied research. (1) Field experiments to de- Taxonomy termine optimal crop densities, dates of sowing, etc.; (2) identification of suitable trap crops for Applied research. (1) Sympatric existence of different Striga species; (3) study Striga chasers Striga species and its implications; (2) clarifica- like coriander, etc.; (4) determine seasonal emer- tion of relationships among closely linked gence patterns. species.

Basic research. (1) Striga seed behavior under Basic research. (1) Genetic diversity and ge- different conditions in the soil; (2) determine fac- netic interchanges among and within the Striga tors such as soil, host, parasite, and climate that species by inter- and intra-specific hybridiza- dictate narrow and broad spectrum adaptation tion, chromosome studies, and seed morphol- of Striga; (3) develop growth models, etc. ogy, floral morphology, and breeding systems. s

Chemicals Biology

Adaptive research. (1) Demonstrate value of 2, Applied research (1) Field experimentation 4-D in sole crops; (2) demonstrate other her- with antitranspirants. bicides in crop mixtures. Basic research (1) Relationship between Striga Applied research. (1) Conduct trials with anti- seed germination and host crop phenological transpirants and systemic herbicides. development; (2) photosynthesis; (3) transpira- tion; (4) water relationships; (5) chemical com- Basic research. (1) Synthesis of stable stimu- pounds produced by the host that initiate lants based on strigol, ethylene, or other com- parasitic shoot growth; (6) Striga-specific her- pounds; (2) study combinations of stimulant and bicides; (7) toxins of Striga, and antitoxins of herbicide compounds. host plants.

Biological control Outlook

Applied research. (1) Controlled exchanges of Striga research in Africa is inadequate, and the insects between India and Africa; (2) survey of volume of work required for rapid progress in oxganisms parasitic or pathologic on Striga; (3) Striga research is almost certainly beyond pre- use of the gall-forming weevil Smicronyx urn- sent resources. Even so, increased effort is essen- brinus in reducing Striga seed production. tial if the Striga problem is to be managed. Considerable manpower will be required to Basic research. (1) Develop bioherbicides intensify work on the series of research and ex- based on biological "enemies" of Striga; (2) tension needs identified here. Intensive training study taxonomy of African Smicronyx spp; (3) of national research and extension staff, espe- study diseases and other pests of Striga. cially those in endemic areas for Striga is essen-

198 tial; involvement with international scientists in Doggett, H. 1965. Striga hermonthica on sorghum this effort for regional cooperation would be in East Africa. Journal of Agricultural Science required. 65(2):183-194. Practical research results should be made available to the farmers as soon as practical Eplee, R.B., and Norris, P.S. 1987a. Chemical through pilot projects demonstrating the value control of Striga. Pages 173-182 in Parasitic of simple recommended cultural practices. Plants in Agriculture. Vol I. Striga (Musselman, There is need for continued cooperation and L.J., ed.). Boca Raton, FL, USA: CRC Press Inc. collaboration among all groups working on the Striga problem in Africa and elsewhere; these Eplee, R.B., and Norris, P.S. 1987b. Control of include IARCs (International Agricultural Re- Striga asiatica F (L) Kuntze with systemic her- search Centers), NARSs (National Agricultural bicides. Pages 183-186 in Parasitic Flowering Research Systems), funding agencies, and coor- Plants (Weber, H.C., and Forstreuter, W., eds.). dinating organizations. Hesse, Germany: Marburg an der Lahn. A continental-action program in the form of a "Striga Research and Control Network" with Housley, T.L., Ejeta, H., Sherif-ari, (X, Netzley, strong regional components and strong national D.H., and Butler, L.J. 1987. Progress towards an commitments to research, training, and exten- understanding of sorghum resistance to Striga. sion would be of genuine value, with payoff at Pages 411-419. in Parasitic Flowering Plants the farmer level. (Weber, H. C. and Forstreuter, W., eds.). Hesse, Germany: Marburg an der Lahn.

References ICRISAT (International Crops Research Insti- tute for the Semi-Arid Tropics). 1986. ICRISAT/ Andrews, F.W. 1945. The parasitism of Striga Burkina Faso Annual Report, 1985. Integrated hermonthica Benth. on sorghum spp. under irri- Striga management, Burkina Faso. Ouaga- gation. I. Preliminary results and the effect of dougou, Burkina Faso: ICRISAT. heavy and light irrigation on Striga attack. An- nals of Applied Biology 32(3):193-200. ICRISAT. (International Crops Research Insti- tute for the Semi-Arid Tropics).1987. Annual Re- Babikar, A.G. J., Hamdoun, A.M., and Mansi, port, 1986. Patancheru, A. P. 502 324, India: M.G. 1987. Influence of some soil and environ- ICRISAT mental factors on response of Striga hermonthica (Del.) Benth. seeds to selected germination stim- Musselman, L.J. 1987. Taxonomy of witch- ulants, Pages 53-56 in Parasitic flowering plants weeds. Pages 3-12 in Parasitic Weeds in Agricul- (Weber, H.C., and Forstreuter, W., eds.). Hesse, ture. Vol I. Striga (Musselman, L.J., ed.). Boca Germany: Marburg an der Lahn. Raton, FL, USA: CRC Press Inc.

Bebawi, F.E 1987. Cultural practices in witch- Musselman, L.J., and Parker, C. 1983. Bio- weed management. Pages 159-172 in Parasitic systematic Research in the genus Striga (Scro- weeds in agriculture, Vol I. Striga (Musselman, phulariaceae). Pages 19-24 in Proceedings of the L.J. ed.) Boca Raton, FL, USA: CRC Press Inc. Second International Workshop on Striga. (Ramaiah, K.V., and Vasudeva Rao, M.J., eds.), Burtt-Davey, J. 1905. Botanical notes: Witch- Patancheru, India; International Crops Research weed. Transvaal Agricultural Journal 3:130. Institute for the Semi-Arid Tropics.

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199 Obilana, A.T. 1983b. Striga studies and control Press, M.C., Tuohy, J.M., and Stewart, G.R. in Nigeria. Pages 87-98 in Proceedings of 1987a. Gas-exchange characteristics of the sor- the Second International Workshop on Striga ghum-Striga host-parasite association. Plant (Ramaiah, K.V., and Vasudeva Rao, M.J., eds.). Physiology 84:814-819. Patancheru, India; International Crops Research Institute for the Semi-Arid Tropics. Press, M.C., Shah, N., Tuohy, J.M., and Stewart G.R. (In press). Carbon isotope ratios demon- Obilana, A.T. 1987. Breeding cowpeas for Striga strate carbon flux from C4 host to C3 parasite. resistance. Pages 243-253. In Parasitic Weeds in Plant Physiology. Agriculture (Vol. I) (Musselman, L.J., ed.). Boca Raton, FL, USA: CRC Press Inc. Press, M.C., Mallaburn, P., Nour, J., and Stew- art, G.R. 1987b. The characterization of resis- Obilana, A.T., Knepper, D., and Musselman, tance and tolerance mechanism against the L.J. 1988. Striga (witchweeds) in Zimbabwe. Pa- parasitic angiosperm Striga hermonthica. Annual per presented at the Fourth Regional SADCC/ Report 1986-87. Research project R 4022. Lon- ICRISAT Sorghum and Millet Improvement Pro- don WCIE 6BT: University College London. gram Workshop, 21-24 Sep 1987, Matapos, Zimbabwe. Ralston, D.M., Riches, C.R., and Musselman, L.J. 1986. Morphology, floral syndrome, and Obgorn, J.E.A. 1984. Research priorities in host specificity oi Striga spp in Botswana. OICD agronomy. Pages 195-212 in Proceedings, Work- Report. P.B. 0033 Gaborone, Botswana: Dept. of shop on the Biology and Control of Striga, 14-17 Agricultural Research. 41 pp. Nov 1983, Dakar, Senegal, (Ayensu et al., eds.). ICSU Press/IDRC. Ramaiah, K.V. 1984. Patterns of Striga resistance in sorghum and millets with special emphasis Ogborn, J.E.A. 1987. Striga control under peas- on Africa. Pages 71-92 in Striga: Biology and ant Farming conditions. Pages 145-158 in Parasi- Control, Proceedings Workshop on the biology tic weeds in agriculture Vol I. Striga (Mussel- and control of Striga, 14-17 Nov 1983, Dakar, man, L.J., ed.). Boca Raton, FL, USA: CRC Press Senegal. ICSU Press/IDRC. Inc. Ramaiah, K.V. 1985. Hand pulling of Striga her- Parker, C. 1983. Striga: analysis of past research monthica on Pearl Millet. Tropical Pest Manage- and summary of the problem. Pages 145-158 in ment 31(44):326-327. Proceedings of the Second International Work- shop on Striga, 5-8 Oct 1981, IDRC/ICRISAT, Ramaiah, K.V. 1987. Control of Striga and Oro- Ouagadougou, Burkina Faso. Patancheru, A.P. banche species - a review. Pages 637-664 in Parasi- 502 324, India: International Crops Research In- tic Flowering Plants (Weber, H.C. and Forstreuter, stitute for the Semi-Arid Tropics. W., eds.). Hesse, Germany: Marburg an der Lahn.

Pearson, H.H.W. 1913. The problem of the Ramaiah, K.V., Parker, G, Vasudeva Rao, M.J., witchweed. South African Department of Agri- and Musselman, L.J. 1983. Striga Identification culture. Agricultural Journal 4:34. and Control Hand Book. Information Bulletin no. 15. Patancheru, A.P. 502 324, India: Interna- Porters, R. 1952. Linear cultural sequences in tional Crops Research Institute for the Semi- primitive systems of agriculture in Africa and Arid Tropics. 52 pp. their significance. Sols Air. 2:15. Raynal-Roques, A. 1987. The genus Striga (Scro- Press, M.C., and Stewart, G.P. (In press). phulariaceae) in western and central Africa - a Growth and photosynthesis in survey. Pages 675-689 in Parasitic Flowering infected with Striga hermonthica. Annals of Plants (Weber, H.C. and Forstreuter, W., eds.). Botany. Hesse, Germany: Marburg an der Lahn.

200 Riches, C.R., de Milliano, W.A.J., Obilana, Photosynthesis and stomatal characteristics of A.T., and House, L.R. 1987. Witchweeds (Striga Striga hermonthica in relation to its parasitic spp) of sorghum and pearl millet in the SADCC habit. Physiologia Plantarum 69:699-703. Region—Distribution and Control In Proceed- ings, Third Regional SADCC/ICRISAT Sor- Timson, S.D. 1931. Witchweed. Progress report ghum and Millet Improvement Project Work- and a warning. Rhodesia Agricultural Journal shop, Lusaka, Zambia, 20-25 Sep 1986. 28:1101-1111.

Saunders, A.R. 1933. Studies on phanerogamic Timson, S.D. 1934. Witchweed. Rhodesia Agri- parasitism with particular reference to Striga cultural Journal 31:792-401. lutea. Bulletin No. 128. Pretoria, South Africa: Department of Agricultural Science, South Af- Vasudeva Rao, M.J., Chidley, V.L., and House, rica. 56 pp. L.R. (In press). Estimates of grain yield losses caused in sorghum (Sorghum bicolor L. Moench) Saunders, A.R. 1942. Field experiments at Pot- by Striga asiatica (L.) Kuntz obtained using the re- chefstroom. A summary of investigations con- gression approach. ICRISAT Journal Article JA 718. ducted during the period 1903-1940. South African Department of Agriculture and Forestry Visser, J.H., and Botha, P.J. 1974. Chroma- Science Bulletin 214:19-31,49-50. tographic Investigation of the Striga seed ger- mination stimulant. Zeitschrift fur Shah, N., Smirnoff, N., and Stewart, G.R. 1987. Pflamzenphysiologie 72:352-358.

201