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Misunderstandings on parasitic weed control and research in the world with the references of Striga tolerance in maize

Soon-Kwon Kim1, Mamodou I. Ouattara2, Mahama Ouedraogo2, Victor Adetimirin3, Charles The4, Segun Toyosi Lagoke5, Louise Akanvou6, Mashark S. Abdulahi7, Gualbert Gbehounou8, N’Tji. Coulibaly9, Abebe Menkir10 and Badu B. Badu-Apraku11

1Kyungpook National University (KNU), Daegu, S. Korea, kimsk@knu.ac.kr;
2SAFGRAD of African Union, Ouagadougou, Burkina Faso;
3University of Ibadan, Nigeria;
4IRAD, Yaounde, Cameroon;
5University of Abeokuta, Abeokuta, Nigeria and the Coordinator of the PASCON of FAO;
6CNRA, Ferkessedougou, Cote d’Ivoire;
7SARI, Tamale, Ghana;
8INRAB, Porto-Novo, Republique du Benin;
9IER, Bamako, Mali;
10IITA, Ibadan, Nigeria; and
11WECAMAN, Bouke, Cote d’Ivoire, respectively.

Abstract

Striga species infect millions of hectares of arable land in Africa. They cause loss of seven billion US dollars of food production annually. The objectives of this paper are to present the results of on-farm testing of tolerant maize under Striga hermonthica infection in West and Central Africa during six years and critical reviews on parasitic weed control worldwide. The program was supported by the KOICA of Korea, SAFGRAD of African Union and PASCON of FAO (Food and Agriculture Organization). Approximately 5,000 farms from six countries (Benin, Cameroon, Cote d’Ivoire, Ghana, Mali, and Nigeria) participated. The results show that genetic tolerance is the key for the stability of maize yield. Legume intercropping and rotation helped to reduce the parasite infection and increase maize yield by improvement of soil fertility. Sorghum appears to be the primary host for seed population increase of the parasite. The tolerance technology can be called as the genetic integrated pest management (IPM). The same principle can be applicable to control Striga species of sorghum, millet, rice, cowpea, and Orobanche species of faba beans, sunflower, and tomato.

Media summary

Results of on-farm demonstration of maize against Striga hermonthica in Africa show that the parasite can be controlled by a tolerant cereal variety with a legume inter-cropping or rotation. Sorghum is the primary host responsible for increasing Striga seed population. The same principle can be applicable not only to control of Striga species in other crops, but also other parasitic weeds.

Key Words

Parasitic weed Striga, Africa, tolerance, Genetic IPM, maize, cereal + legume package

Introduction

The Striga species of parasitic weeds are considered the No. 1 biotic stress of cereal crop production in Africa. Four important species infecting maize, sorghum, millet, rice and sugarcane are S. hermonthica, S. asiatica, S. aspera and S. forbesii. Another species, S. gesnerioides affects cowpea. Striga is primarily a ‘poor farmers’ problem and its infection is associated with low soil fertility, lack of fertilizer application, continuous cropping of a cereal crop and unavailability of Striga tolerant and resistant (STR) cultivars (Kim 2003 a & b; Kim and Adetimirin 2002). Africa is the center of origin of most species of parasitic weeds. Research on the parasite has been carried out by national, international and advanced institutions for a century. International centers such as ICRISAT, IITA and CIMMYT have worked on the parasite for three decades with emphasis on the development of high resistance and integrated cultural method. Considerable time over the last half century has been spent arguing terms on resistance and tolerance, whether to call the plants weeds as parasites, or diseases, whether to take a biotech or conventional approaches, whether to count emerged plants of the parasite, or collect host damage ratings, cultivar x environmental interactions, whether to use a farming system approach or integrated control method, what is responsible for distribution of parasites, identification of different species and level of yield reduction and all that mixed with the politics of understanding the parasite at national programs, international centers and donor agencies.

With the financial supports of IDRC in Canada, ICRISAT had worked to develop sorghum and millet resistance to Striga species for two decades in Africa. In the early 1980s, the ICRISAT’s Board concluded that it was impossible to control the parasite through host resistance. IITA scientists started their research on Striga with cowpea in the early 1980s. The team searched for high level of vertical resistance (VR), even though resistance had broken down in West Africa.

The maize program at IITA had the philosophy of horizontal resistance (HR) against biotic stresses in Africa and started Striga research in 1982. The team had experienced with tolerance with Puccinia polysora rust, maize streak virus (MSV), Exserohilum turcicum, Bipolaris maydis, downy mildew, and stem borers. Their first target was to develop a uniform infestation technique for the parasite. In 1987, the team developed a simple and uniform method of field infestation with a large number of locally collected Striga seeds from cereal crop hosts (sorghum, millet and maize) (Kim 2003a; Kim and Adetimirin 2002). With this technique, they screened over 5,000 germplasm lines, hybrids, open-pollinated families and varieties, annually. The team also studied interactions of cereal hosts with seed population, nitrogen, water, crop specific Striga strains, Striga species and their interactions. By 1995, IITA scientists developed 50 cultivars of tolerant open-pollinated cultivars and hybrids that have different Striga infection zones in sub-Saharan Africa (Kim 2003b). Most of the cultivars confer combined tolerance to MSV, drought and other biotic stresses. On-farm demonstration trials of tolerant cultivars under both natural and artificial infestations were carried out since 1988. However, the spread of tolerant cultivars has been limited due to various reasons, including misunderstandings on host tolerance, VR association of low counting, and inter-center and inter-scientist politics. While poor farmers have suffered much due to the lack of new tolerant cultivars.

Materials and Methods

Tolerant maize cultivars (OP and hybrids) developed at IITA and Cameroon national program with IITA were used for three different types (STR variety, STR + legume inter-cropping and STR + legume rotation) of on-farm demonstration trials in six countries. Institutions responsible for the trials were the Institut National des Recherches Agricoles du Benin (INRAB), the Institut de la Recherche Agricole pour le Development (IRAD) in Cameroon, the Centre National de Recherche Agronomique (CNRA) in Cote d’Ivoire, the Savannah Agricultural Research Institute (SARI) in Ghana, the Institut d’Economic Rurale (IER) in Mali and the Institute of Agricultural Research, ABU, Zaria (IAR/ABU) and the University of Agriculture, Abeokuta (UNAAB) in Nigeria. Three tolerant cultivars (ACR TZEComp.5, NCRE STR-Y, Oba Super #1) were compared with susceptible local cultivars grown with a legume crop, either cowpea, soybean or groundnut. Plot size was approximately 20 x 20 m. Seeds and management fund were provided to the program. Data taken were degree of Striga infestation, level of host tolerance, plant height, grain yield and farmers’ response. A monitoring tour was organized by the expert groups and some countries organized field days with the participating farmers.

Results and conclusion

The results of on-farm demonstration and the monitoring tours showed that the northern regions of Cameroon, Ghana, Nigeria, Benin and Cote d’Ivoire are areas with the highest infection of Striga hermonthica. STR maize + legume inter-cropping demonstration increased maize yield 26% (2.17 vs 1.72 t/ha) with 20% less host damage, and 47% lesser Striga plants emerged to farmer variety, respectively (Table 1). This system additionally produced more than 500kg/ha of legume grain, which could compensate for maize yield reduction due to the intercropping. While, STR maize + legume rotation demonstration increased maize yield 14% (2.63 vs 2.31 t/ha) with 34% less Striga plants emerged to farmer variety. Yield of maize was further compensated by legume yield. STR maize variety demonstration without legume increased an average 40% (2.70 vs 1.92 t/ha) of maize grain yield with 62% less host damage and 24% less Striga plants emerged to farmer variety, respectively, but this continuous maize cropping system is considered not sustainable. In the large scale and high labor cost areas, STR maize + legume rotation system may be easier to operate than the inter-cropping system.

We found that sorghum is a key host plant for increasing Striga seed populations in the regions (e.g., Cameroon). The erratic nature of Striga observations and research data are due to the quantitative nature of host tolerance, variations in Striga infestation and environmental conditions (rainfall, soil fertility, farming systems, interactions). To control the parasite and sustain yield, the host crop must confer combined tolerance to major biotic and abiotic stresses including drought.

Data for Striga counting are unreliable and caused confusion in expression of host crop response to the parasite. Scientists still look for high resistance even after the first report of the breakdown of high (vertical) resistance of a sunflower cultivar to Orobanche cernua in Russia in 1912. With over 1,000 publications and 30 international workshops, researchers are still wasting time and resources arguing terminology, while resource poor farmers are suffering because of parasite.

Table 1. Results of On-Farm Maize/Legume intercropping demonstration of Striga tolerance maize in six ecological zones of five countries in Africa, 2003

Parameters
measured

Technology
demonstrated

Participating Country

  

Means

Benin

Cameroon
GS

Cameroon
SS

Mali

Nigeria

Burkina
Faso

∙ Maize grain
yield (t/ha)

-STR maize ass
-Local maize
-STR maize alone
-% local

2.98
2.59
-
115

2.54
2.41
-
105

2.32
2.03
-
114

0.77
0.65
1.34
118

2.24
0.93
-
240

0.48
0.51
-
363

2.17
1.72
-
126

∙ Number of Striga plants emerged (no/ha)

-STR maize ass
-Local maize
-STR maize alone
% local

30000*
35000*
-
86

5650
9350
-
60

9450
11700
-
81

4400
15700
6200
28

702
1187
-
59

10667*
-
42666
25

5051
9484

53

∙ Leguminuous crop yield (t/ha)

-Sole
-In STR maize
-In local maize

0.84
0.69
0.60

-
-
-

-
-
-

0.64
0.32
-

-
-
-

0.46
0.58
-

0.64
0.53
0.60

∙ Maize damage score (1-9)

-STR maize ass
-Local maize
% local

-
-

-
-

-
-

-
-

2.8
3.5
80

-
-

2.8
3.5
80

*: not included in over all mean

Tolerant cereal cultivars plus a legume inter-cropping should be the most economic, sustainable and pro-environmental package to control parasitic weeds for small scale farms. The principle of tolerant cultivar can be called as genetic IPM that is based on horizontal resistance without any use of chemicals. This quantitative nature of tolerance must be understood first by scientists and research staff, followed by administrators from international centers, donor agencies and national programs. Introduction of a legume crop in cereal field shall increase soil fertility and also reduce Striga infection.

The program has developing a video tape and information sheets to describe the simple control package (a tolerant cereal + legume intercropping or rotation) with farmers’ participatory on-farm demonstrations. The program emphasizes seed multiplication of tolerant cultivars, training, of farmers and on getting more regions and countries involved (Kim et al 2001; SAGRAD 2003).

References

Kim SK (2003a). Dr. Soon-Kwon Kim’s efforts at combating hunger in Africa and Asia (1969-2003). Kyungpook National University Press, Daegu, Korea, 360 pp.

Kim SK (2003b). Maize germplasm developed and studied by Dr. Soon-Kwon Kim and his colleagues for Africa, Asia and USA (1969-2003). KNU Press, Daegu, Korea. 216 pp.

Kim SK and Adetimirin VO (2002). Maize breeding and related research for the control of Striga in sub-Saharan Africa. A collection of published work carried out by S. K. Kim and his colleagues. KNU Press, Taegu, Korea. 192 pp.

Kim SK, Adetimirin VO, Bezuneh T, The C, Lagoke STO, Kling J, Badu-Apraku B, and Fajemisin J (2001). Maize breeding and on-farm testings of Striga species in Africa. ASA, CSSA and SSSA Annual Meetings, Charlotte, North Carolina, Oct. 21-25.

SAFGRAD (2003). Annual report 2003, KOICA (Government of Korea) AU/SAFGRAD African Striga Control Programme, SAFGRAD, Ouagadougou, Burkina Faso. 28 pp.

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