Publications: ISAAA Briefs
No. 16 - 2000
Edited by Florence Wambugu and John Wafula
Originating in Central America and introduced into Africa by the Portuguese in the 16th century, maize has become Africa’s most important staple food crop. Both large and small-scale farmers grow maize in various agroecological zones. It is increasingly replacing traditional food crops, including cereals such as sorghum and millet. Maize is so important that in most of sub-Saharan Africa poor maize yields are usually linked to food shortages and famine.
Yet although maize is a crucial staple food crop, the average yield per hectare in Africa is the lowest in the world. Furthermore, annual maize yield increases in Africa have not kept pace with population growth (1.9 vs. 2.9%, respectively, for the years 1961-65/1984-88). Unless this situation is reversed, food dependency will increase throughout much of the continent. According to the Food and Agriculture Organization (FAO), the worldwide average maize yield per hectare is about 4 t, but in Africa it is 1.7 t, less than half the global average. The yield is 8 t/ha in a highly industrialized nation such as the USA. African maize yields are clearly far below the crop’s genetic capacity: on-farm yields range from only 10 to 35% of the potential. In Kenya, for example, on-farm yields were 2.3-4.7 t/ha below potential. In Malawi, on-farm hybrid maize yields reached a mere 2.5 t/ha compared with 8 t/ha during trials at agricultural stations. When open-pollinated maize was used, the difference was 10-fold (0.6 t/ha compared with 6 t/ha). These numbers show how important it is to raise maize yields to improve food security for households in both rural and urban Africa.
One of the major problems facing maize farmers in Kenya and other east African nations is maize streak virus (MSV). Indigenous to Africa and its offshore islands, MSV causes yield losses of up to 100% even in high potential agricultural zones. A good example of this is the Githunguri region in Kenya’s Kiambu district. It is an agricultural zone with adequate rainfall and fertile red soil. MSV is wreaking havoc in this area. Since 71% of Kenya’s land is arid or semiarid, such serious crop losses in high potential zones can devastate the national economy and threaten food security, which is already precarious. Currently, Kenya’s annual average maize production is 2.7 million t, slightly lower than the 3 million tons consumed each year— and consumption is expected to increase in the future.
As a 1990 sub-Saharan Africa survey by UK’s Natural Resources Institute (NRI) showed, MSV is one of the two most important biotic constraints affecting maize production in Africa (the other is maize mottle virus). The most important abiotic factor is drought, and when weather conditions combine with MSV epidemics, maize shortages can produce famines in Kenya’s semiarid regions. The large fluctuations in maize production also generate large price swings. Prior to May 1999, for example, prices for maize flour were low, averaging under 30 Kenyan shillings for 2-kg bags sold in rural retail shops and major urban supermarkets. But despite Kenya’s relatively well-developed agricultural infrastructure, May prices suddenly increased to 55 shillings per 2-kg packet—an increase of more than 80%. This rise in price was due in part to an outbreak of MSV disease.
Given Kenya’s fertile agricultural regions, farming should be a profitable business for maize producers (two-thirds of whom are women according to experts at KARI—a significant measure of how important maize is to families’ food security). But MSV strikes at the heart of the region’s farming system, destroying the main crop that small-scale farmers subsist on. And the problem is becoming more acute. From the 1930s to the 1980s, MSV epidemics were commonly reported, but in the last decade their frequency and destruction have increased. At the same time, subsistence farmers lack options to control the disease. Applying systemic insecticides helps reduce yield loss by reducing vector populations, but this is not an option for most farmers in Africa, especially small-scale farmers who cannot afford to buy chemicals.
The devastating impact of MSV is not confined to Kenya alone. Serious MSV epidemics have been reported in at least 20 African nations, including Nigeria, Ethiopia, Sudan, Tanzania, Zimbabwe, Zambia, Angola, Mozambique, Malawi, Madagascar, Senegal, Ghana, Cameroon, Togo, Benin, Sao Tome, and Burkina Faso. MSV disease has been reported in all areas of the continent except the north.
For farmers who suffer major yield losses due to MSV in the high yield potential areas of Kenya and other African countries, scientific data and surveys are no substitute for the practical solutions that they urgently need. Pauline Ruiru, a small-scale farmer from Githunguri in the fertile highlands north of Nairobi, put the question plainly to a visiting team of scientific experts: "Can you please tell me what I should do?"
Today, no one can provide farmers such as Pauline Ruiru with a direct, clear-cut answer to her MSV problem. But according to Dr. Anatole Krattiger, executive director of the International Service for the Acquisition of Agri-biotech Applications (ISAAA), the solution lies in biotechnology: "Millions of small-scale farmers can seize the promise of better lives through the altruistic dissemination of such biotechnology." Indeed, we cannot afford to ignore biotechnology applications that can help African maize farmers control MSV in a cost-effective, agriculturally sustainable, and environmentally protective way.
In 1996, ISAAA launched a project to help national research groups in Africa combat MSV through improved resistance breeding. The collaborative project aims to provide a better understanding of the epidemiology of MSV, its insect vectors, and genetic resistance to MSV in maize varieties. It may involve developing appropriate molecular markers for breeding, and increasing the capacity of national programs to breed and adapt MSV resistant maize. Dr. Florence Wambugu, director of ISAAA’s AfriCenter, believes that a solution can be found within 5 years.
Much of the efforts to improve maize yields have so far focused on breeding for high-yielding varieties (HYV). Diseases and insects, which easily reduce yields from such varieties by over 50%—and much more if post-harvest losses are included—tended to be sidelined. It was probably assumed that farmers would use chemical pesticides and take agronomic measures to control diseases and vectors. This era of "pure plant breeding" also tended to exclude plant pathologists, entomologists, and other scientific specialists who possessed important information about maize and its cultivation. Furthermore, the promise of biotechnology was overlooked or underestimated. Yet it is increasingly clear that both small-and large-scale farmers stand to gain from various aspects of biotechnology, particularly its low-input requirements. This is because biotech packages the technology "in the seeds," a delivery system that farmers have handled since the emergence of agriculture 10,000 years ago.
Kenya’s past emphasis on breeding to improve yields excluded other factors that reduce yields. This has resulted in high-yielding maize varieties that are equally highly susceptible to MSV, such as the popular 511 hybrid. Farmers can lose the whole season’s crop if the MSV infection occurs at an early stage. Before 1980, losses due to MSV in Africa were around 10%. In the 1990s the trend worsened: average MSV yield losses range today from 30 to 50%—this in a continent that imports 25% of its grain. Robert W. Herdt, director for Agricultural Sciences at the Rockefeller Foundation, has pointed out that dependency on imported grains has increased over the last three decades—and a quarter of the food imported is donation or food aid. In short, there is an urgent need to control major crop constraints such as MSV so that Africa can become self-sufficient in food production.
Africa, with maize yields that are less than half the global average, never fully plunged into the Green Revolution that enabled other developing nations in Latin America and Asia to become self-sufficient in grain production through the increased use of high-yielding seed varieties, pesticides, fertilizers, and irrigation. Biotechnology now offers the continent a major opportunity to curb hunger and unacceptable levels of food dependency. while at the same time reducing the use of toxic pesticides.
Identifying and breeding maize varieties that are resistant to MSV is a very practical contribution in the war against food shortage and famine that stalk parts of Africa at the dawn of this new millennium.
The procedures and capacity for breeding maize are well established in several African nations, including Kenya, Zimbabwe, South Africa, Nigeria, Reunion Island, and others. But the technology and expertise to understand the pathology of MSV and the modes and genetic basis of resistance are missing. All these are needed before effective resistance screening and breeding strategies can be developed to conquer MSV.
Such efforts will improve the living standards of rural families and poor urban populations—those who are hit hardest by food price increases and shortages. Results already indicated that MSV can be controlled through resistant hybrids adapted to Kenya’s ecological zones.
Indigenous to Africa and adjacent islands, MSV is believed to have evolved with native grasses. It is transmitted by leafhoppers belonging to the genus Cicadulina, a species which varies in its ability to transmit the virus. Transmission, however, is inherited, dominant, and sex linked, with males being heterozygous. Species known to transmit MSV include C. mbila, C. similis, C. storeyi, C. triangula, C. arachidis, C. latens, C. bipunctata, C. ghauri, and C. parazea. C. mbila is considered the most common vector.
A gemini virus, MSV is one of seven viruses that attack maize crops in Africa (worldwide 32 maize viruses have been identified). Eighteen grasses serve as alternate hosts—most of them are annual species. In maize, the virus causes stunting, bareness, interveinal necrosis, and death. The younger the crop at the time of germination the higher the yield loss. In fact, yield losses easily reach 100% if MSV infects a maize crop in its first 3 weeks. In Kenya, MSV outbreaks cause serious losses in various agroecological zones, including the coastal lowlands, central highlands, and the lake basin. MSV exists in mixed populations with isolates that have different virulence.
The single-stranded DNA virus requires leafhoppers (Cicadulina spp.) for transmission; neither the virion nor the viral DNA can be mechanically transmitted. The viral sequence or genome, however, can be delivered into the plant by agroinoculation techniques via Agrobacterium tumefaciens. Agroinoculation makes it easy to distinguish between various levels or mechanisms of MSV resistance, especially when coupled with insect transmission tests. Developed by the John Innes Centre (JIC), UK, agroinoculation enables researchers to differentiate between resistance against the insect vector, resistance against the transmission of the virus, resistance against the virus spreading within the plant, and host infection or immunity.
1.3 Collaborating to Combat MSV
The MSV project is an example of a unique, well-coordinated collaborative effort at global, regional, and national levels to solve an urgent need of resource-poor farmers. Facilitated by ISAAA, the MSV project involves international collaboration from both public institutions and the private sector.
These institutions include the Kenya Agricultural research Institute (KARI), which hosted the project, International Center for Insect Physiology and Ecology (ICIPE) for insect vector studies, the John Innes Centre for molecular marker studies and agroinoculation, and the University of Cape Town (UCT) for expertise in virology needed to identify MSV isolates. Other organizations also made important contributions, including the International Institute of Tropical Agriculture (IITA) in Nigeria, the Centro Internacional de Mejoramiento de Maiz y Trigo (CIMMYT) in Mexico, the Grain Crops Institute, PANNAR Seeds, Kenya Seed Company, and CIRAD.
KARI, one of Africa’s leading national agricultural research service institutions, has direct and indirect networks that can be used to share and disseminate research results within the region. Dr. C. Ndiritu, KARI’s director, points out that "once the capacity for crop biotechnology, research, and development has been well established, Kenya will be able to share its technology and know-how with neighboring countries, including Uganda, Tanzania, Rwanda, and others with resource-poor farmers." Working on MSV for two decades, KARI has taken a multidisciplinary approach that now includes pathologists, entomologists, and plant breeders. KARI’s Dr. Jane Ininda, for example, has been receiving advanced training in mapping gene(s) with resistance to MSV at JIC. Dr. Benjamin Onudi has also been trained at JIC in agroinoculation techniques used for screening maize genotypes with resistance to MSV. Other top experts include JIC’s Dr. Peter Markham and Dr. Edward Rybicki of UCT. JIC is a centre of excellence that conducts research on plant genomes. Its facilities can also maintain the vectors needed to carry out transmission studies.
1.4 Objectives of the MSV Workshop
In bringing together scientists, policymakers government leaders, and others, the workshop sought to accomplish the following objectives:
Overall, the workshop and its proceedings revealed the extent of the progress made towards developing maize varieties that are resistant to MSV disease. Partly by bringing together insights from other MSV programs in the region, the workshop also helped to develop a strategy for future research and development (R&D) of MSV-resistant maize varieties. KARI scientists gained from the peer review and close analysis of the project by experts from the international community. Institutions that had provided MSV-resistant maize germplasm, which included CIRAD’s C390, IITA’s Tzi3, CIMMYT’s CML202, PANNAR’s AO76, and KARI’s Embu 11, took a keen interest in the project results.
For Africa, the workshop was a big step towards controlling or eradicating MSV, which would greatly strengthen food security and alleviate poverty. Significantly, sources of resistance to MSV were identified in maize varieties Reunion Yellow and Arkells Hickory as early as 1931. But 60 years after recognizing MSV disease, Kenyan farmers still have no maize hybrids with MSV resistance. The workshop’s theme, "Advances in MSV Disease Research in Eastern and Southern Africa," reflects the new and increasingly practical efforts used to control MSV disease in a sustainable way. As Dr. Krattiger pointed out, "farmers need practical advice about how to protect their crops from MSV— they do not need a parade of high scientific learning."
The workshop was divided into six sessions: disease epidemiology, vector studies, MSV status reports in Africa and individual countries, breeding and biotechnology, seed production, and MSV in the private sector. Research results from the project show that maize varieties like CIMMYT’s OSU23I, CIRAD’s C390, and IITA’s Tzi3 are resistant to MSV. It should therefore be possible to identify MSV resistance genes using advanced biotechnology skills. Their location will help clarify the relationship among various resistance sources and facilitate the incorporation of MSV resistance genes into hybrids or commercial genotypes. Some of the most exciting results of the project were from a study evaluating the "Expression of disease symptoms among F 3 lines of different parental sources of MSV resistance." KARI maize breeders already have their eyes on short-, medium-, and long-term commercial products that use MSV-resistant populations as a source of future inbred lines.
Workshop participants were also provided with data and reports of achievements of phase I of the MSV Project, including epidemiology studies that are continuing into phase II. These include:
These data suggest that the potential to incorporate MSV resistance into various hybrids is high. According to CIMMYT’s Dr. Alpha Diallo, there is no data to support the widespread notion that resistance to MSV breaks down under various environmental conditions—although there may be variability in resistance levels due to other factors. Analysis of his lead paper on "MSV status in Africa" shows that by 1998 CIMMYT Harare had developed 54 MSV-resistant/tolerant inbred lines that are available to maize researchers upon request. But participants agreed that the final focus should be full immunity against MSV—not tolerance—because the project can avail of advanced biotechnology tools and skills such as marker-assisted selection (MAS), agroinoculation, and genome mapping.
According to Prof. Peter Markham, however, if this is the goal many unanswered questions still prevent scientists from reaching it. Where does the virus go after harvest? Where does it come from? What are the factors that control vector migration or movements? Is the epidemic linked to mutation or new strains that are more virulent? His remarks made clear that much remains to be studied about the relationship between the virus and its hosts.
1.5 Workshop Summary: Achievements of Phase I of the MSV Project
The workshop also provided an opportunity to share information about the project’s structure, programs, and achievements:
Overall, this project has made considerable progress in understanding MSV, its vectors, its epidemiology, and basis for resistance. Work in phase II will focus more on efforts to determine the locations of MSV-resistant genes. As this report of the Workshop indicates, there is a lot of excitement about the preliminary data supplied by phase II.