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Your Enemy
A worsening problem Pauline Ruiru lives near Githunguri, in the nutrient rich highlands north of Nairobi. Blessed with favorable rainfall and fertile red soils, these highlands should be a land of plenty, brimming copiously with cash crops such as tea, pyrethrum, and the many fruits and vegetables in great demand in nearby urban markets. Farming should be a profitable business here, but the prevalence of maize streak virus (MSV) disease strikes at the heart of the farming system, destroying the main crop on which small-scale farmers rely for subsistence. Farmers like Ruiru watch their young plants anxiously for the tell tale symptoms that give the disease its name. Chlorotic streaks appear on new leaves, presaging stunted growth and death. Even if the plant survives, flowering is impeded and little seed is set. Indigenous to Africa and its offshore islands, maize streak is caused by a virus present in many of the region's native grasses. There it might have stayed, were it not for the feeding habits of an insect known as Cicadulina, a member of the leafhopper family The insect, which feeds on a wide range of crops besides maize, acts as a vector for the disease, transmitting the virus through its saliva. "The problem is getting worse each year," Ruiru laments. She points sadly at the withered stalks, a shadow of a crop. This is all that remains for her. "I used to get at least some yield from my maize. Now I'm not getting any at all."
Her experience corresponds with what past studies of the disease in Kenya have shown. Incidence rarely exceeded 10% before 1980, but since then epidemics have become recurrent, pervasive, and more destructive. The pattern in Kenya repeats itself throughout sub-Saharan Africa. Reports indicate that MSV is becoming an increasingly serious problem in a swathe of countries from South Africa, where it was first identified, to Senegal in the west and Ethiopia in the east. Over the past decade, epidemics have occurred not only in Kenya but also in Angola, Mozambique, Zambia, and several other countries. Several factors have helped spread the disease. The area of land devoted to the cultivation of maize has grown, especially in the moist, mid-altitude grasslands where leafhoppers breed. Where farmers use irrigation to produce a second crop during the dry season, the plants become particularly susceptible because with few other plant hosts available large numbers of leafhoppers descend upon the crops. Furthermore, most farmers grow the traditional open pollinated varieties of maize, which MSV quickly destroys. But modern plant breeding hasn't been able to contain the problem either: many of the hybrids introduced over the past decade are now susceptible to the disease. The common staggering of sowing dates also increases the severity of the disease. Maize streak is harmless if it occurs more than eight weeks after the maize crop has germinated. But if it attacks within three weeks of emergence, losses approach 100%. In areas with longer growing seasons, where farmers sow maize on two or three different dates, the disease easily spreads from older to younger crops as the season progresses, leaving behind rows and rows of stunted, withered stalks.
Control options Farmers currently have few options for controlling maize streak. Insecticides could be used to destroy the leafhopper vector, but they are too expensive for the resource-poor farmers who am hardest hit by MSV. Besides, the relief provided by chemicals would present a hazard to human health and the environment. It would also be short-lived. Insects resistant to commercially available products would soon evolve, as Helicoverpa and many other pests have done.
Early sowing can help the crop reach a safe age before populations of the leafhopper build up. But in areas with multiple sowing dates— especially those with two growing seasons— this strategy does not work. Viral inoculum and vector populations accumulate during the first season or after the first sowing, providing a ready source of infection for the second. The best hope of protection lies in genetic resistance. Sources of resistance to MSV have been known and used in breeding programs since the 1930s. Among the public-sector research institutions that have reported progress in developing resistant materials are the Centro Internacional de Mejoramiento de Maiz y Trigo (CIMMYT), the International Institute of Tropical Agriculture (IITA), the Centre de Cooperation Internationale en Recherche Agronomique pour le Developpement (CIRAD), and the Kenya Agricultural Research Institute (KARI). In the private sector, companies such as Pannar (South Africa) and the Seed Coop (Zimbabwe) also stock resistant varieties. But despite their availability, few resistant materials have been widely tested and virtually none of the farmers use them in eastern Africa. The reasons for this are not fully understood, but the major factors seem to be that resistance either breaks down or is accompanied by unacceptably low yields. "All the institutions with breeding programs claim, with some justification, to have solved the maize streak problem. But their ‘resistant' materials usually succumb to the disease somewhere in Kenya," observes Jackson Njuguna, plant pathologist with KARI.
Unanswered questions Breeding for resistance is hampered by inadequate knowledge in three major areas. First, the pathogen possesses considerable variability. Effective screening and breeding require accurate characterization of viruses and their strains as well as the early detection of any changes in them. The University of Cape Town in South Africa has generated much of the knowledge available about MSV at the regional level. Scientists at the university warn that only a small proportion of the overall diversity of MSV in Africa has been sampled, and that its relations with other viruses are little understood. Njuguna's research in Kenya described three different agro-ecological zones, each containing pathogens of different severity. This study provides a useful foundation on which further research can build, but much more needs to be done. Indeed, countries other than Kenya have yet to make a start on characterization. The second area is our insufficient understanding of the genetic basis of resistance, which is highly complex. Studies so far suggest that there are three kinds of resistance. First, the plant may be able to deter the insect from feeding on it, thereby preventing infection from taking place at all. Secondly, the insect may feed on the host plant, but for some reason the virus fails to transmit itself. Thirdly, the replication of the virus within the plant may be blocked. Previous research on resistance has failed to distinguish between these three types, so we rarely know which mechanism is at work in specific instances. This has made it almost impossible to devise an effective and practical screening program. It is also unclear whether one gene or several genes confer resistance in each case. The work of a Ugandan student at Ohio State University, in the USA, suggested that the trait was monogenic, but researchers at IITA and in Malawi indicate that it is polygenic. The insect vector constitutes a third area in need of further investigation. Of the 18 species of Cicadulina present in Africa, nine are known to transmit the disease. But how many of the others also do? What am the factors that make some species more effective at transmission than others? What role does the weather (rainfall and wind) play in the buildup and migration of pest populations? And what opportunities for biological control do the insect's life cycle, its natural enemies, and alternate plant hosts offer? The establishment of effective breeding and screening programs as well as the development and deployment of possible transgenic resistance depend upon finding answers to these questions.
Answering the questions Under this ISAAA-brokered project, four institutions are pooling their expertise in a fresh attempt to answer these questions. Underpinning the project are field studies conducted in Kenya by KARI and proprietary technology developed by the John Innes Centre (JIC) in the UK. These provide the raw material for laboratory research in the UK and South Africa, research that KARI scientists also participate in. The KARI team began by asking all the institutions involved in resistance breeding to send their most resistant lines for inclusion in a combined multilocational screening trial. They received germplasm from CIMMYT, CIRAD, IITA, Pannar, and others. A trial to determine the extent of resistance to MSV in these lines is currently under way at eight locations across the country, locations specially selected for the probability of exposing the germplasm to different isolates of the virus. During the trial the scientists will save samples of each line affected by MSV at each location. After extracting DNA from each sample, two members of the team will take the virus isolates to the University of Cape Town for characterization. Under the direction of Dr. Edward Rybicki in the Department of Microbiology, they will learn how to use different genetic markers, such as restriction fragment length polymorphisms (RFLPs) and polymerase chain reaction (PCR), to determine variability in the virus. This research will yield useful information about how stable these fines' resistance might be across Africa. Until recently, breeders have not been able to distinguish between resistance to the insect and resistance to the virus. Scientists at JIC have overcome this problem by developing the techniques of agro-infection and cell bombardment. In agroinfection, a proprietary technique developed by the centre's scientists, the insect vector found in nature is replaced by Agrobacterium tumefaciens, a bacterium into which the DNA of the virus is inserted. Using Agrobacterium as a vector eliminates plant palatability as a factor in transmission, thus revealing genuine cases of resistance to the virus. Further characterization involving cell bombardment enables the scientist to detect whether the virus can replicate within individual cells. Both techniques are accompanied by tests of insect feeding behavior, which detect cases of resistance to the insect. Under the project, two maize breeders from KARI will visit JIC to learn and apply the techniques. They will benefit from the considerable expertise acquired by JIC's scientists, led by Dr. Peter Markham and Dr. Margaret Boulton, who now have more than a decade's experience of working on molecular and other aspects of MSV. Through the agro-infection technique the KARI scientists will also have access to a patented technology, the benefits of which JIC has agreed to share with KARI and other institutions in Africa. Meanwhile, scientists at Kenya's International Centre for Insect Physiology and Ecology (ICIPE) continue to contribute their expertise to the development of a better understanding of the insect vector. They will collaborate with KARI's pathologists and entomologists in a study of the distribution patterns and population dynamics of all the Cicadulina species found at each trial location. An important part of this research will be the identification of alternate plant hosts. ICIPE will also host a Kenyan postdoctoral research fellow to work with its scientists on testing different leafhoppers for their effectiveness in transmitting the virus. The results of the field trial will also serve as the basis for more advanced research that will develop molecular markers for resistance. KARI scientists plan to cross a susceptible line with two resistant lines of contrasting parentage. A member of the team will spend several months at JIC studying the distribution of the trait in the progeny, to assess whether resistance appears to be controlled by one or by several genes. RFLPs will be used to analyze the genome of the progeny in preparation for making a molecular map of resistance.
Joint action plan The four institutions (KARI, JIC, ICIPE, and the University of Cape Town) will present the initial results of their collaboration on MSV at an international workshop to be held in Kenya in late 1998. Everyone who has contributed germplasm or who has an interest in MSV in the region and elsewhere will be invited, including scientists from national institutions in neighboring countries. Thus the project will optimize benefits by sharing the critical information it generates as widely as possible. In addition to keeping participants up to date with developments in the field, the workshop will also serve as a forum for formulating a joint plan of action for future collaborative research, development, and technology transfer. In so doing it will play an important part in overcoming the serious yet fragmentary efforts that have held back progress in the past.
Eventually, a combination of improved screening against a broader range of viral strains and a better understanding of the mechanisms of resistance should enable a cross-institutional team of scientists to 'pyramid' the relevant genes in new, higher-yielding varieties with more stable resistance. The varieties could form the central component of an integrated pest/ disease management program. The scientists have a long way to go before they can offer farmers such as Ruiru a definitive solution to maize streak disease. But by working together to increase our understanding of the disease, they are taking big steps in the right direction. |
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