Tanzania and partners tackle cassava disease

Tanzania’s Ministry of Agriculture, Food Security and Cooperatives (MAFC) and the Bill & Melinda Gates Foundation have launched three new projects to support efforts to develop cassava varieties with resistance to Cassava Mosaic Disease (CMD) and Cassava Brown Streak Disease (CBSD) and to establish more sustainable seed systems to provide smallholder farmers better access to such varieties.

The projects were launched during a cassava value chain event in Dar es Salaam that brought together representatives from the government, donor community, private sector and development partners.

Farmers in Tanzania and the region need access to planting materials of new improved varieties released in the country. Cassava is a very important crop not only for food security but it also has great potential as a cash crop through processing. The two diseases, especially CBSD, are a major problem and need to be urgently addressed.

The Cassava Varieties and Clean Seed to Combat CBSD and CMD (5CP) project will facilitate sharing of five of the best varieties from Tanzania, Kenya, Malawi, Mozambique, and Uganda for regional testing across the countries to speed up the development of varieties with dual resistance to the two diseases.

Transgenics in crop improvement research

Leena Tripathi (l.tripathi@cgiar.org)
Biotechnologist, IITA, Nairobi, Kenya

Biotechnology has opened unprecedented avenues for exploring biological systems. Transgenics is one of the key techniques particularly useful for the genetic improvement of crops that are not amenable to conventional breeding, such as those that are vegetatively propagated. In IITA, transgenic technologies are being used for improving banana/plantain (Musa sp.), cassava (Manihot esculenta), and yam (Dioscorea sp.).

Harvested bunch of transgenic banana, Kampala, Uganda. Photo by L. Tripathi.
Harvested bunch of transgenic banana, Kampala, Uganda. Photo by L. Tripathi.
Genetic transformation platform
An efficient protocol for plant regeneration and transformation is a prerequisite for the successful use of transgenic technologies. Despite the technical difficulties in transforming monocot species, efficient transformation protocols that are embryogenic cell suspension based and Agrobacterium mediated have been established for many cultivars of banana/plantain. This system, however, is a lengthy process and cultivar dependent. Therefore, a transformation protocol using meristematic tissues was also established which is rapid and genotype independent. These protocols have paved a way for the genetic manipulation of banana/plantain by incorporating agronomically important traits such as those conferring resistance to diseases or pests as well as tolerance to abiotic stress factors.

Agrobacterium-mediated transformation protocols for three popular cassava varieties preferred by African farmers were established through somatic embryogenesis. A regeneration and transformation protocol is also established for yam (Dioscorea rotundata and D. alata) using nodal explants, but transformation efficiency needs to be improved. A transformation protocol using somatic embryogenic callus for yam is under development.

Development of disease- and pest-resistant transgenic crops
Banana Xanthomonas wilt (BXW), caused by the bacterium Xanthomonas campestris pv. musacearum (Xcm), is the most devastating disease of banana in the Great Lakes region of Africa. In the absence of natural host plant resistance, IITA, in partnership with NARO-Uganda and the African Agricultural Technology Foundation, has developed transgenic banana by constitutively expressing the Hypersensitive Response Assisting Protein (Hrap) or plant ferredoxin-like protein (Pflp) gene from sweet pepper (Capsicum annuum). The transgenic plants have exhibited strong resistance to BXW in the laboratory and screenhouse tests. The best 65 resistant lines were planted in a confined field trial at the National Agricultural Research Laboratories (NARL), Kawanda, Uganda, for further evaluation.

Transgenic technologies provide a platform for controlling diseases in banana, cassava, and cowpea. Photo by IITA.
Transgenic technologies provide a platform for controlling diseases in banana, cassava, and cowpea. Photo by IITA.
Based on results from mother plants and their first ratoon plants, 12 lines were identified that show absolute resistance. The plant phenotype and the bunch weight and size of transgenic lines are similar to those of nontransgenic plants. These lines will be further tested in a multilocation trial in Uganda. They will be evaluated for environmental and food safety in compliance with Uganda’s biosafety regulations, risk assessment and management, and procedures for seed registration and release, and are expected to be released to farmers in 2017.

Cassava brown streak disease (CBSD) has emerged as the biggest threat to cassava cultivation in East Africa. As known sources of resistance are difficult to introgress by conventional methods into the cultivars that farmers prefer, the integration of resistance traits via transgenics holds a significant potential to address CBSD. Of the available transgenic approaches, RNA silencing is a very promising strategy that has been successfully employed to control viral diseases. IITA, in collaboration with Donald Danforth Plant Science Centre (DDPSC), USA, is developing CBSD-resistant cassava for East Africa.

Nematodes pose severe production constraints, with losses estimated at about 20% worldwide. Locally, however, losses of 40% or more occur frequently, particularly in areas prone to tropical storms that topple the banana plants. IITA, in collaboration with the University of Leeds, UK, has generated transgenic plantain using maize cystatin that limits the digestion of dietary protein by nematodes, synthetic peptide that disrupts chemoreception, or both of these traits. These lines expressing the transgenes were challenged in a replicated screenhouse trial with a mixed population of the banana nematodes, Radopholus similis and Helicotylenchus multicinctus. Many lines were significantly resistant to nematodes compared with nontransgenic controls. The promising transgenic lines showing high resistance will be planted in confined fields in Uganda for further evaluation in mid-2012.

Transgenic technologies for abiotic stress tolerance
Cassava roots undergo rapid deterioration within 24–48 hours after harvest, the so-called postharvest physiological deterioration (PPD), which renders the roots unpalatable and unmarketable. IITA, in collaboration with the Swiss Federal Institute of Technology (ETH) Zurich, is developing cassava tolerant of PPD through the modification of ROS (reactive oxygen species) scavenging systems. The potential is being assessed of various ROS production and scavenging enzymes, such as superoxide dismutase, dehydroascorbate reductase, nucleoside diphosphate kinase 2, and abscisic acid responsive element-binding protein 9 genes, to reduce the oxidative stress and the extent of PPD in transgenic cassava plants.

Future road map
Efforts at IITA over the last 10 years to establish transformation protocols for all the IITA crops have been paying off and have led to the establishment of a genetic transformation platform for cassava, banana/plantain, and yam―the three most important food crops in sub-Saharan Africa. These technologies have contributed to significant advances in incorporating resistance to pests and diseases in banana and cassava. Some of these technologies have the potential to offer additional benefits. For instance, the transgenic technology to control Xanthomonas wilt may also provide an effective control of other bacterial diseases of banana (Moko, blood, and bugtok diseases), and of bacterial blight in other crops such as cassava and cowpea.

Combating the threat of CBSD

Cassava brown streak disease (CBSD) is a virus disease that has emerged as a serious threat to production in Eastern and Southern Africa.

Brown streak-affected cassava. Photos by L. Kumar, IITA.
Brown streak-affected cassava. Photos by L. Kumar, IITA.

Two virus species, Cassava brown streak virus and Cassava brown streak Uganda virus, the cassava brown streak viruses or CBSVs, have been recognized to cause CBSD. The infection results in mosaic symptoms on leaves, brown streaks on stems, and a corky necrosis in tuberous roots.

Root necrosis has the most damaging effects on the use and marketability of the tubers and thus affects the livelihoods of cassava farmers. It can make susceptible varieties unusable if the roots are left in the ground for over 9 months.

CBSVs are spread through the planting of infected stem cuttings and also by a vector, a whitefly, Bemisia tabaci. The foliar symptoms of CBSD are less conspicuous and farmers are often unaware of the problem until they harvest the roots and the corky, yellow-brown necrotic rot becomes evident.
There is no cure for the disease. Once plants become infected, the only option for growers is to uproot and destroy them. The use of virus-free planting material and the cultivation of resistant varieties are the only options for the control of CBSD.

Where is it and where is it heading?
CBSD is endemic in Kenya, Malawi, Mozambique, Tanzania, and Uganda and its occurrence is suspected in Burundi, Gabon, Madagascar, DRC, and Rwanda. Available evidence suggests a westward spread of the disease.

What is IITA doing about it?
IITA has adopted a multipronged strategy to tackle CBSD, to reduce the effects on cassava in epidemic areas, and prevent a further spread of the disease. Its efforts begin with informing governments about the threat. The four technical pillars of this strategy are as follows.

-Monitor disease spread and assess its impact: Key outputs include (a) the development of disease distribution maps, (b) estimates of yield loss, and (c) identification of targets for development.

-Understand disease etiology and epidemiology; develop tools for monitoring and forewarning: Key outputs include (a) understanding the effects of the viruses in cassava, (b) examining the characteristics of virus spread, (c) creating diagnostic tools for CBSVs, and (d) using digital-enabled field surveillance tools for real time reporting and a monitoring network.

-Develop and disseminate durable CBSD-resistant cassava cultivars: Key outputs include (a) screening and selecting over 40 elite cassava cultivars with dual resistance/tolerance to CBSD and cassava mosaic disease (CMD) appropriate for various countries, (b) deploying tolerant varieties for farmers to cultivate in East Africa, (c) developing molecular markers and modern molecular breeding tools for the accelerated development of CBSD-resistant varieties, (d) pre-breeding in areas currently not affected by the viruses, and (e) developing clean seed systems for the multiplication and dissemination of virus-free planting material.

-Capacity building through the transfer of knowledge, technology, and products to stakeholders: IITA has (a) built a coalition of international teams to combat CBSD, (b) trained scientists, extension workers, and plant quarantine officials in disease recognition, monitoring, and diagnostics, (c) established regional diagnostic labs, (d) created awareness through the use of the mass media, and (e) provided technical backstopping to national efforts in combating CBSD.

A suite of knowledge, technologies, and products derived so far from IITA’s R4D efforts is playing a vital role in checking the spread of the disease and has contributed to reviving cassava production in areas affected by the epidemic. However, complete recovery and the prevention of any further spread of CBSD are still a long way off. They require a strong commitment from national and international communities to sustain the ongoing and emerging research and development efforts that are devising effective and eco-friendly technologies for sub-Saharan Africa.

Prof Mike Thresh, Scientist Emeritus, Natural Resources Institute, UK (right), during his visit to IITA-Ibadan. Photo by L. Kumar, IITA.
Prof Mike Thresh, Scientist Emeritus, Natural Resources Institute, UK (right), during his visit to IITA-Ibadan. Photo by L. Kumar, IITA.

Advice to stakeholders
In countries where CBSD is already established, IITA recommends that governments require the use of available CBSD control programs, including the adoption of promising CBSD-resistant cultivars, and the production and distribution of clean cassava planting material.

Countries not yet affected need to increase their vigilance and develop the capacity to recognize CBSD and deploy eradication programs; establish plans for preemptive action to reduce the risk of CBSD spreading from affected regions; and put in place programs to produce and distribute clean planting material.

All the cassava-producing countries in Africa should:

-Organize large-scale awareness creation programs to inform farmers, extension workers, CSOs, and national research entities about CBSD, the eradication of infected plants, and the steps for disease control.
-Strengthen the monitoring capacity of the national quarantine authorities and other relevant bodies including the establishment of communication systems for a rapid response to prevent disease and eradicate infections where they are identified.
-Develop resistant varieties most urgently, through breeding, using both conventional and transgenic approaches.
-Put in place a strategy for the production and distribution of clean cassava planting material, and adopt improved varieties with resistance to CBSD and CMD.
-Affirm financial and political support for collaboration, cooperation, and coordination to prevent the further spread of CBSD in tropical Africa.

DEWN: a novel surveillance system

Innocent Ndyetabula*, indyetabura@yahoo.com and James Legg, j.legg@cgiar.org
*Maruku Agricultural Research Institute, PO Box 127, Bukoba, Tanzania

Researchers inspect cassava plants for disease incidence. Photo by IITA.
Researchers inspect cassava plants for disease incidence. Photo by IITA.

Pandemics of cassava mosaic disease (CMD) and cassava brown streak disease (CBSD) are the most important biotic constraints to cassava production in East and Central Africa.

For several years, researchers have tracked these two diseases and monitored patterns of pandemic expansion. However, costs have been high, and the visits made once a year have barely kept pace with the rate of disease spread.

Hence, researchers working to control these problems resolved to explore other monitoring options. During early discussions, two themes were frequently highlighted: community participation and new technology. Could both of these be incorporated into an alternative approach to monitoring disease spread in such a way that the system would provide an early warning of new outbreaks?

The result was the Digital Early Warning Network or DEWN. After extensive consultation, a plan was developed for its pilot-level implementation. This system works with six farmers’ groups in each of 10 disease-threatened districts of northwestern Tanzania, and provides them with a system based on the use of the mobile phone for reporting incidences of CMD and CBSD in their farms. By communicating monthly with farmers’ groups, it was expected that new outbreaks would be identified quickly, allowing the timely implementation of control measures.

Partnerships
The pilot phase of DEWN has been primarily implemented by the Lake Zone Agricultural Research Institute (LZARDI), under the IITA-coordinated Disease Objective of the Great Lakes Cassava Initiative (GLCI). GLCI is funded by the Bill and Melinda Gates Foundation (BMGF) and is led by the Catholic Relief Services (CRS). The partners of GLCI in the DEWN target districts included several local NGOs (TAHEA, MRHP, KUMKUMAKA, RUDDO, and TCRS) as well as the local government agricultural advisory system.

Training
At the outset, it was essential to train all participating farmers’ groups to recognize the symptoms of the two virus diseases, and introduce the SMS-based communication system. A total of 1281 farmers were trained in the 60 groups, and district partners were provided with a GPS unit and digital camera to record field locations and any unusual disease symptoms.

Each of the farmers’ groups was provided with a basic GSM phone and SIM card and introduced to the simple texting system for sending monthly disease reports. A straightforward text format was used for the farmers’ groups to provide information on how many farmers had observed each of the two diseases in their fields that month, and for how many farmers each disease had become more severe, less severe, or stayed the same. Once reports had been compiled at the farmers’ group level, they were sent as a single text to the LZARDI modem.

Outcomes
Validation visit. A follow-up visit was made after 6 months to validate farmers’ reports. A refresher course was provided, but the farmers generally indicated a good knowledge of the main symptoms of both diseases. Partly as a consequence of their new understanding of the significance of CMD and CBSD, there was a strong demand from participating farmers for improved varieties.

Voice of the Farmer reports. Participating farmers were linked to the Voice of the Farmer project (VOF). This is a project that is executed by Synovate and financed by BMGF. It aims to use a network of call centers to provide monitoring and evaluation support to existing BMGF programs.

Map based on farmers and researchers' report of CMD occurrence in Lake Zone districts of Tanzania.
Map based on farmers and researchers' report of CMD occurrence in Lake Zone districts of Tanzania.

DEWN provided a means for VOF to communicate directly with many of the participating farmers. This enabled VOF to conduct two surveys to assess the effectiveness of DEWN’s training program on the identification and management of cassava pests and diseases. Participating farmers were called directly by VOF call center staff and were asked a series of short questions in Swahili. Although farmers’ responses indicated a good general knowledge of CMD and CBSD, some confusion about symptoms was evident, highlighting the need for further training support. The VOF–DEWN reports are available online at www.vof.synovate.co.ke.

Mapping new disease outbreaks. Information obtained from the DEWN reports received from farmers’ groups was used to generate maps. One of the most significant findings was that CBSD, reported by farmers via SMS, was then confirmed by researchers’ visits in two districts (Bukombe and Urambo) in which CBSD had not previously been reported. This has allowed project teams to focus extra disease mitigation efforts on these areas.

Extending DEWN. Recognizing the potential value of DEWN for providing communities with a means of doing their own monitoring of crop disease, the GLCI cassava team in Rwanda decided to start a similar scheme. Farmers’ representatives from Rwanda visited DEWN partners in Tanzania in October 2010 and were introduced to the approach and given training in recognizing CBSD and CMD. The Rwanda team will initiate its own DEWN program in 2011.

Map based on farmers and researchers' report of CBSD occurrence in Lake Zone districts of Tanzania.
Map based on farmers and researchers' report of CBSD occurrence in Lake Zone districts of Tanzania.

DEWN has provided an innovative, informative, and relatively cheap means for involving communities in monitoring the health of their own crops. Farmers’ participation has been enthusiastic, and some important practical outcomes have been achieved. Two of the greatest challenges which remain, however, are the accurate diagnosis of CBSD, which has cryptic or unrecognized symptoms and the regular provision of feedback to participating communities.

Plans are already being developed to address these problems. As these difficulties are overcome and as connectivity in rural areas continues to expand, it seems certain that there is great potential for the more widespread use of digital networks such as DEWN for the community-based monitoring of crop diseases.

Developing clean seed systems for cassava

James Legg, j.legg@cgiar.org

Cassava stems for future crop. Photo by L. Kumar, IITA.
Cassava stems for future crop. Photo by L. Kumar, IITA.

Cassava is one of those crops that uses part of the plant for propagation. It is very convenient to use vegetative material from a previous crop to plant a new one. This is one of the beauties of vegetatively propagated crops. However, this convenience comes at a price. The use of planting material from a previous generation to establish the next provides an easy way for disease-causing pathogens, particularly viruses, to pass directly from one plant generation to another. So, while they offer convenience, vegetatively-propagated crops are often more widely affected by pathogens than those planted in the form of true seeds.

In Africa, cassava is the most widely cultivated of the vegetatively-propagated crops, being grown on more than 12 million ha across the continent. The exotic pest introductions, cassava mealybug and cassava green mite, caused great damage to Africa’s cassava crop in the 1980s and 1990s, but both have been effectively managed through the implementation of a classical biological control program.

The fungal diseases, cassava bacterial blight (Xanthomonas axonopodis pv. manihotis) and cassava anthracnose (Colletotrichum gloeosporioides f. sp. manihotis) are locally important. The greatest current constraints to cassava production, however, are the virus diseases, cassava mosaic disease (CMD) caused by cassava mosaic geminiviruses (CMGs) and cassava brown streak disease (CBSD) caused by cassava brown streak viruses (CBSVs), which together cause crop losses worth more than US$1 billion annually.

One of the most important approaches to controlling these virus diseases, as well as other pathogens of cassava, is through the avoidance of infection. This can be achieved by starting out with pathogen-tested plants, and then bulking the planting material through a series of quality controlled multiplication steps. Although it sounds very simple, this can be difficult to achieve in practice.

Pathogen testing requires well-equipped laboratories run by adequately trained staff. Quality management in the field requires extensive grassroots knowledge of disease symptoms and the involvement of an appropriately trained and resourced national plant protection organization. In many parts of sub-Saharan Africa, capacity for these functions remains insufficient to meet the demands.

IITA and its partners have made significant progress in developing and implementing new systems to maintain the health of cassava through seed systems. For instance, through the Great Lakes Cassava Initiative (GLCI), a multi-partnered project implemented from 2007 to the present in Burundi, Democratic Republic of Congo, Kenya, Rwanda, Tanzania, and Uganda, a rigorous system has been put in place to assure the health of cassava planting material. This has been particularly important in view of the rapid recent spread of a devastating pandemic of CBSD in East Africa.

Healthy cassava plant. Photo by IITA.
Healthy cassava plant. Photo by IITA.

The key components of the quality and health management system are as follows: Primary (centralized seed production sites) managed by researchers or qualified seed producers, secondary, and tertiary multiplication sites (usually in farmers’ fields) are all assessed, at least once in a year, using the Quality Management Protocol (QMP). This sets out quality levels, primarily in terms of disease and pest incidence and material quality that must be met if the field is to “pass”.

The QMP standards for CMD and CBSD incidences ascertained by diagnostic tests are <10% for primary and secondary sites and <20% for tertiary sites in endemic areas. Planting materials from fields that fail to meet QMP standards are not distributed or used for further multiplication, although the tuberous roots can be used by the growers for consumption. Fields that meet the QMP standard and test negative for CBSVs are approved for more widespread dissemination.

This is the first time that this level of rigor has been applied to maintaining the health of cassava through multiplication programs in sub-Saharan Africa. It has been invaluable in assuring the health of the planting material provided to more than half a million beneficiaries in six countries, and provides an important model for other current and future cassava development programs.

Much remains to be done before such an approach can be used in a more sustainable way. Most importantly, basic capacity needs to be strengthened in most countries. Key elements of this include the laboratory and human capacity for virus indexing, as well as the knowledge of QMP and the capacity of the national plant quarantine organization to monitor cassava seed systems.

In addition, the management of cassava diseases could be greatly enhanced by the establishment of isolated nuclear multiplication sites planted with virus-tested cassava plantlets derived from tissue culture, as well as by raising awareness among growers about the importance of establishing the next crop with healthy planting material.

A long-term goal, as the commercial value of cassava increases, will be to provide a mechanism through which planting material certified through the QMP attracts a price premium. Creating added value is certain to be the key to the future development of clean seed systems for cassava in Africa. IITA and its partners are strongly committed to reaching this goal.

Dual-resistance cassava

IITA scientists are a step closer to making a breakthrough in developing cassava that is resistant to both the cassava brown streak disease (CBSD) and the cassava mosaic disease (CMD) in Eastern and Central Africa. The two diseases are the biggest threats to cassava production in the region putting at risk the food security and livelihoods of over 200 million people.

According to Edward Kanju, IITA cassava breeder, 14 types of the crop under research are very promising. Kanju’s team had just harvested an advanced trial of such cassava in Uganda.

This is the fourth year of trials for dual-resistance cassava for the mid-altitudes in Uganda. The trials are being conducted at Mukono, near Kampala, an area regarded as a hot spot for CBSD and CMD. The breeding work started with over 5,000 true seeds of parents with tolerance to CBSD from Tanzania for crossing with IITA varieties that are resistant to CMD.

Farmers have also been involved in the selection process to ensure that the varieties meet their preferences on cooking, taste, texture, and yield. Breeding for dual-resistance cassava is also being conducted in Tanzania and DR Congo.

CBSD: Enemy number 1

Caroline Herron, c.herron@cgiar.org

Tertiary vein chloroses of CBSD. Photo by C. Herron
Tertiary vein chloroses of CBSD. Photo by C. Herron
A major disease is ravaging cassava production in the lowland areas of Eastern Africa. The culprit is the cassava brown streak disease (CBSD), which has been spreading to higher altitude areas.

CBSD was first described in detail in Tanzania in the early 20th century and for decades, has been causing severe economic losses in coastal and low altitude Eastern Africa where cassava mosaic disease (CMD) is endemic.

Symptoms
CBSD symptoms are not as distinct in cassava foliage as those of CMD. The main differences are that in CBSD the leaves are not distorted in shape (no epinasty); and the myriad of chlorotic mosaics and blotches in the leaves commonly start as tertiary vein chloroses and are also often very insignificant compared with those from CMD.

Two symptoms in the same plant: CMD (top left) and CBSD (bottom right). Photo by C. Herron
Two symptoms in the same plant: CMD (top left) and CBSD (bottom right). Photo by C. Herron
CBSD leaf symptoms may be limited to the lower leaves only, depending on the cultivar. If the above-ground symptoms are in the lower leaves only, then during plant maturity these often senesce, and thus symptoms can be missed. Other CBSD symptoms in very susceptible cultivars are necrotic longitudinal streaks on the stem (from where the disease derives its name)—which eventually coalesce from the shoot downwards, plant shoot dieback, and petiole necroses.

In the tuberous storage roots of susceptible cultivars, CBSD’s dry necrotic lesions may develop from approximately 5 months onwards, depending on the cultivar, with or without any external root symptoms. Root lesions vary widely; in a cross-section of roots these symptoms may range from dry dark brown radially aligned lesions to lesions with a dry white firm interior and a necrotic exterior. The time of first appearance of root symptoms and the tissues in which the symptoms appear are also variable.

External symptoms in some cultivars appear as radial root constrictions, creating abnormal root shapes. Root symptoms may also be irregularly distributed across fields planted with the same cultivar and also within roots of the same plant. CBSD in the roots can lead to significant reductions in quality and yield, or complete spoilage.

Various CBSD root symptoms on cultivars from Tanzania. Photo by C. Herron, IITA
Various CBSD root symptoms on cultivars from Tanzania. Photo by C. Herron, IITA
Economic costs
Alleviating the CBSD problem would reduce human burden and toil during the growing season and during processing, and enhance livelihoods.

On average, cassava yields are approximately 3.5 t/ha in CBSD-infested fields compared to 10 t/ha in fields without CBSD. The estimated economic loss due to CBSD is about US$130/ha, based on a sample in eastern Tanzania. Extrapolating to the entire cassava-producing regions of the country shows a loss of $45 million/year based on a conservative 14% yield loss from early harvesting to escape heavy CBSD losses. If the farmers’ estimated yield loss of 64% at full physiological maturity is taken into consideration, the estimated annual loss for the country is as high as $202 million (Manyong et al. 2008).

Current management strategies
The use of field-resistant cultivars is the recommended disease management strategy. Many field-resistant cultivars should be deployed at any one time in every cassava cultivation area where CBSD is endemic. Susceptible cultivars are not recommended for these areas.

Across the region, many national agricultural research systems (NARS) partners are involved in ongoing cassava breeding programs and multilocational field trials. True seeds from lines of resistant materials identified in CBSD-infested areas (coastal Tanzania) over many growing seasons have been distributed to surrounding countries for incorporation of resistance in farmer-acceptable cultivars. This management strategy has allowed farmers in some of the worst-affected CBSD areas, who had given up growing cassava, to grow and thrive from cassava once more.

Annual monitoring and evaluation of the CBSV presence and CBSV strains over production areas should be combined with this approach. Other management strategies are also being evaluated currently for usefulness, such as use of virus-free propagative materials.

Sustaining disease management
Sustaining CBSD management through planting field-resistant cultivars means that scientists cannot afford to be complacent. While these cultivars are proving to be an excellent and the sole current first line of defense against the pathogen, the situation is dynamic, and CBSD damage may be expected over time. This is because genetically diverse pathogens, if allowed to exist widely and in large numbers, can eventually cause damage to earlier tolerant cultivars. Certain pathogen strains or isolates become more “fit” within the population, eventually leading to disease damage or “resistance breakdown”.

Ideally, a continuous “pipeline” of cultivars may be needed in every cassava-growing area, together with large-scale detection and monitoring of CBSV strains predominant in the population in any given area on an annual basis. This will provide an early warning detection system for the CBSV strain population shift or resistance breakdown. Long-term management strategies and the development of true resistant cassava cultivars are still desirable.

To date, all new cassava breeding lines tested in Tanzania in field trials have CBSV in many tissues. This indicates that CBSV immunity may not be present within currently used materials. Other attempts must thus be made to provide useful and different mechanisms of resistance to the pathogen. Hybridization between cassava and wild relatives and use of pathogen-mediated resistance could provide various types of resistance to CBSV.

cassava_tissue_cultureThe transformation approach
Constraints with the traditional approach make the use of transformation a viable alternative in incorporating virus resistance to cassava. The Mikocheni Agricultural Research Institute (MARI) under the Ministry of Agriculture, Food Security and Co-operatives, and IITA are undertaking cassava genetic transformation in Tanzania. The project is funded by the Rockefeller Foundation, but will continue to be funded in 2009 by the Bill and Melinda Gates Foundation. Other partners include the Plant Biosafety Office, Tropical Pesticides Research Institute; Agricultural Biosafety Scientific Advisory Committee; National Biosafety Focal Point; and National Biotechnology Advisory Committee.

To date, work on incorporating resistance to the East African cassava mosaic virus and CBSV in cassava cultivars is ongoing. Twenty-six farmer-preferred cultivars from Tanzania that are high yielding but susceptible to CMD and CBSV have been micropropagated. From these cultivars, a total of 1014 plantlets had been produced by end-September 2008. So far, cassava cultivar Kibandameno produced the highest number of plantlets (282) followed by Karatasi (58), while Pikipiki and Katakya from the Lake Victoria zone produced the least (4). These tissue culture plantlets will be used for producing embryogenic cultures for transformation.

Reference
Manyong V.M, E.E. Kanju, G. Mkamilo, H. Saleh, and V.J. Rweyendela. 2008. Baseline study on livelihood status and technology adoption levels in Cassava Brown Streak Disease (CBSD)-infected areas of Eastern Tanzania and Zanzibar. Technical Report. IITA-Tanzania. 37 pages.