Tackling Cassava Mosaic Disease in Southeast Asia

In December 2015, a journal publication reported evidence of Sri Lankan cassava mosaic virus (SLCMV) in a single plantation in Ratanakiri province in northeast Cambodia. This was the first time that Cassava Mosaic Disease (CMD) was reported in Southeast Asia. Until then, outbreaks of CMD had only been reported in Africa and the Indian subcontinent.

Government representatives, development partners, and cassava researchers from Cambodia, Laos, Thailand, and Vietnam gathered to devise a regional plan to control the potentially devastating Cassava Mosaic Disease (CMD) in Southeast Asia.

Why is cassava such an important crop globally? What is the status of CMD in Southeast Asia right now?  What are the expected repercussions of cassava mosaic disease? What are the primary potential solutions to cassava mosaic disease? What would be the challenges to implement those solutions? What are the projected impacts of cassava mosaic disease on farmers’ livelihoods? 

This series of videos produced during the meeting held in Phnom Penh, Cambodia, on 18–20 September 2018, features these and other questions addressed by experts attending the meeting.

Luis Augusto Becerra
Cassava Program Leader, CIAT

Jonathan Newby
Agricultural Economist, CIAT

Claude Fauquet,
Joint Director Global Cassava Partnerships for the 21st Century

Hernan Ceballos
Cassava Breeder, CIAT

Boonmee Wattanaruangrong
Secretary General at Thai Tapioca Starch Association, Thailand

Viengkham Sengsoulivong
Plant Protection Center, Department of Agriculture, Ministry of Agriculture and Forestry, Laos

Le Huy Ham
Chairman of Science Council Institute of Agricultural Genetics, Vietnam

 

Stakeholders design a motley plan of action to stop the spread of cassava mosaic disease in Southeast Asia

At 250 to 370 Cambodian riel per kilogram during the February harvest, Toun Nath was getting double to triple the price of cassava fresh roots compared to last year’s prices. However, that does not mean that her income had increased. Yields had decreased from 18 tonnes per hectare in 2016 to 15 tonnes per hectare in 2017, and while she has not completed harvesting at the time of this conversation, she said she was anticipating further reduction in yields.

“Most of the plants are diseased,” Toun says, rather pointing out the obvious when we’re standing in the middle of the unharvested field amidst the cassava plants’ broom-like top, and mottled and twisted leaves. The pile of newly dug up fresh roots nearby, many of them smaller than usual, offers another proof of disease.

This section chronicles the efforts of stakeholders in Southeast Asia as they deal with the disease and try to stem its potentially devastating impact on millions of smallholder livelihoods and billion dollars of investments in the region.

What is Cassava Mosaic Disease?

The mottling and twisting of cassava leaves at Toun’s field is a manifestation of Cassava Mosaic Disease or CMD.

CMD is caused by a virus belonging to the virus family, Geminiviridae, and could be transmitted through two ways: through stakes from infected cassava mother plants, or, through the whitefly vector – the insect, Bemisia tabaci.

If infected, cassava plants display a number of symptoms including stunted growth, as well as mosaic, deformed, twisted, and smaller, leaves. Depending on a number of factors including cassava variety, virus isolate, age of the plant at the time of infection, and mode of infection, CMD-affected plants produce few or no roots. Infection through cuttings is known to be more damaging than infection through whiteflies.

The cassava mosaic virus in Cambodia – and mostly likely the one that devastated Toun’s field – has been molecularly identified as belonging in the Sri Lankan cassava mosaic virus (SLCMV) species, which is known to be present in Sri Lanka and India. As the other viruses in the Geminiviridae family, the virus only infects cassava and is not dangerous for animals and humans.

 

The cassava mosaic disease found in Southeast Asia has been linked to the Sri Lankan cassava mosaic virus also present
in India and Sri Lanka.

Where in Southeast Asia is CMD?

The journey of a disease

In December 2015, researchers from Zhejiang University and Holley Group Co. in China published on Plant Disease journal a report of Sri Lankan cassava mosaic virus (SLCMV) in a single plantation in Ratanakiri province in northeast Cambodia. Until then, Cassava Mosaic Disease (CMD) had only been known to be found in Africa and parts of the Indian subcontinent.

Two months after that initial publication, researchers at the International Center for Tropical Agriculture (CIAT) and their collaborators initiated a larger field survey to determine the geographical distribution of CMD in the country. They also carried out laboratory diagnostics to confirm the disease’s pathogen identity.

Laboratory diagnostics confirmed the identity of the virus as SLCMV and signaled its presence in a number of other Cambodian provinces. Also, preliminary results showed that the virus was widespread in Vietnam although no symptoms had been observed in cassava fields.

Later surveys by different research groups completed the picture of the spread of CMD in the region.  To date, Cambodian and Vietnamese authorities have officially reported the presence of CMD in seven provinces in eastern and central Cambodia, and ten provinces in southern Vietnam.

Sites in Cambodia and Vietnam where samples have been collected (green) and where CMD has been confirmed present (red) are shown here on PestDisPlace (https://pestdisplace.org/), a geo-referenced global database on pest and disease occurrence in vegetative propagated crops, including cassava, developed by CIAT and curated by a team of CGIAR researchers.

How did cassava mosaic virus get to Southeast Asia, and how is it spreading around the region?

Experts are not clear at this time as to where the disease in Southeast Asia had originated from.

The distribution of the disease in the plants and in the fields suggest that infections are both occurring from whitefly transmission and infected cuttings. In Cambodia, CIAT researchers found plants showing non-systemic SLCMV symptoms – mosaic only in upper leaves – indicating insect vectoring. On the other hand, they also found plants showing systemic symptoms in heavily infested fields. From the disease distribution, it appears that the spread of infection is more due to stakes rather than insect transmission.

Large amounts of cassava stakes are currently being traded over long distances of up to several hundred kilometers to meet farmer demand. Infection through cuttings is known to be more damaging than infection through whiteflies.

In 2016 CIAT studied existing cassava seed systems in the cassava growing regions of Cambodia and Vietnam to understand how preventing the movement of diseased stakes, and introducing sources of clean material could help to halt the spread of CMD in Southeast Asia. They found that seed exchange ran through farmer-organized ‘informal’ networks, which are complex, connected over multiple scales, and include links between sites as distant as up to more than 250 km from each other. This even while use of their own saved seed accounted for 47 percent and 64 percent of seed use cases in Cambodia and Vietnam, respectively.

While movement within communes was prevalent, with 82 percent and 78 percent of seed provided to others being exchanged between family and acquaintances within the commune in Cambodia and Vietnam, respectively, a proportion of seed flows, facilitated by community collection points (in Cambodia) and mediated mostly by traders (in Vietnam), formed a robust inter-provincial and international exchange network.

In Vietnam’s Tay Ninh, for example, which is a highly commercial production area with dozens of traders operating, a high number of trader-mediated seed exchanges were recorded, many of which eventually fell into trade networks that have a large national and cross-border character. Stakes from Vietnam get transported deep into Cambodia, covering larger distances as they ride along the same transport routes as do root products, and firmly demonstrating Cambodia’s role as a sink rather than a source of seeds.

In Cambodia, 20 percent of seed acquisitions were imported from abroad, particularly, neighboring Vietnam and Thailand.

The cost of CMD in Asia

If cassava mosaic disease (CMD) were to devastate Southeast Asia, not only will the food security of many upland subsistence-oriented households be threatened, but global value chains, billion dollars of investments, and several millions of smallholder livelihoods will face challenges caused by a disruption in the 55 million-tonne supply coming from the region annually.

Dried cassava trade figures, 2012-2016. China accounts for a large share of global cassava trade through the importation of dried cassava chips for animal feed and ethanol production. Thailand and Vietnam are the primary sources of chips with fresh and dried cassava from Cambodia and Lao PDR entering the market predominately through border trade.

 

Cassava starch trade figures, 2012-2016. Thailand and Vietnam exported USD1.76 billion of cassava starch in 2017. Fresh cassava roots from Cambodia currently enter the processing market in Vietnam and Thailand. While current imports are dominated by other Asian countries, there is growing global demand for cassava starch due to its favorable functional traits, as well as changing consumer preferences.

The third most produced crop in Southeast Asia after sugarcane and rice, cassava is a key ingredient in multiple industries such as biofuel, animal feed, and starch, from which a wide range of products including paper, textiles, alcohol, sweeteners, and others, are made.

In sub-Saharan Africa, where the disease ravaged – and continues to do so – since CMD was first reported over a century ago, storage root yield loss across the region were estimated to be between 15 and 24 percent annually – equivalent to 12 to 23 million tons or an annual loss of US$1.2 to 2.3 billion, according to experts.

The market for cassava-based products has continued to expand, particularly as incomes in Asia increased and consumption patterns changed. At the same time, new markets are developing as the demand for ‘clean label’ and other differentiated products – gluten-free, GMO-free – continues to grow.

“Cassava starch (or tapioca) has always had superior functional properties in many applications sought after by food processors and manufacturers, with new breeding efforts enhancing these characteristics adding to the competitiveness of cassava in the starch market,” said Dr. Jonathan Newby, agricultural economist at CIAT. “However, farm level productivity remains critical to ensure competitiveness in the market.”

While the market outlook for cassava will depend on how quickly the disease is brought under control, Dr. Newby highlights the severity of short-term livelihood impacts. “Cassava has many great characteristics in terms of smallholder inclusion. However, the poorest and most vulnerable households are highly exposed to the impacts of this disease. Crop failures associated with CMD will have deep and long lasting impacts on many livelihoods in the region.”

CMD threatens, Southeast Asia responds

Preliminary research identifying the speed and extent of CMD spread in the region had been performed by CIAT, with funding support from the CGIAR Roots, Tubers and Bananas (RTB) Research Program, and the Australian Center for International Agriculture Research (ACIAR), and in coordination with the Cambodian General Directorate on Agriculture (GDA) and Vietnam’s Agriculture Genetics Institute (AGI). CIAT has transferred sources of resistance – CMD1 and CMD2 – to Southeast Asia in order to begin breeding CMD-resistance into cassava varieties preferred by farmers in the region, but more options are needed.

“Beginning 2017, CMD symptoms have become more prevalent, increasing the need for immediate action at the regional level to effectively control a CMD pandemic,” noted Dr. Luis Augusto Becerra, head of CIAT’s cassava program.

The need for a regional response was earlier articulated at the GCP21 (Global Cassava Partnerships for the 21st Century) international conference, participated in by more than 400 representatives of GCP21’s over 40 member organizations globally, held in Benin in June this year.

“Cassava producers in Southeast Asia may not appreciate the threat of viral diseases such as CMD because they had never been exposed to them. But if unabated, it is a disaster waiting to happen, particularly for the millions of poor farmers producing the roots, and the people working in the hundreds of factories processing these roots,” said Dr. Claude Fauquet, GCP 21’s Adjunct Director. “An international response to this threat is therefore necessary to elaborate the best possible plan of CMD control, and a coordinated regional team effort is required to deploy the plan.”

The biggest source of risk for long distance CMD spread from one region to another is the exchange of infected cassava cuttings. According to research done by CIAT, large amounts of cassava stakes are currently being traded in Cambodia and Vietnam over long distances (several hundred kilometers) to meet farmer’s demand. A proportion of seed flows in these countries form a robust inter-provincial and international exchange network, facilitated for the most part, by cassava traders through informal networks.

Efforts to keep currently disease-free areas CMD-free, by restricting movement of cuttings from infected districts to other districts, are necessary and will be discussed with Cambodian and Vietnamese authorities.

Governments in the region, together with their development partners, and researchers, agree that Southeast Asia needs a regional platform to coordinate joint cassava disease monitoring and response. The regional platform is envisaged to be an information exchange avenue where real-time and useful information pertaining to the disease are shared amongst members, as well as a space for planning joint and effective coordinated response to the disease.

“The emergence of CMD in Southeast Asia sets off alarm bells for us to watch out for any signs of other known cassava diseases in the region such as cassava witches’ broom (CWB) or others not previously seen in the region, for example, cassava brown streak disease (CBSD),” noted Dr. Wilmer Cuellar, lead virologist at CIAT.

Plans of action in the short term will include: propagating and distributing virus-free stakes or planting materials of existing varieties, as well as improved cassava varieties; testing and dissemination of CMD-resistant cassava varieties; and implementing information and communication campaigns with national extension services to encourage cassava traders and farmers to help stop the spreading of the disease by trading and using only virus-free planting materials.

At the regional meeting, the various stakeholders will look into the following possible course of action:

Immediate term

  • Integrate ongoing surveillance data collected by different research groups working in the region to identify currently affected and immediately threatened areas.
  • Map variety distribution in areas of high CMD incidence using an agreed molecular markers based diagnostic approach.
  • Disseminate information among extension services and farmers to improve recognition and management of the disease by good farming practices (i.e. application of balanced fertilizer and timely weeding).
  • Do rapid multiplication of and distribute virus-free cuttings to farmers in affected parts of the region.

Mid-term

  • Import, test, and multiply existing CMD-resistant material in several locations in the region.
  • Develop by introgression the source of CMD resistance present in African varieties into improved materials in Southeast Asia.

Long-term

  • Introgression of virus resistance genes into varieties that meet regional needs.

Behind the scenes… time to talk about (not so) serious issues

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