Understanding cassava seed networks to curb disease spread in Southeast Asia

If you have a garden, you probably buy seeds from the store, or maybe even swap some varieties with your friends or relatives. Exchanging seed can be a very simple act. But how is it accomplished when billions of plants are involved, and exchanges take place at the national or international scale? A new publication asks this question for Southeast Asian cassava, the backbone of production for millions of smallholder farmers.

The 3.5 million hectares of cassava planted every year in Southeast Asia requires around 40 billion plants to be established in the right place, and at the right time. But how the seed gets there without the existence of a formal seed sector remains surprisingly poorly understood. Unravelling how varieties, diseases and information move between farmers is critical to effectively target plant health surveillance and interventions.

Cassava stem traders unload trucks in Eastern Cambodia. Photo: CIAT

We often glimpse the silhouettes of seed movement by observing side effects – a new variety appears far from where it originated, or a slow-moving pest or disease hopscotches between distant farms. These hints have indicated that cassava stems are being moved widely in Southeast Asia, but how and by whom remained a mystery until recently, when researchers from the International Center for Tropical Agriculture (CIAT) and the CGIAR Research Program on Roots, Tubers and Bananas (RTB) undertook the first comprehensive evaluation of cassava seed use and exchange in Vietnam and Cambodia.

Cassava production over time (1961-2014). Mouse over the data for more detail, or click on the legend to switch regions on or off and take a closer look.

How is the spread of pests and diseases related to cassava seed? To explain we need a quick refresher on plant biology.

In agriculture the word ‘seed’ is often a catchall for the different types of plant tissues used to produce new crops. These include actual botanical seeds, but also other plant parts such as stems, roots, and rhizomes. For many root and tuber crops, including cassava, farmers have mostly done away with the use of true seeds altogether through the special ability of ‘vegetative propagation’.

Thousands of years ago, our ancestors discovered that some plants could be snipped, planted, and re-sprout identical little copies. But the repetitive cycle of cutting and replanting also poses risks. Mother plants may acquire diseases during their lifetime, and then pass them on to daughter plants in the following generation. Insect pests can easily hitchhike on the big, bulky planting stems. The gradual decline in seed quality caused by these accumulating factors – in particular viruses – is referred to as ‘seed degeneration,’ and makes studying seed systems very important.

Cassava planting stakes. Photo: CIAT

In Vietnam and Cambodia, we interviewed 840 farmers to gather information on seed use in 31 districts. We asked farmers about their seed use during the past year, including all exchanges they made with others (both who they acquired seed from, and who they gave it to). The interactive graphs below allows you to explore some data from four contrasting provinces with different production characteristics:

Farmer seed acquisitions in 2016-17. Interact with the data by clicking and dragging the categories.

We found that seed exchanges in the high production intensity sites in both countries are overwhelmingly managed by male farmers, while the low intensity production sites have quite equal male and female participation. The result shows that male farmers manage most of the seed acquisition from traders, and only men interacted with the other ‘commercial’ seed sources. This shows that, in addition to the biological importance of seed systems, social factors such as gender, trust, and accessibility influence seed exchange.

Farmer provision of seed to others in 2016-17

In this figure we can clearly see that one province (Tay Ninh) is responsible for most of the seed supply to traders, indicating the presence of a stronger stem export network. It is also interesting to note that only 9% of seed supply was provided to farmers that the respondent did not personally know.

The data also reveals elaborate regional flows of seed between different types of actors in the seed system, including farmers, cassava root collection centers, and specialized traders. To better understand the spatial structure of the network, we constructed a map showing the seed exchange network at the provincial level, providing a snapshot of seed transport across the region.

Cassava planting material exchange in Vietnam and Cambodia (2016-17). The country squares indicate cases where planting material came from a given country, but the reciepient did not know which province. usually this occurs when a stem trader transports the material.

In just a single season, cassava seed travelled long distances to and from the farmers in our sample, moving around the region and freely crossing international borders. There is a lot to unpack in the data, but here are five major takeaways about local seed systems from the recent publication:

  1. The cassava seed system is farmer and trader-driven and regulated through social networks, with very little ‘formal’ seed production or quality control. 
  2. Frequent seed exchange occurs between farmers in the same community, but re-use of the farmer’s own seed supply remains the most common source of seed. 
  3. Volumes reaching several thousand planting stakes moved distances exceeding 300km in the study year. 
  4. Dedicated cassava traders have a robust network, and play an especially important role in cross-border transactions. 
  5. International movement of cassava seed is common, supplying 20% of stakes used at the national scale in Cambodia. All of the exchanges recorded went into Cambodia from neighboring countries; none went out.

These findings give us new insights on how varieties, diseases, and information spread through farmer networks. The results are particularly timely, as a spate of seed-vectored pests and diseases (including the newly arrived cassava mosaic virus) are making inroads in cassava fields across the region.

An increased understanding of seed use patterns helps us to identify key points for plant health surveillance and interventions, and to explore the potential for quality controlled seed production systems to gain a foothold in Southeast Asia. Work is ongoing to document the functioning of official sources of seed, like government multiplication centers and NGO or private sector sponsored activities. Evaluating the costs of seed production and mechanisms of seed dissemination will help to create strong interfaces with farmer seed networks, improving access to healthy seed across the region.

Blog contributed by Erik Delaquis, Associate Researcher, CIAT

The full manuscript is available for free at this open-access link (Delaquis et al., 2018).

This research was undertaken as part of the CGIAR Research Program on Roots, Tubers and Bananas (RTB), with funding support provided by the Australian Centre for International Agricultural Research (ACIAR) and CGIAR Trust Fund contributors.