Kwame Ogero

Seed degeneration caused by viruses is a major constraint to sweetpotato production at the Lake Zone, Tanzania. Dual infection with sweet potato feathery mottle virus (SPFMV) and sweet potato chlorotic stunt virus (SPCSV) can cause up to 100% yield losses. Use of cleaned-up, virus-tested vines disseminated through a formal system is one of the strategies to address this constraint. However, clean seed can get infected once exposed to virus-spreading vectors in the field leading to degeneration and yield losses therefore requiring replacement. There is a need to understand the interactions between viruses, the crop and the environment better. This will enable farmers to know the pattern of degeneration of planting material in their localities, which control measures to use and when to purchase new planting material. Understanding variations in yields of clean planting material after several generations of field propagation defines how seed producers can sustain the supply of quality planting material. Several practices have been promoted to aid on-farm management of sweetpotato viruses during seed production. These include use of insect-proof net tunnels to protect clean seed from virus vectors. However, their effectiveness in reducing virus infection is not well understood. Moreover, successful uptake of clean seed requires an understanding of the current seed sourcing strategies among farmers in Tanzania. This includes understanding how they perceive and react to degeneration.

This thesis sought to assess how cleaned-up, virus-tested seed can be incorporated into smallholder farming systems to address degeneration in sweetpotato. The first chapter is a General Introduction on seed degeneration, sweetpotato production in Tanzania and the challenges posed by virus-related seed degeneration. Chapters 2 to 5 are research chapters that sought to answer the following questions:
a) How does sweetpotato seed sourced from clean virus-tested plants degenerate in different agroecologies following several seasons of on-farm propagation? (Chapter 2)
b) Can insect-proof net tunnels limit virus infection of clean sweetpotato planting material? (Chapter 3)
c) How does ratooning affect vine production in insect-proof net tunnels? (Chapter 4)
d) What are farmers’ experiences with and actions towards degeneration including seed sourcing strategies? (Chapter 5).

Chapter 2 was based on experiments conducted over five seasons in a high- and low-virus-pressure environment. The experiments compared degeneration of seed sourced from clean virus-tested plants and farmer-sourced seed over the five seasons. The analysis in this chapter shows that clean virus-tested seed slows down degeneration especially for susceptible varieties grown in high-virus-pressure environments. It also shows yield stability for farmer-sourced material of resistant varieties. This provides more evidence on why farmers recycle planting material. This chapter shows that it is important to consider cultivar resistance and agroecology when promoting adoption of clean virus-tested seed.

Chapter 3 captures the effectiveness of insect-proof net tunnels in reducing seed degeneration. It presents findings from a 21-month experiment assessing virus infection on vines grown in net tunnels and open fields. In addition, a SeedHealth model (see https://tools4seedsystems.org/) was used to model percentages of yield loss over ten seasons in scenario analyses for a high-virus-pressure site. The chapter shows that insect-proof net tunnels can prevent infection in a high-virus-pressure area for up to 20 months. The SeedHealth modeling showed that plants sourced from net tunnels would still attain more than 60% of the expected yields after 10 generations while those sourced from open fields would not do so after only 4 generations. The findings form a basis for recommending adoption of net-protected structures such as screenhouses in high-virus-pressure environments to reduce rapid infection of clean seed. These can be adopted by well-resourced farmers producing at medium or large scale.

Following the successful demonstration by Chapter 3 that insect-proof net tunnels can reduce the rate of virus infection in clean seed it was necessary to assess how farmers can successfully manage the plants to avoid losses not related to virus infection. This included evaluating the effect of ratooning in vine production in the net tunnels. Chapter 4 captures findings from a 14-month experiment comparing a ratoon cropping technique and a replanting technique in net tunnels and open fields. This chapter shows that ratooning increases vine production. However, the technique leads to higher virus incidences on plants grown in the open. Ratooning is recommended for vine production in net tunnels and screenhouses. If used in the open fields, it should be accompanied with virus control measures.

To understand farmers’ experiences with and actions towards degeneration, a ‘small N’ survey was conducted with 37 farmers (20 female and 17 male). The findings captured in Chapter 5 show that more women reported seed degeneration than men. Those experiencing degeneration said that the main signs were production of fewer and smaller roots. This is indicative of declining yields. The Chapter also shows that farmers associate degeneration with yellowing and stunting of plants – key signs of virus infection.

In addition, Chapter 5 shows the main action to address degeneration was to seek a new variety rather than getting cleaner planting material of the degenerated variety. This indicates the need to inform farmers that degenerated varieties can be cleaned-up and returned into the system especially if they are market-preferred. Chapter 5 also indicates that sweetpotato planting material is shared within very close networks (friends, neighbours and relatives) and that on-farm seed recycling still dominates.

Chapter 6 puts the results of Chapters 2 up to and including 5 in a broader context. It also draws some comparisons with seed degeneration in other root, tuber and banana crops. Chapter 6 notes the importance of getting more information on farmers’ experiences with degeneration. Such data can be combined with data from biophysical experiments to come up with better management strategies.

The findings captured in this thesis show that using clean seed can help improve seed quality in sweetpotato across generations. However, it is important to consider other important factors that can influence its adoption. These include farmer seed sourcing strategies and risk of re-infection. Characterizing the farmer seed exchange networks can give a detailed description of and bring to the forefront the value embedded in them and how they can be tapped to disseminate clean seed. It can also help understand how different types of farmers have access to different types, sources and origins of planting material.