Degeneration of sweetpotato seed in Tanzania: can cleaned-up, virus-tested seed help?

Abstract

Low yields caused by poor phytosanitary quality of seed vines are a major challenge to sweetpotato farming in Tanzania. Poor seed quality is largely caused by viruses, especially the co-infection of sweet potato feathery mottle virus (SPFMV) and sweet potato chlorotic stunt virus (SPCSV). These two viruses are the most devastating ones in East Africa and when combined cause the sweetpotato virus disease (SPVD), often leading to 56 − 100% yield losses. Being vegetatively propagated, sweetpotato accumulates viruses across generations. The use of virus-infected material obtained from the previous crop is the most common source of viruses. The accumulation of viruses in the propagating material leads to progressive yield loss and is called seed degeneration. Use of clean seed disseminated through a formal system is one of the options to address this. However, clean seed can get infected once in the field and it is not known how it will perform following several seasons of on-farm propagation. Moreover, successful use of clean seed requires an understanding of the current practices that farmers use when sourcing sweetpotato seed. This research started by assessing the performance of clean seed and farmer-sourced seed of varieties that are susceptible or relatively resistant to viruses grown in environments with high or low virus pressure. The results showed that starting propagation with clean seed slows down degeneration especially in susceptible varieties grown in high-viruspressure environments. The findings also showed yield stability for farmer-sourced material of virus resistant varieties. The yield stability for farmer-sourced material of the relatively virus-resistant variety illustrates why farmers recycle planting material. The findings also showed that regular replacement by clean seed is especially economic in areas with high virus pressure and for susceptible varieties. In addition, it is important to consider cultivar resistance and agroecology when promoting adoption of clean seed.To understand farmers' experiences with degeneration a 'Small-N' survey was conducted with 37 respondents. We found that female farmers reported degeneration more frequent than male farmers. The main action to address degeneration was to acquire a new variety, mostly from close sources in the farmers' social networks (friends, neighbors and relatives). Planting material was mostly acquired and shared as gifts. Farm seed recycling still dominates. These networks will continue to play an important role in sweetpotato seed flows and therefore it is important to devise innovative ways of linking them with the formal seed system for access to clean seed and improved varieties.Clean seed is not immune to viruses and can get infected once planted in the open fields. We investigated the efficiency of insect-proof net tunnels in reducing degeneration of clean seed at seed producer level. Results from a 21-month experiment showed that insect-proof net tunnels can prevent infection in an area with high virus pressure for up to 20 months....