Understanding seed potato selection practices in Uganda

Potato (Solanum tuberosum L.) is an important crop for food security and cash income for  smallholder farmers in Uganda. However, the national mean potato yield has been in decline to less than 5 Mg ha-1 in 2016. Low productivity of potato might be associated with poor and diverse adoption of innovative crop management practices. Smallholder farmers in Uganda commonly use seed potato tubers from the informal sector, especially by seed recycling over several generations. Therefore, seed tubers are highly degenerated with viruses and other diseases, resulting in poor yield and quality of the produce. Over one cycle of multiplication, the degeneration management by positive seed selection was found to be efficient in reducing virus diseases compared with the farmers’ method of selection. The aim of this thesis was to provide novel information regarding understanding positive seed selection by investigating it across multiple cycles of multiplication with an interdisciplinary approach.

To identify potato farms that are homogeneous in uptake of innovations (use of fertilizer, organic input, fungicides, pesticides, seed selection methods, seed renewal by using quality declared seed, and sole cropping), a farm typology was used and socio-economic characteristics, access to agricultural extension services, memberships of farmers’ groups, yield levels of potato and economic return rates were assessed. A farm household survey (n=270) was carried out and principal component analysis and cluster analysis were used to identify types of farms differing in adoption of innovations. Four farm types were identified that demonstrated significant differences in uptake of innovation practices; these differences in uptake were associated with small but significant differences in yield and further in land ownership, availability of labourers and cash, economic return, and access to knowledge. The farm type with relatively high frequencies of using organic input, fungicide input, pesticide input, seed plot technology or positive selection, quality declared seed and sole cropping achieved highest potato productivity; the farm type with relatively frequent use of fungicide input and no use of pesticides was associated with the lowest potato yield.

To assess to what extent positive selection over several seasons can reduce incidences of six different viruses in seed lots of different starting quality, multi-seasonal trials were carried out in three locations, with five seed lots from four sources and three cultivars. Detection of viruses was based on DAS-ELISA and Luminex xMAP technology. Results showed fluctuations in some viruses over seasons, with lower Potato leafroll virus (PLRV) and Potato virus X (PVX) incidences in lots from positive selection compared with lots from farmers’ selection. Some seed lots were initially highly infected with Potato virus S (PVS) and Potato virus M (PVM) and showed no reduction in virus incidence through positive selection. In general, little infection with Potato virus Y (PVY) and Potato virus A (PVA) was found.

To investigate how effectively positive selection enhances yield and underlying crop characteristics, positive selection was compared with farmers’ seed selection for up to three seasons in three field trials at different locations. Across all experiments, seasons and seed lots, yields were higher under positive selection than under farmers’ selection. The average yield increase resulting from positive selection was 12%, but yield increases were variable, ranging from –5.7% to +36.9%, and in the individual experiments often not significant. These yield increases were associated with higher yields per plant, and mostly higher weights per tuber, whereas the numbers of tubers per plant were not significantly different. Experimentation and yield assessment were hampered by a varying number of plants that could not be harvested because plants had to be rogued from the experimental plots because of bacterial wilt (more frequent under farmers’ selection than under positive selection), plants disappeared from the experimental field and sometimes plants did not emerge.

To evaluate costs and benefits of positive selection in order to assess its feasibility and affordability, data from the smallholder farms in the four farm types were used for an economic analysis. It showed that farms that already adopted positive selection, invested on average 1.2 extra days (i.e. 2.7 extra labourer days) per acre in positive selection, with an average of 4.0% extra labour costs. A scenario study among the non-adopters of positive selection, assuming a 10% extra yield by carrying out positive selection, showed that a marginal rate of return of adopting positive selection of far above 100% was achieved in every farm type. Gross and net benefit varied because of different yield increases and different selling prices of potatoes in the different farm types, indicating that some farm households benefitted more than others.

The present study shows that positive selection does fit in the current seed system for smallholder farmers and has the capacity to increase yield and reduce viruses with visible symptoms compared to farmers’ selection. Positive selection being part of the informal and integrated seed sector will help improve seed quality and seed health in farmers’ networks.