Response of local sweet potato varieties to virus infection

Abstract

Sweet potato is an important crop in Kenya not only as a staple food in many communities, but also as a food security crop. The full production potential of sweet potato has however not been realized due to numerous production constraints with pests and diseases being rated most important. Lack of quantitative information regarding relationships between disease severity and the corresponding yield losses has been a major impediment in the evaluation of control programs for many disease problems. Therefore, the aim of this study was to analyze the effect of virus infection on root yield, vine production and source-sink interaction of six local sweet potato varieties (Bungoma, Nyathi Odiewo, Kemb 10, KSP 36, SPK 004 and CPT 560). The specific objectives of this study were (i) to determine the effect of single virus infections with sweet potato feathery mottle virus (SPFMV) and sweet potato chlorotic stunt virus (SPCSV) on growth, vigour and root yield of sweet potato (ii) to determine the effect of dual infection with SPFMV and SPCSV i.e. SPVD on growth, vigour and root yield of sweet potato and (iii) to determine the effect of single viruses (SPFMV and SPCSV) and the combination (SPVD) on, relative chlorophyll content, specific leaf area (SLA) and leaf area index (LAI) of sweet potato. The study was conducted at Jomo Kenyatta University of Agriculture and Technology. The experiment was set up in randomized complete block laid out as a two factor split plot replicated three times. The data collected during growth included vine number, vine length and leaf number. The experiment was terminated five months after planting by carefully harvesting the roots and vines. Data collected at harvest included, specific leaf area, leaf area index, fresh root weight, number of marketable and unmarketable roots, dry weight of shoots and roots, biomass and harvest index. Data collected were subjected to ANOV A using Proc GLM and means separated using SKN in the SAS statistical package. Findings from this study show . that single virus infection with SPFMV had the least severe effect on growth, vigour (number of vines, vine length and leaf number) and yield (fresh weight of roots, the number of marketable roots and harvest index) in all the six sweet potato varieties. Single virus infection with SPCSV reduced growth and vigour notably in all the six sweet potato varieties. Varieties were not significantly (P= 0.05) different from each other in the way they responded to virus infection in most of the parameters analyzed. Nyathi Odiewo however, revealed considerable virus tolerance as it was associated with the least reduction on growth (13%) and yield (37%) as compared to other varieties. Dual virus infection with both SPFMV and SPCSV i.e. (SPVD) consistently caused the most severe reduction on growth, vigour (56-74%) and yield (89-93%) in all the sweet potato varieties. Single virus infection with SPFMV did not drastically reduce relative chlorophyll content. SPCSV on the other hand, significantly reduced chlorophyll content and increased specific leaf area by (9- 38%).SPVD caused the most severe reduction on relative chlorophyll content (20- 30%) and had the highest disease severity scores. The range of symptoms caused by sweet potato viruses therefore, indicate the extent to which they interfere with assimilate production and partitioning. Furthermore, viruses compete and divert host resources for their own growth and replication, disrupting the source-sink relationships in the infected plant. This study underscored the fact that dual virus infection with the SPFMV and SPCSV has devastating effects on growth, vigour and yield of sweet potato as opposed to the single virus effects which were less severe. It is therefore recommended that field trials be done in many regions with a wider range of varieties to elucidate yield stability in the respective regions. Variety Nyathi Odiewo should be given special focus to assess the reason behind its high virus ?tolerance and thus be recommended to farmers.