Farmers’ perceptions and coping strategies with sweet potato weevil and characterization of sweet potato genotypes for diversity and resistance to Cylas puncticollis Boheman in Kenya

Abstract

Sweet potato (Ipomoea batatas (L.) Lam.) contributes significantly to food security and income of subsistence farmers in Kenya. However, productivity of the crop is constrained by several biotic, abiotic and socio-economic factors. Amongst the biotic constraints, insect pests such as the sweet potato weevil (Cylas spp.) cause significant yield losses. However, in Kenya, there is limited information on farmers’ perception and management of Cylas spp. and on diversity among cultivated sweet potato genotypes. The objectives of this study were to: (i) Assess farmers’ perceptions and coping strategies to the sweet potato weevil; (ii) Analyze variation among selected sweet potato genotypes using agro-morphological, molecular and nutritional characters, and; (iii) Screen selected sweet potato (Ipomea batatas L.) genotypes for resistance to the sweet potato weevil. Firstly, participatory rural appraisal approach was conducted in the year 2012 in Homa Bay County where 269 farmers were interviewed on farmers’ perceptions and coping strategies against the sweet potato weevil (Cylas spp.). This study revealed that Cylas spp. was the most problematic (93.3%) pest. Many farmers (90.7%) were not aware of sweet potato genotypes that had field resistance to Cylas spp. The most commonly used methods by farmers to manage Cylas spp. were re-ridging during weeding (21.2%) followed by covering exposed roots with soil (12.6%). Secondly, field experiments were conducted on-station in 2014 at two sites (ATC -Miyare and -KALRO Embu) using 68 sweet potato genotypes arranged in a Randomized Complete Block Design. Data were recorded on variation in agro-morphological, molecular and nutritional characters. On the basis of quantitative agro-morphological traits, Analysis of variance revealed significant (p≤0.05) differences among sweet potato genotypes. Genotypes Nyautenge (16.82 t/ha) and Kemb 10 (17.04 t/ha) had the highest average root yield at ATC Miyare and KALRO Embu respectively while, genotypes 56682-03 (0.84 t/ha) and K/KA/2004/215 (1.07 t/ha) had the least average root yield at ATC -Miyare and KALRO -Embu respectively. The genotypes were variable in respect to all qualitative traits studied. Both quantitative and qualitative based dendrograms did not group the genotypes according to geographical area of origin or shared names. On molecular approach, 13 simple sequence repeat (SSR) markers were used to determine genetic relationship among the sweet potato genotypes. The SSR markers were highly polymorphic (0.2723) and cluster analysis divided the genotypes into two major groups. However, the genotypes did not form specific groups according to geographic regions or shared names. Nutrionally, the genotypes significantly (p≤0.0001) differed in dry matter, root protein, root carotenoids, root sucrose and root starch contents but dendrograms did not group the genotypes in relation to their origin or shared names. Genotype Nyautenge had a stable high yield (16.82 t/ha at ATC -Miyare and 15.23 t/ha at KALRO -Embu) and high dry matter content (40.14% at ATC -Miyare and 32.26% at KALRO -Embu) at both sites. However, the same genotype rated very low in other equally important nutrients like total carotenoids and sucrose contents at ATC -Miyare and KALRO -Embu. Genotypes Kenspot 1, Saly boro, 91/2187, 9 Nduma, Kenspot 3 and Kenspot 2 had high dry matter contents at both sites and hence recommended for inclusion in future breeding programmes. Thirdly, fifty-one selected sweet potato genotypes were evaluated for their resistance to Cylas punticollis Boheman (Coleoptera: Brentidae) in a controlled experiment of no-choice arena from November, 2015 to February, 2016. The 51 evaluated genotypes were significantly (p≤0.0001) different in their resistance to C. puncticollis damage. The study revealed that no genotype was completely resistant to weevils but genotypes Obugi (5.00 adults) and 5 Nyandere (5.00 adults) were the highly resistant to C. puncticollis while genotypes Tainung (25 adults), Naspot 1 (24.33 adults), Kenspot 5 (22.67 adults) and Fundukhusia (22.67 adults) were the most susceptible to C. puncticollis damage. Resistance to weevils was negatively correlated (-0.71) to dry matter content and positively correlated to starch (0.46) and sucrose (0.48) contents. In conclusion, genotype Obugi is a stable high yield performer (9.21 t/ha and 9.55 t/ha in ATC -Miyare and KALRO -Embu respectively) and has a high resistance to C. puncticollis as compared to Naspot 1 (susceptible check) and Santo Amaro (resistant check). Other genotypes rated in this study as medium resistant to C. puncticollis and had earlier recorded high yields at KALRO -Embu are Santo Amaro (11.49 t/ha) and Wera (9.22 t/ha). However, these genotypes recorded low yields in ATC -Miyare and thus may not be suitable for ATC -Miyare site and its surrounding. Genotype Tainung was found to be the most inadequate since it was the most susceptible (25 adults) to weevils, low performing in yield (1.44 t/ha at ATC -Miyare and 5.70 t/ha at KALRO -Embu) and was low in dry matter content (21.40% at ATC -Miyare and 24.39% at KALRO -Embu) as compared to others. All the above-mentioned traits can make the genotype not to be preferred by many farmers. Nonetheless, the genotype has got a high carotenoid content (27.55 µg/g at ATC Miyare and 30.57 µg/g at KALRO Embu) as compared to other genotypes and thus suitable for addressing vitamin A defficiency in the society.