GENETIC STUDIES ON ORANGE-FLESHED SWEETPOTATO FOR HIGH DRY MATTER CONTENT AND RESISTANCE TO SWEETPOTATO VIRUS DISEASE
In order to develop acceptable orange-fleshed sweetpotato (OFSP) to replace the low dry matter content and sweetpotato virus disease (SPVD)-susceptible varieties that are facing low adoption, studies were conducted to identify sweetpotato farmer-preferred traits that influence adoption, and to use the information generated to develop acceptable orange-fleshed sweetpotato genotypes capable of combating vitamin A deficiency. Surveys were conducted using structured questionnaires in sweetpotato producing communities in two states from different agro-ecologies. Sweetpotato germplasm comprising local cultivars, released varieties and exotic genotypes from the International Potato Center (CIP), Peru conserved at the National Root Crops Research Institute, Umudike, Nigeria, was evaluated in two locations for their agronomic, disease resistance, dry matter content and starch content. Parents were selected from the evaluated germplasm and crosses were made with the aim of developing OFSP genotypes with high dry matter content and resistance to SPVD, and to study the inheritance of dry matter and resistance to SPVD. The survey showed that key root quality traits that influence sweetpotato farmers’ preferences were root yield, large root size, root storability, high dry matter content and early maturity. Sweetpotato virus diseases (SPVD) and the sweetpotato weevil were the main biological constraints reported by the respondents. Low dry matter of the OFSPs was a major utilization constraint as it affects oil absorption of fried roots and too soft texture of boiled roots. Evaluation of the 52 germplasm collections revealed wide variability among the genotypes evaluated for yield, yield components, dry matter content and SPVD resistance. The moderate broad-sense heritability (HB)estimates of 0.39 – 0.44 for yield and yield components, and high H2 estimates of 0.71 and 0.80 for SPVD incidence and severity, and 0.87, 0.82 and 0.76 for dry matter content, starch content and peel-loss, with the high genetic advance (% mean) suggest that
moderate to high genetic gain was possible through selection. Five OFSP, five white-fleshed, and five yellow fleshed genotypes based on their yield, carotenoid content, dry matter content and resistance to SPVD were thus selected from the germplasm as parents for hybridization. Following hybridization and progeny evaluation, 31 superior F1 genotypes with fresh root yield range of 10.30 – 20.69 tons/ha and SPVD severity score of 0.21 – 3.22 (where 5 = severely damaged) were identified. Sixteen orange-fleshed (pro vitamin A) genotypes with high root yield (10.20 – 20.65 tons/ha), high beta-carotene (1.5 – 11.03 mg/100g fw) and high dry matter (28.50 – 37.69) were selected as promising OFSP genotypes. On the inheritance study, the genetic control of SPVD incidence and severity, dry matter content and starch content was under both additive and no-additive gene effect, with additive gene effect more pronounced. All the traits were heritable with high genetic advance, and all the traits showed heterosis. The selected thirty-one genotypes that cut across all flesh colours, and sixteen specific OFSP genotypes with desired traits need to be evaluated further in more locations under bigger plot size and more replications to know their true agronomic performance.