Genetic enhancement of sweetpotato for weevil (Cylas spp.) resistance, storage root yield and yield-related traits.

Sweetpotato (Ipomoea batatas [L.] Lam.) is an important crop providing livelihood and economic opportunities for millions of smallholder farmers in sub-Saharan Africa (SSA). Weevil infestation caused by sweetpotato weevil (Cylas spp.) is one of the main factors contributing to the low storage yields of the crop in SAA, including Tanzania. Field sanitation, early planting, early harvesting and chemical treatments are the suggested control options to manage sweetpotato weevils. However, these strategies are less effective and unsustainable. Breeding sweetpotato varieties with durable resistance to weevils and enhanced yield and yield components is advocated as an economic and environmentally friendly strategy to control weevils and to boost production and productivity of the crop. Therefore, the objectives of this study were: (i) to identify farmers’ perceptions on sweetpotato weevil damage, production constraints and criteria used to select and grow the best sweetpotato varieties in western Tanzania, (ii) to determine variation among Tanzanian sweetpotato germplasm for dry yield and yield-related traits, (iii) to assess Tanzanian sweetpotato germplasm for resistance to weevils in western Tanzania and (iv) to determine inheritance of weevil resistance, yield and yield-related traits in newly developed sweetpotato clones. The first study was undertaken using a participatory rural appraisal (PRA) method in four selected districts of western Tanzania. Data were collected using a structured questionnaire, focus group discussions and transect walk involving 122 sweetpotato farmers. Farmers identified weevil damage to be the overriding constraint to sweetpotato production followed by sweetpotato diseases and drought. Farmers’ preferred agronomic traits of sweetpotato included high yield, drought tolerance and disease and weevil resistance. Dry matter content was the most important quality trait followed by reduced cooking time, taste and fiber content. Seventy six sweetpotato accessions collected from Tanzania and 20 accessions received from International Potato Centre (CIP) in Lima/Peru were field characterized in two seasons using a 16 x 6 triple lattice design to determine their genetic diversity. Genotypes New Kawogo, Kiti cha Nyerere and Kisu cha Masai had the highest root yields of 10.14, 9.85, 9.67 t/ha, respectively The following genotypes were identified with high dry matter content: Ngw’anangusa (43.50%), Rugomoka (43.30%) and Secondary (43.30%). Two major genetic groups with genetic diversity of 0.54 were distinguished for further selection. A total of 96 sweetpotato genotypes were screened for weevil resistance and yield and yield-related traits in two selected sites in western Tanzania using a 12 x 8 lattice design with three replications at each site. Nine sweetpotato genotypes (Kibandule, Malulumba, Utitiri, 3-CIP, Madebe, Magunhwa, 5-CIP, Kafu and Chuchu ya nesi) expressing weevil resistance and 10 genotypes (Nyamvuva, sengi, 22-CIP, Rugomoka, Tumauma, Ejumla, Carot C, New Kawogo 2, Haraka and 4-CIP) with moderate resistance to weevils were identified. Magunhwa, Chuchu ya Nesi, Rugomoka, Tumauma and New Kawogo were the best five genotypes selected displaying promising weevil resistance and desirable yield and yield-related traits. The last experiment determined gene action and heritability of weevil resistance, yield and yield components and dry matter content and selected best parents and families. Six weevil resistant and six susceptible parents were crossed using a 6 x 6 North Carolina Design II mating design. The 36 families were evaluated at three locations using a 3 x 12 lattice design with two replications. Results showed that additive gene action showing a >0.5 general predicted ratio (GPR) was more influential for total root number (TRN), root yield (RY), dry matter content (DMC), percentage of infested root number (PIRN), percentage infested root yield (PIRY) and weevil damage score (WDS). Non-additive gene action was more influential for percentage of marketable root number (PMRN) and percentage of marketable root yield (PMRY) with a <0.5 GPR. The narrow sense heritability for TRN, RY, DMC, PIRN and WDS were 0.24, 0.56, 0.84, 0.62 and 0.62, while the broad sense heritability for these traits were 0.58, 0.72, 0.93, 0.78 and 0.77, in that order. Good combiner parents for RY were Simama, 2-CIP, 8-CIP and 17-CIP, while good combiner parents for DMC were Burenda, Kasinia, Masinia and 8-CIP. Genotypes Burenda, Kasinia, Masinia, 4-CIP and 5-CIP were good combiners for weevil resistance assessed through WDS. The best selected families for RY were Kasinia x 8-CIP, Simama x 2-CIP, Jewel x 5-CIP and Masinia x 18. The following families: Burenda x 2-CIP, Kasinia x 8-CIP, Masinia x 17-CIP and Simama x 17-CIP were superior for high DMC. The families Jewel x 18-CIP, Simama x 4-CIP, Masinia x 2-CIP and Kasinia x 5-CIP were selected for improved WDS. In general, the study selected good combining sweetpotato parents that can be used to develop recombinant populations for future breeding. Further, best families were selected with promising root yield, dry matter content and weevil resistance which will be subjected to multi-environmental evaluation for cultivar release in western Tanzania.