ABSTRACT

Farmers in Nigeria usually use sweetpotato vine cuttings from previous crops or volunteer plants as planting materials. This results in accumulation of systemic pathogens, especially viruses,and information on the rate of degeneration due to this practice is very meagre. Seventeen orange-fleshed sweetpotato genotypes (Amelia, Delvia, Gloria, Irene, Jane, Lourdes, Melinda, Namanga, Sumaia, Tiojoe, Umuspol, Umuspo 3, Bela, Cecelia, Erica, Esther and Ininda) were evaluated during the 2015 and 2016 planting seasons at Iresi in southwest and Umudike in southeast Nigeria, to determine the rate of cultivar degeneration over three generations. The experiment was a split- plot in randomized complete block design with three replications. The two locations (Iresi and Umudike) constituted the main plot and the genotypes were assigned to the subplots. The trials started with virus-free planting material and succeeding trials retained planting material from previous crops. The observed virus symptoms were small twisted leaves with reduced leaf-size, vein chlorosis, purpling of mature leaves and stunted growth. There was higher virus incidence at Iresi in the first year of cropping while Umudike had higher disease incidence in the second year. At 8 WAP, Cecelia and Esther had higher virus incidence than other genotypes and along with Bela, Erica and Ininda , did not survive beyond the first season of cropping. Genotype Jane had the highest virus incidence and severity while Umuspo 1, Umuspo 3, Delvia, Tiojoe and Sumaia had the least in the second year of cropping. Vine length at 8 to 12 WAP, number of branches at 10 to 12 WAP, number of storage roots per plant and storage root yield were significantly higher at Iresi than at Umudike. In the first season of cropping, Umuspo 3, Ininda and Namanga produced significantly higher storage root yields than other genotypes, except Umuspo 1, Erica, Tiojoe and Gloria. In the second year of cropping, however, genotypes Umuspo 1 and Umuspo 3 produced the highest storage root yield, followed by Namanga, Jane and Delvia. Genotypes that contributed to the significantly higher storage root yield of the second year cropping were Umuspo 1, Umuspo 3, Sumaia, Namanga, Irene and Delvia. Storage root yields were negatively and significantly correlated with virus incidence and severity but positively and significantly correlated with number of branches at 12 WAP, number of storage roots and root weight.