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Nicholas A. George, Kenneth V. Pecota, and G. Craig Yencho

Sweetpotato (Ipomoea batatas) is one of the world’s most important and widely grown starch crops. It is usually produced for direct human consumption but can be readily converted to simple sugars that then have industrial end uses. The objective of this study was to compare the carbohydrate yield of the conventional sweetpotato cultivar, Beauregard, with new clones selected specifically for higher carbohydrate production. Ten sweetpotato clones were grown from both slips and root pieces at five locations, over 2 years, in North Carolina. A sweetpotato clone selected for high carbohydrate production, and planted as slips, yielded on average 4150 kg·ha−1 of carbohydrates, 10% to 15% higher than Beauregard. The chemical composition of roots was unaffected by planting method, but slips usually outyielded the same clone grown from root pieces. Carbohydrate yield was significantly impacted by genotype × environment effects in both slips and root pieces. We conclude that further work will be needed to develop sweetpotato clones with both high carbohydrate content and high yield potential, and that are also adapted to planting from root pieces. Any breeding and development work will need to take into account genotype × environment effects.

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Wanda W. Collins, Kenneth V. Pecota, and G. Craig Yencho

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Ryan N. Contreras, Thomas G. Ranney, Susana R. Milla-Lewis, and G. Craig Yencho

Morphological analysis historically has been used to determine parentage of unknown hybrids. This can be difficult when potential parents have similar appearance, as in the case of three azaleodendron cultivars, Rhododendron L. ‘Fragrans’, ‘Fragrans Affinity’, and ‘Fragrant Affinity’. These cultivars are similar in name and appearance, and all are purported hybrids of R. catawbiense Michx. or R. ponticum L. and R. viscosum (L.) Torr. Amplified fragment length polymorphism (AFLP) analysis was conducted to determine whether the cultivars are synonyms or distinct clones and to elucidate the parental species. The three cultivars, suspected to be hybrids between taxa in subgenera Hymenanthes (Blume) K.Koch (evergreen rhododendrons) and Pentanthera (G.Don) Pojarkova (deciduous azaleas), and related taxa from each subgenus were evaluated using 31 AFLP primer combinations. Genetic similarity, calculated using Jaccard's coefficient, among the hybrids ranged from 53% to 71%, indicating that they are distinct cultivars and not a single clone. Genetic similarity was highest between the hybrids and R. ponticum among the evergreen rhododendrons, and R. viscosum among the deciduous azaleas. A dendrogram generated using the genetic similarity matrix grouped taxa into their respective subgenera, with the three cultivars nested intermediately between subgenera but more closely with subgenus Hymenanthes and particularly R. ponticum, suggesting it is the evergreen rhododendron parent. Furthermore, principle components grouped R. ponticum more closely with the hybrids and there were 18 AFLP fragments unique to R. ponticum and the hybrids. However, no unique AFLP bands were shared exclusively among the hybrids and the purported deciduous azalea parent, R. viscosum, suggesting that the original azalea parents may have been hybrids.

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Steven M. Todd, Van-Den Truong, Kenneth V. Pecota, and G. Craig Yencho

Interest in the potential of sweetpotato (Ipomoea batatas) for the production of industrial products is increasing. As part of an effort to evaluate the potential of sweetpotatoes for starch and anthocyanin production in the southeastern United States, a 5 × 5 North Carolina mating design II (NCII mating design) was developed to estimate the relative importance of general and specific combining abilities for dry matter (DM) content, total monomeric anthocyanin (TMA) concentration, fresh yield, and total DM and anthocyanin yields. All five traits had significant general combining abilities (GCA). Yield and DM yield had significant specific combining abilities. Significant differences among parents were observed for all traits. Yield, DM content, DM yield, and TMA yield were significantly impacted by spatial gradients within the field, but TMA concentration was not. Many trait-pairs of interest had either genotypic and/or phenotypic correlations. Phenotypic and family mean correlations among yield, DM content, and DM yield; as well as among yield, TMA, and TMA yield suggested that improving one trait will not negatively impact other traits of importance.

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Nicholas A. George, Mark Shankle, Jeff Main, Kenneth V. Pecota, Consuelo Arellano, and G. Craig Yencho

Various workers have attempted to develop a root piece planting system for sweetpotato, similar to the system used commercially for potato, but attempts to select and breed sweetpotato clones adapted to root piece planting have met with mixed success. It has been hypothesized this is the result of significant genotype × environment effects, which are complicating phenotype screening. The aim of this work was to investigate genotype × environment interactions and yield stability of sweetpotato grown from cut root pieces. Ten sweetpotato clones were grown from cut root pieces in three locations over three seasons at sites in North Carolina and Mississippi. The study found sweetpotato clones grown from root pieces were influenced by both genetic and environmental factors and that the interaction was often complicated and dependent on the trait being measured. A significant genotype × environment interaction and yield instability were found to be present. Further work will be required to understand the nature of the genotype × environment effects; however, the results suggest programs aiming to develop sweetpotato clones adapted to root piece planting will need to use appropriate multienvironment screening so as to account for genotype × environment effects.

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Lucia E. Villavicencio, Sylvia M. Blankenship, G. Craig Yencho, Judith F. Thomas, and C. David Raper

Sweetpotatoes [Ipomoea batatas (L.) Lam.] often experience significant epidermal loss during harvest and postharvest handling. Skin loss causes weight loss, shriveling of the root surface, and increased susceptibility to pathogen attack as well as poor appearance. It is not known if sweetpotatoes show variation in skin adhesion, cell wall enzyme activity and components, and growth parameters with growth temperature or if skin loss can be explained on the basis of variation among these variables. Skin adhesion, polygalacturonase (PG) and pectin methylesterase (PME) activity, lignin, anthocyanin, and dry matter content were measured in the periderm of ‘Beauregard’ roots grown at various temperatures under controlled conditions. Biomass dry matter content, storage root yield, root length, diameter, and weight at harvest were recorded. Histochemical and anatomical characteristics of periderm of roots were studied. Growth temperature affected skin adhesion, PG and PME activity, periderm and biomass dry matter content, yield, storage root weight, and diameter. High temperatures (34/31 °C day/night) yielded roots that were smaller and more resistant to skin loss. These roots had a periderm composed of more cell layers with a lower dry matter content than roots grown at lower and intermediate temperatures (27/24 °C and 20/17 °C). In cured roots, the correlation between skin adhesion and PG activity was negative (r = 0.544, P = 0.0006) and positive between skin adhesion and PME (r = 0.319, P = 0.05). For most of the variables studied, the interaction between growing temperature and curing was significant. Curing improved skin adhesion, but the effect of curing was dependent on the root growth temperature. The periderm of roots grown at higher temperatures was thicker and had more layers than that of roots grown at lower temperatures. Histochemical studies of the periderm of sweetpotato showed that the anatomical and structural composition of the cell walls differ depending on growth temperature.

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Nicholas A. George, Kenneth V. Pecota, Blake D. Bowen, Jonathan R. Schultheis, and G. Craig Yencho

Sweetpotato (Ipomoea batatas) is traditionally grown for fresh consumption, particularly in developed nations, but it is increasingly being used for alternative markets such as processed foods and industrial products. Sweetpotato is well suited for these end uses but its utilization is limited due to high production costs. These costs are primarily the result of high labor inputs. As a vegetatively propagated crop, sweetpotato is typically planted using unrooted plant cuttings, or “slips,” which requires hand labor at several stages. Consequently, planting costs can be as high as 20% of total production costs. As an alternative to slips, sweetpotato can be established using root pieces, similar to the seed piece system used for potato (Solanum tuberosum). This system can be readily mechanized and therefore has the potential to reduce labor demands. Root piece planting has been investigated several times since the 1940s but is not reported to be in large-scale commercial use anywhere in the world. In this work, we review the research literature relating to root piece planting in sweetpotato. This literature demonstrates that it is possible for sweetpotato root pieces to produce yields comparable to slips, but that in most cases yields from root pieces are usually lower than from slips. We conclude that given suitable cultural management and appropriate varieties, it may be possible to successfully produce sweetpotato using root pieces. More work is necessary to develop root piece planting as a viable alternative to slips in sweetpotato production. This work should include the selection and breeding of adapted varieties, evaluation of the economics of sweetpotato production using root pieces, development of planting equipment suited to sweetpotato root pieces, and examination of chemical treatments to improve success of root piece planting.

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Jim C. Cervantes-Flores, G. Craig Yencho, Kenneth V. Pecota, Bryon Sosinski, and Robert O.M. Mwanga

Resistance to root-knot nematodes [Meloidogyne incognita (Kofoid & White) Chitwood] in sweetpotato [Ipomoea batatas (L.) Lam.] was studied in a mapping population consisting of 240 progeny derived from a cross between ‘Beauregard’, the predominant cultivar in the United States, and ‘Tanzania’, an African landrace. Quantitative trait loci (QTL) analyses to locate markers associated with resistance to root-knot nematodes (RKN) were performed using genetic maps based on parental segregation in ‘Beauregard’ and ‘Tanzania’ consisting of 726 and 947 single-dose amplified fragment length polymorphism (AFLP) markers, respectively. RKN resistance in the progeny was highly skewed with most of the progeny exhibiting medium to high levels of resistance. Single-point analysis of variance and interval mapping revealed seven consistently significant QTL in ‘Tanzania’ and two significant QTL in ‘Beauregard’. In ‘Tanzania’, three QTL were associated with reduction in resistance as measured by the number of RKN egg masses and explained ≈20% of the variation. Another four QTL had positive effects on resistance and explained ≈21% of the variation. Other minor QTL explained ≈2% or less of the variation but were not always consistent across geographical locations. In ‘Beauregard’, two QTL had positive effects on RKN resistance and explained ≈6% of the observed variation. Based on molecular and phenotypic data, RKN resistance in sweetpotato is hypothesized to be conferred by several genes, but at least nine AFLP markers (seven from ‘Tanzania’ and two from ‘Beauregard’) are associated with genomic regions that have the biggest effect in the number of egg masses of RKN produced in the root system.

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Benard Yada, Gina Brown-Guedira, Agnes Alajo, Gorrettie N. Ssemakula, Robert O.M. Mwanga, and G. Craig Yencho

Genetic diversity is critical in sweetpotato improvement as it is the source of genes for desired genetic gains. Knowledge of the level of genetic diversity in a segregating family contributes to our understanding of the genetic diversity present in crosses and helps breeders to make selections for population improvement and cultivar release. Simple sequence repeat (SSR) markers have become widely used markers for diversity and linkage analysis in plants. In this study, we screened 405 sweetpotato SSR markers for polymorphism on the parents and progeny of a biparental cross of New Kawogo × Beauregard cultivars. Thereafter, we used the informative markers to analyze the diversity in this population. A total of 250 markers were polymorphic on the parents and selected progeny; of these, 133 were informative and used for diversity analysis. The polymorphic information content (PIC) values of the 133 markers ranged from 0.1 to 0.9 with an average of 0.7, an indication of high level of informativeness. The pairwise genetic distances among the progeny and parents ranged from 0.2 to 0.9, and they were grouped into five main clusters. The 133 SSR primers were informative and are recommended for use in sweetpotato diversity and linkage analysis.

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G. Craig Yencho, Kenneth V. Pecota, Jonathan R. Schultheis, Zvezdana-Pesic VanEsbroeck, Gerald J. Holmes, Billy E. Little, Allan C. Thornton, and Van-Den Truong

‘Covington’ is an orange-fleshed, smooth-skinned, rose-colored, table-stock sweetpotato [Ipomoea batatas (L.) Lam.] developed by North Carolina State University (NCSU). ‘Covington’, named after the late Henry M. Covington, an esteemed sweetpotato scientist at North Carolina State, was evaluated as NC98-608 in multiple state and regional yield trials during 2001 to 2006. ‘Covington’ produces yields equal to ‘Beauregard’, a dominant sweetpotato variety produced in the United States, but it is typically 5 to 10 days later in maturity. ‘Covington’ typically sizes its storage roots more evenly than ‘Beauregard’ resulting in fewer jumbo class roots and a higher percentage of number one roots. Total yields are similar for the two clones with the dry matter content of ‘Covington’ storage roots typically being 1 to 2 points higher than that of ‘Beauregard’. ‘Covington’ is resistant to fusarium wilt [Fusarium oxysporum Schlect. f.sp. batatas (Wollenw.) Snyd. & Hans.], southern root-knot nematode [Meloidogyne incognita (Kofoid & White 1919) Chitwood 1949 race 3], and moderately resistant to streptomyces soil rot [Streptomyces ipomoeae (Person & W.J. Martin) Wakswan & Henrici]. Symptoms of the russet crack strain of Sweet Potato Feathery Mottle Virus have not been observed in ‘Covington’. The flavor of the baked storage roots of ‘Covington’ has been rated as very good by standardized and informal taste panels and typically scores as well or better in this regard when compared with ‘Beauregard’.