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P. J. Ndolo and E. G. Rhoden

Root growth of sweet potato [Ipomoea batatas (L) Lam.] cvs `TI-82-155', `Centennial' and `Rojo Blanco' in coarse fritted clay soil, was investigated under greenhouse conditions. The sweet potato cultivars were harvested at 41 and 82 days after planting. Dry weight of fibrous roots of all cultivars were similar at day 41. Fibrous root weight of `Rojo Blanco' increased by 5% while those of the other cultivars increased by 168%. Mean fibrous root length per centimeter depth was not significantly different among cultivars. Although fresh weight of storage roots of `Rojo Blanco' was significantly lower than those of the other cultivars, their dry weights were similar. `TI-82-155' and `Rojo Blanco' had fewer storage roots compared to the other cultivars, however, storage root length of `TI-82-155' or `Rojo Blanco' was greater than that of `Georgia Jet' or `Centennial'. Length to diameter ratio of the storage root of `Rojo Blanco' was significantly greater than that of `TI-82-155' and `Georgia Jet'.

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A. Golmirzaie, D.P. Zhang, L. Nopo, C.A. Newell, A. Vera, and F. Cisneros

Euscepes postfasciatus is one of the most important sweetpotato pests in the South Pacific, Caribbean basin, and some countries of Central and South America. Development of host resistance will greatly improve the effects of integrated pest management (IPM) for this pest. Ten transgenic clones of `Jewel' sweetpotato with cowpea trypsin inhibitors and snowdrop lectin, developed by Axis Agri. Genetics, Ltd., were assayed for weevil resistance using a no-choice bioassay. A replicated experiment was conducted in the screenhouse. Five storage roots from each clone were infested with five pairs of adults. Non-transformed `Jewel' was used as a check. Resistance was assessed 60 days after infestation by estimating the percentage of internal damage and the weevil population in the storage roots. A five-grade damage index was recorded. The experiment was repeated twice. Significant enhancement of resistance was found in the transgenic clones. Clone CTI-13 with cowpea trypsin inhibitor and clone PCG-7 with both cowpea trypsin inhibitor and snowdrop lectin demonstrated moderate resistance to E. postfasciatus, whereas the non-transformed `Jewel' was susceptible. This result shows that resistance to Euscepes postfasciatus can be achieved through genetic transformation.

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W.A. Mulkey, W.B. McLemore III, and T.P. Talbot

In preplant nitrogen studies with the `Beauregard' Variety maximum yields of U.S. No. 1 grade roots are produced using 50.4 kg/ha N. In 1992 studies were initiated to determine the effect of preplant N rates on storage root set and yield. Preplant N rates ranged from 0 to 84 kg/ha in 16.8 kg/ha increments. Two plots each of 0, 16.8 and 33.6 kg/ha were included so sidedress applications could be made to bring one of the treatments to the 50.4 kg/ha N level 30 days after transplanting. The 0 kg/ha N treatment had significantly more storage roots per plant than all other treatments 21 days after transplanting and more vine growth measured by weight. The 0 and 16.8 kg/ha treatments had significantly more storage roots 26 days after transplanting. At 26 and 35 days after transplanting vine growth was greater in 16.8 kg/ha N treatment. The 0 plus 50.4 kg/ha N sidedress treatment produced the highest yield of U.S. No. 1 grade roots with the highest number of marketable roots per plant (5.0). Similar results were obtained in the 1993 studies.

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A.D. Bryan, Z. Pesic-VanEsbroeck, J.R. Schultheis, K.V. Pecota, W.H. Swallow, and G.C. Yencho

Decline in sweetpotato yield and storage root quality has been attributed to the accumulation of viruses, pathogens and mutations. To document the effects of decline on yield and storage root quality, two micropropagated, virus-indexed, greenhouse produced G1 `Beauregard' meristem-tip cultured clones, B94-14 and B94-34, were compared with 1) micropropagated B94-14 and B94-34 clones propagated adventitiously up to five years in the field (G2, G3, G4, G5); and 2) nonmicropropagated, unimproved stock of `Beauregard' seed in field trials during 1997 to 2001. At least three trials were located each year in sweetpotato producing regions in North Carolina. In 2000 and 2001, two trials were monitored weekly for foliar symptoms of Sweet potato feathery mottle virus (SPFMV) and other potyviruses, and virus-indexed for selected viruses using Ipomoea setosa and nitrocellulose enzyme linked immunosorbant assays (NCM-ELISA). Only SPFMV was detected in field samples using NCM-ELISA, but this does not rule out the presence of newly described viruses infecting sweetpotato for which tests were unavailable. Monitoring indicated that all G1 plants became infected with SPFMV by the end of the growing season, and that G2 to G5 plants were probably infected in their initial growing season. G1 plants consistently produced higher total yield, total marketable yield (TMY), U.S. No. 1 root yield and percent No. 1 yield than G2 to G5 plants. G1 plants also produced storage roots with more uniform shapes and better overall appearance than storage roots produced from G2 to G5 plants. Also, G2 to G5 storage roots tended to be longer than G1 storage roots. Rank mean yield and storage root quality measurements of each location were consistent with means averaged over locations per year and suggested a decrease in yield and storage root quality with successive seasons of adventitious propagation. Linear regression analysis used to model yield and storage root quality measurements of seed generations G1 to G5 indicated that total yield, TMY, No. 1 yield, percent No. 1 yield, shape uniformity, and overall appearance decreased gradually, and that length/diameter ratios increased gradually with generation. The rate of decline in No. 1 yield was greater for B94-34 compared to B94-14. Both viruses and mutations of adventitious sprouts arising from storage roots probably contribute to cultivar decline in sweetpotato, but further studies are needed to determine their relative importance. A simple profitability analysis for G1 vs. G2-G4 planting material conducted to facilitate better understanding of the economics of using micropropagated planting material to produce a crop in North Carolina revealed that growers have a potential net return of $2203/ha for G1 plants, $5030/ha for G2 plants, and $4394/ha for G5 plants. Thus, while G1 plants generally produce higher No. 1 yields, a greater monetary return can be achieved using G2 planting materials because of the high costs associated with producing G1 plants. Based on this analysis, the best returns are accrued when growers plant their crop using G2 and/or G3 seed.

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D.G. Mortley, P.A. Loretan, W.A. Hill, C.K. Bonsi, C.E. Morris, R. Hall, and D. Sullen

`Georgia Red' peanut (Arachis hypogaea L.) and TU-82-155 sweetpotato [Ipomoea batatas (L.) Lam] were grown in monocultured or intercropped recirculating hydroponic systems in a greenhouse using the nutrient film technique (NFT). The objective was to determine whether growth and subsequent yield would be affected by intercropping. Treatments were sweetpotato monoculture (SP), peanut monoculture (PN), and sweetpotato and peanut grown in separate NFT channels but sharing a common nutrient solution (SP-PN). Greenhouse conditions ranged from 24 to 33 °C, 60% to 90% relative humidity (RH), and photosynthetic photon flux (PPF) of 200 to 1700 μmol·m-2·s-1. Sweetpotato cuttings (15 cm long) and 14-day-old seedlings of peanuts were planted into growth channels (0.15 × 0.15 × 1.2 m). Plants were spaced 25 cm apart within and 25 cm apart between growing channels. A modified half-Hoagland solution with a 1 N : 2.4 K ratio was used. Solution pH was maintained between 5.5 and 6.0 for treatments involving SP and 6.4 and 6.7 for PN. Electrical conductivity (EC) ranged between 1100 and 1200 μS·cm-1. The number of storage roots per sweetpotato plant was similar for both SP and SP-PN. Storage root fresh and dry mass were 29% and 36% greater, respectively, for plants in the SP-PN treatment than for plants in the SP treatment. The percent dry mass of the storage roots, dry mass of fibrous and pencil roots, and the length-to-diameter ratio of storage roots were similar for SP and SP-PN sweetpotato plants. Likewise, foliage fresh and dry mass and harvest index were not significantly influenced by treatment. Total dry mass was 37% greater for PN than for SP-PN peanut plants, and pod dry mass was 82% higher. Mature and total seed dry mass and fibrous root dry mass were significantly greater for PN than for SP-PN plants. Harvest index (HI) was similar for both treatments. Root length tended to be lower for seedlings grown in the nutrient solution from the SP-PN treatment.

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P.P. David, A. Almazan, C.K. Bonsi, D.G. Mortley, A.A. Trotman, and G.W. Carver

Studies were conducted in a greenhouse to determine the effects of harvesting sweetpotato (Ipomea batatas L. (Lam.) cv. “TI-82-155”) shoot tips (top 10 cm) at biweekly intervals beginning 42 days after transplanting on yield of storage root and nutrient content of harvested shoot tips. Plants were grown hydroponically from vine cuttings of 15 cm length, planted in 0.15 × 0.15 × 0.12 m growth channels using a closed NFT system. Nutrient was supplied in a modified half-strength Hoagland's solution with a N:K ratio of 1:2.4. Final harvest was at 120 days when shoot biomass and yield of storage roots were measured. Biweekly topping did not affect storage root yield or shoot biomass. However, harvesting time had a significant effect on dry matter and nutrient content of shoot tips.

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A.Q. Villordon, J.M. Cannon, H.L. Carroll, J.W. Franklin, C.A. Clark, and D.R. LaBonte

Yield tests and evaluation of selected storage root and vine characters were conducted among 12 `Beauregard' sweetpotato [Ipomoea batatas (L.) Lam.] mericlones. Maximum yield differences were 43%, 48%, 79%, and 40% for U.S. #1, canners, jumbos, and total marketable yield, respectively. Additive main effect and multiplicative interaction (AMMI) biplot analysis was useful in graphically presenting the yield differences and stability patterns of mericlones. Differences were also detected in vine length, internode diameter, and internode length. Digital image analysis of U.S. #1 storage roots also revealed differences in storage root minor axis length, roundness, and elongation attributes. The results provide valuable information for enhancing current methods of evaluation and selection of mericlones for inclusion in sweetpotato foundation seed programs.

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P. D. Dukes, J. R. Bohac, and J. D. Mueller

A root-knot nematode (Meloldogyne incognita) project was initiated in a field of infested sandy loam (EREC) in 1991 and continued. There were ten sweetpotato entries consisting of six cultivars (Beauregard. Excel, Georgia Jet, Jewel, Red Jewel, and Sumor), three advanced lines (W-270, W-274, and W-279) and PI 399161 which were selected for their diversity in disease reactions and other traits. Each entry was planted in the same plots each year to monitor effects of continuous cropping, disease reactions, yield and population shifts of the pathogen. Marketable yields were reduced each year for Georgia Jet and Red Jewel, but not for Beauregard. Internal necrosis in the storage roots was most severe for Beauregard. Several of the highly resistant entries, especially Sumor and W-279, performed well each year, including high yields, good quality. and little or no nematode reproduction. This study demonstrates the considerable economic benefits of a high level of durable resistance to root knot in sweetpotato.

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D. Mortley, P. Loretan, C. Bonsi, and W. Hill

An experiment was conducted in environmental growth chambers to study the response of sweet potato to relative humidity (RH). Twenty-four vine cuttings of `TI-155' sweet potato were planted in growth channels in a modified half Hoagland's solution using the nutrient film technique. Plants were exposed to constant RH levels of 50% or 85%. Temperature regimes of 20/22 C were maintained during the light/dark periods with an irradiance level of 600 umol m-2 s1, and a 14 hr/10 hr photoperiod. Plants were harvested 120 days after planting and yield data was taken. High RH (85%) resulted in significant increases in number of storage roots/plant, storage root fresh and dry weight, single leaf photosynthesis and stomatal conductance than at 50% RH. Foliage dry weight and leaf temperature was higher at 50% than 85% RH.

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Paul G. Thompson, John C. Schneider, and Boyett Graves

One hundred one accessions from the U.S. germplasm collection were evaluated in field trials for sweetpotato weevil resistance. Weevils were collected from 4 separate Mississippi locations during the winter of 1992-93. They were increased in culture and 6 adult females and 6 males were applied to the crown of each plant percentage of uninjured storage roots ranged from 53 to 99. The most highly resistant control, Regal, had 79% and the most susceptible, Centennial, 60% uninjured roots. Uninjured root numbers ranged from 0.03 to 3.82 per plant. Regal had 2.1 and Centennial 1.88 uninjured roots per plant. Seventy-five accessions produced higher percentages of uninjured roots than Regal. However, 48 of those accessions produced less than one root per plant and previous results indicated that estimates with low storage root numbers lack precision. Fourteen accessions produced as many or more roots than Regal and also higher percentages and numbers of uninjured roots.