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  • Author or Editor: Paul G. Thompson x
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A breeding population including parents with known moderate levels of weevil resistance was evaluated for resistance in a field experiment with few naturally occurring weevils. Weevils were collected from several Mississippi locations and intermated for increase. One male and one female were applied to the crown of each plant 73 days after transplanting. Seventeen parents and their progenies plus 6 additional clones or families were grown in 5 plant plots replicated 8 times. Genetic variability for resistance to weevil injury was observed. Mean percentage injured roots ranged from 25 to 85 and there were differences in resistance to injury among genotypes. Additive genetic variance was moderate so increased resistance levels should result from selection and intermating for high resistance.

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RAPD marker analyses were completed on parents and progeny of two sweetpotato [Ipomoea batatas (L.) Lam.] crosses to determine the feasibility of genetic linkage map construction. A total of 100 primers was tested and 96 produced amplified genomic DNA fragments. The average number of polymorphisms per primer was 0.69. A total of 134 polyphorphic markers was observed and 74 (60%) segregated 1 band present : 1 band absent as needed for use in genetic linkage mapping of polyploids. The 60% of RAPD markers that segregated 1:1 shows that genetic linkage mapping of the hexaploid sweetpotato by RAPD marker analysis is feasible. Linkage was determined for all markers that segregated 1:1 and five pairs of linked markers were found. These were the first linked molecular markers found in sweetpotato and they show that construction of a genetic linkage map is feasible. A genetic linkage map will be a valuable tool to assist in genetic improvements.

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Responses of sweet potato (Ipomoea batatas (L>) Lam) to irrigation rates were evaluated under line-source irrigation systems on Tifton loamy sand soil in Georgia and on a Bude silt loam soil in Mississippi. Total water (rainfall plus irrigation) rates ranged from about 55% to 160% of pan evaporation (Epan). Marketable yields increased with irrigation rate until total water was about 75% of Epan then decreased rapidly with greater irrigation rates. Sweet potato yields were more sensitive to excessive water rates when grown on a silt loam than on a sandy loam soil. Storage loss and quality of cooked 'Jewel' sweet potato roots also increased as the irrigation rate increased until total water was 75% to 95% of Epan then decreased rapidly at water rates of 135 to 160% of Epan.

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The sweetpotato weevil is the most-destructive worldwide pest of sweetpotato and only low to moderate levels of resistance to the insect are available in acceptable cultivars. No sources of high resistance levels have been identified; consequently, there is a need to identify additional sources of resistance genes to develop high resistance levels. To begin a search for sources of resistance, plant introductions were evaluated for injury levels. In 1993, 100 plant introductions were evaluated for sweetpotato weevil injury and 62 of the least injured were tested again in 1994. In 1995, 36 of the least injured in 1993 and 1994, plus 24 additional PIs were evaluated. Control cultivars included `Regal', moderately resistant; `Jewel', intermediate; and `Beauregard' and `Centennial', susceptible. Measurements of injury were percentage of roots injured, and, stem and root injury scores based on a 0–5 scale, with 0 being no injury. First year results indicated that a low level of resistance to stem injury is available in the PIs tested. Stem injury was more severe in the following year and no differences were found. Lower weevil populations will be required to screen for low levels of stem injury resistance. Percentage injured roots and root injury scores were lower over the 3 years for five PIs than for `Regal'.

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A breeding program to develop improved sweetpotato genotypes with increased sweetpotato weevil resistance was started in 1990. Germplasm, including plant introductions, cultivars, and breeding lines with reported insect resistance, was field tested for injury levels by applying low numbers of weevils. Low levels of resistance were found and `Regal' was among the highest. Top performing lines were selected and intermated. After 2 selection cycles the most highly resistant selection produced 89% uninjured roots compared to 28% in `Regal'. Severity of injury score was 16 times lower in the most resistant selection (0.15) compared to `Regal' (2.40).

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A line-source irrigation design was used to provide continuously increasing amounts of irrigation at each application to sweetpotato [Ipomoea batatas (L.) Lam]. Marketable yields increased with applied irrigation amounts until a total water application of 76% of pan evaporation (Epan) was reached and then decreased rapidly with applied irrigation amounts. Weight loss and decay of roots during storage showed quadratic responses to irrigation amounts and were minimal at the irrigation level of maximum yields. Contents of dextrins and maltose increased with irrigation amounts. Glucose content was maximum at a total water amount of 94% Epan and fructose content decreased with increased amounts of irrigation. Sensory ratings for appearance, flavor, texture, and preference, and objective color measurements of cooked flesh also reached their highest values near the irrigation amount of maximum yield.

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Narrow-sense heritabilities (h2) for sweetpotato weevil (Cylas formicarius elegantulus) resistance were estimated in 2 breeding populations. Population A included clones from US programs with previously reported moderate levels of weevil resistance. Population B included clones of US origin plusplant introductions from 14 countries. Parents and progenies were included in field evaluations with no wild weevils present. Weevils were cultured and applied to each plant. Population A was evaluated for 2 years and population B for 1. The GXE estimate for population A was also used for population B. Heritabilities were estimated by parent offspring regression and variance component analysis. Average h2 for percentage noninjured roots were 0.35 and 0.47 for population A and B, respectively. Intermating highest performing genotypes from both populations should increase h2 by increasing frequencies of resistance genes. A moderate rate of increase in resistance levels should result from selecting and intermating resistant genotypes.

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A factorial test was conducted to evaluate the potential of screening sweetpotato plants to three pathogens simultaneously. The pathogens were Meloidogyne incognita, Fusarium oxysporum, and Streptomyces ipomoea. The test also involved six sweetpotato cultivars and three evaluation times. Evaluation times were 3, 6, and 9 weeks post inoculation. The symptoms evaluated were vascular necrosis, fibrous root necrosis, and gall and egg mass production. For each of the three pathogens, the ability to separate cultivars with intermediate levels of resistance from those with low levels of resistance decreased as post Inoculation time increased. Simultaneous screening was practical if the goal was to select plants with resistance to all three pathogens. Resistances to individual pathogens could not be identified in plants inoculated with all three pathogens.

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Seedling plants from the three parents `Resisto', `Southern Delight', and `L86-33', along with three pot sizes (3.8-, 10.2-, and 17.7-cm diameters) were evaluated. Root characteristics evaluated in both the greenhouse and field included: number, length, diameter, length diameter ratio (L:D), size, skin color, flesh color, internal cambium ring (color and width), and the number of lateral and secondary roots. After greenhouse evaluation, plants were transplanted to the field. The 3.8-cm pot did not produce enough roots in the greenhouse for evaluation. In the 10.2-cm pots, greenhouse root number was correlated with the yield, root size, and L:D, and negatively correlated with skin color in the field. Flesh color was correlated with smoothness and flesh color in the field. In the 17.8-cm pots, flesh color, smoothness, and skin color in the greenhouse were correlated with the same character in the field. Skin color was also negatively correlated with smoothness in the field. No differences were found in field yield due to pot size. Results from one season showed that the 10.2-cm pot was effective for greenhouse selection of flesh color, skin color, and smoothness in seedling sweetpotato plants.

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One hundred plant introductions (PIs) were evaluated for sweetpotato-weevil resistance in experiment station field trials for 2 years in Beaumont, Miss. Weevil infestation was accomplished by applying adult weevils in year 1 and weevil infested roots in year 2. The percentage of uninjured roots ranged from 38% in `Centennial', the susceptible control, to 93% in PI538288. Severity of root and stem injury were measured in year 2. Stem injury ratings on a scale of 0, for no injury, to 4, for severe injury, ranged from 1.2 in PI564113 to 3.7 in `Beauregard'. Root injury ratings on a scale of 0 to 5 ranged from 0.1 in PI538288 to 4.2 in `Beauregard' (susceptible control). Thirty-five PIs had lower root injury values than `Regal' (resistant control), and the percentage of uninjured roots was higher in 45 PIs than in `Regal'. These results suggest that genes are available in PIs for increased levels of weevil resistance in sweetpotato.

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