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  • Author or Editor: Boyett Graves 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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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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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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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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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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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.

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Narrow-sense heritability for component traits of freedom from weevil injury and yield of sweetpotato were estimated by parent-offspring regression and variance component analysis. Heritability estimates by variance component analysis based on half-sib families for percent and number of uninjured roots were 0.25 and 0.83, respectively. Individual plant heritability estimates for uninjured root percent and number were 0.03 and 0.13, respectively. Heritability estimates by parent-offspring regression for uninjured root percent and number were 0.35 and 0.52, respectively. Genetic variance was mostly additive for all traits except stem diameter. Genetic correlations between total root number, uninjured root number, and percent uninjured roots ranged from 0.66 to 0.87, indicating that selection for uninjured root number should most effectively increase uninjured root number and percent, as well as total root numbers. Predicted gains in uninjured root percent and number were 8.8% and 0.87 in the progeny derived from intermating the highest four out of 19 families for uninjured root number. The 0.87 gain in uninjured root number equals a 24% increase in one breeding cycle.

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Narrow-sense heritability and selection response estimates for weevil resistance were higher based on half-sib family means than on individual plants. Resistance levels in available sources are low and evaluation with low weevil numbers is essential for differentiation. Consequently, highest performing families produce multiple plants with no injured roots and separation of resistance levels among those plants is impossible Since a large amount of additive variance is contained among plants within families and early reduction of plant numbers would increase breeding efficiency, a study was conducted to determine the effectiveness of individual plant selection in combination with half-sib families. Two plants were selected from each of the 10 highest families based on weevil resistance and yield. Selected clones were compareds for resistance levels to means of their families when tested over 2 years. Fifty percent of selected clones had higher resistance levels than their family means indicating that individual plant selection was effective. Optimum methods of combining family and individual plant selection will be discussed.

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One hundred U.S. sweetpotato [Ipomoea batatus (L.) Lam.] plant introductions (PIs) and four control cultivars were screened for insect injury in 1993. Of the least injured by insects, 56 and 31 were tested again in 1994 and 1995, respectively. Among control cultivars, the most highly resistant was `Regal' (moderately resistant), followed by `Beauregard' (susceptible), `Centennial' (susceptible), and `Jewel' (susceptible). Stem and root injury by the sweetpotato weevil (SPW) [Cylas formicarius elegantulus (Summers)] and root injury by the wireworm (Conoderus sp.)–Diabrotica sp. (cucumber beetle)– Systena sp. (flea beetle) (WDS) complex were measured. SPW stem injury was less severe (P ≤ 0.05) in 1994 and 1995 in PIs 508523, 531116, and 564107 than in control cultivars. PIs 508523 and 531116 also suffered less SPW root injury than did `Regal'. In the six PIs with least SPW root injury, PIs 538354, 564149, 508523, 538286, 531116, and 564103, 70% to 85% of the roots were not injured compared with 36% in `Regal' and 6% in `Jewel'. SPW root injury scores (0 = no injury; 5 = severe injury) in those PIs averaged 0.5 vs. 2.3 for `Regal'. Only in PI 538286 was WDS injury to roots less than in `Regal' over 2 years. However, eight additional accessions suffered less WDS injury than `Regal' in 1995 and four of those were among the six with least SPW injury. The lower levels of combined insect injury found in these four PIs (compared to `Regal') show that PIs have potential use for increasing insect resistance in sweetpotato improvement programs.

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