Browsing by Author "Thomas G. Isleib, Committee Member"

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Biology and Epidemiology of Sclerotinia minor on Peanut (Arachis hypogaea L.)
(2004-03-19) Smith, Damon Lee; Barbara B. Shew, Committee Chair; Marc A. Cubeta, Committee Co-Chair; H. David Shew, Committee Member; Thomas G. Isleib, Committee Member
Sclerotinia blight caused by the fungus Sclerotinia minor is a serious disease of cultivated peanut (Arachis hypogaea L.) in North Carolina. Laboratory and field experiments were conducted to gain a better understanding of how environment affects sclerotial germination, mycelial growth, and plant infection in soil. Furthermore, quantitative relationships between disease incidence, severity, environmental factors, and yield loss were examined in order to improve disease forecasting systems in North Carolina. Laboratory and field experiments also were conducted to gain a better understanding of the biology of both the host and fungus. Experiments were conducted to evaluate the relative importance of tissue resistance vs. plant architecture in highperforming cultivars. In the laboratory, soil matric potential (ΨM) and temperature effects were measured for germination of sclerotia, mycelial expansion, and lesion expansion on detached leaflets. Temperature effect on the production of oxalic acid by mycelium of S. minor was also examined. Maximum sclerotial germination occurred at a ΨM of -7.2 kPa and a temperature of 30 C. Rate of mycelial expansion and lesion development and expansion on detached leaflets were maximal at temperatures of 18-22 C. Lesions on detached leaflets developed slowly at temperatures above 26 C, and failed to develop at temperatures of 29 C or greater. Lesions formed if inoculated leaflets were moved from 29 C to a cooler temperature of 18 or 22 C. Oxalic acid production at temperatures above 29 C was negligible. In the field, incidence of Sclerotinia blight was measured at three sites in 2002 and 2003. A gradient of disease levels was established by utilizing one partially resistant cultivar, two susceptible cultivars and the fungicides fluazinam and boscalid at various rates. Weather data were collected in 2003 and modeled in both years. Disease incidence was highest on the susceptible cultivars with no fungicide treatment. Disease incidence of the partially resistant cultivar and of one susceptible cultivar in conjunction with various weather parameters were used as the dependent and independent variables, respectively, in a regression analysis. The following disease prediction model was created: Total Disease = -79.52 + 3.04 relative humidity ? 0.020 relative humidity 2 ? 0.47 soil temperature ? 5.30 leaf wetness. In other laboratory tests, leaflets, pegs, lateral branches, and main stems of two susceptible cultivars and two partially resistant breeding lines were detached from plants grown in the greenhouse, inoculated with mycelial plugs, and placed in moisture chambers. Lesion development and severity on each part were measured for 7 days. In the field three cultivars and one breeding line were planted in replicated plots. Destructive samples of randomly selected plants were made weekly. Numbers of lesions on the four plant parts of interest were counted. Laboratory studies indicated that leaflets and pegs were the most susceptible plant tissues. Main stems and lateral branches were resistant to infection. In the field, however, lesions were found most frequently on lateral branches. Inconsistencies between laboratory and field studies indicate that there are other mechanisms of resistance operating in the field besides physiological resistance. Management strategies should focus on protecting lateral branches from infection by S.minor. Those management strategies may be improved further with the adoption of a Sclerotinia blight disease forecasting system that utilizes remote site-specific weather data with no need for onsite sensors.
Impact of Heterozygosity and Heterogeneity on Cotton Lint Yield Stability
(2007-04-26) Cole, Clay Brady; Thomas G. Isleib, Committee Member; James B. Holland, Committee Member; Christina Cowger, Committee Member; Daryl T. Bowman, Committee Chair
Adequate stability of cotton (Gossypium hirsutum L.) lint yield is an integral criterion for cultivar release; however, the magnitude of lint yield variation today is close to six times greater than variation observed in the 1920's. Yield stability has often been associated with genetic diversity. Observing cotton lint yield in diverse population types containing various levels and kinds of genetic diversity over many environments could reveal information about stability and how it relates to diversity. An 18-environment field study was undertaken to observe lint yield stability in four population types of cotton. These populations were pure lines grown in pure stands (homozygous⁄homogeneous), pure lines grown in blended stands (homozygous⁄heterogeneous), hybrids grown in pure stands (heterozygous⁄homogeneous), and hybrids grown in blended stands (heterozygous⁄heterogeneous). Lint yield components were also observed to determine the contribution each had towards lint yield stability. Differences were determined by observing the coefficient of variation (CV) for mean yield and yield components of population types and over environments. We found the heterozygous populations to be more stable than the homozygous populations. This was attributed to the hybrids and blends of hybrids out-yielding the parents and blends of parents in the low-yielding environments. This advantage was not observed in the high-yielding environments and, in effect, reduced the amount of variation observed over all environments. The number of bolls/hectare was the only yield component that showed definitive differences for stability between population types with the heterozygous populations having significantly higher stability than the homozygous populations. The superior stability of the heterozygous populations was attributed to an increased lint production in the lower yielding environments stemming from an increased number of bolls/hectare.
Seed Quality Issues Associated with High-oleate Peanut (Arachis hypogaea L.)
(2006-12-02) Sun, Minghui; David Jordan, Committee Member; Janet Spears, Committee Chair; Thomas G. Isleib, Committee Member
The high-oleate trait of peanut is of great interest to the peanut processing industry because it produces greater oxidative stability of the oil without adversely affecting flavor. Most US peanut breeding programs have incorporation of the high-oleate trait into existing cultivars and future releases as an objective. While much of the peanut industry is concerned about peanut dietary oil quality, seed technologists are concern that altering peanut seed fatty acid or total lipid composition could influence germination rate, seed and seeding vigor, and seedling survival, especially if the seed are planted in stressful soil conditions. An experiment was designed to evaluate temperature effect on seed oil quality of high-oleate and normal peanut cultivars in controlled greenhouse environment. Two cultivars, NC-V 11 and Gregory, along with their paired backcross-derived high-oleate lines were planted in greenhouses maintained at 22⁄18oC, 26⁄22oC and 30⁄26oC day⁄night temperature. A split-plot experimental design with two replications was used. Peanut kernels were analyzed for fatty acid composition of the whole seed and axis lipids. The whole seed oleic to linoleic acid (O⁄L) ratio of normal peanuts grown in 30⁄26oC, 26/22oC, and 22⁄18oC, measured 1.9, 1.5, and 1.3, respectively. The O⁄L for their high-oleate pairs decreased from 24.7 when grown in 30⁄26oC to 15.9 in 26⁄22oC and to 13.7 in 22⁄18oC. Temperature did not affect the fatty acid composition of axis total lipid or phospholipid fractions. The high-oleate trait was however, expressed in the axis lipids. The average O⁄L of axis from normal peanut was 1.1 while that of high-oleate lines was 4.6. Likewise, axis phospholipids for normal and high-oleate lines were 1.0 and 5.9. Decreased production environment temperature in this study decreased the O⁄L ratio of seed oil of high-oleic peanut lines, and the high-oleate trait expressed in peanut seed storage lipids is also expressed in axis membrane lipids to a lesser degree. The second experiment was designed to determine if the production methods applicable to traditional peanuts will hold for high-oleate cultivars. Six Virginia-type peanut cultivars and their paired backcross-derived high-oleate lines were grown at the Peanut Belt Research Station near Lewiston, NC in 2003 and 2004. A split-plot experimental design was used with 2 x 2 factorial combinations of planting and harvest date as whole plot treatments, and 2 x 6 factorial combinations oleic acid and cultivars as subplot treatments. Seed quality evaluation included standard germination (SG), cool germination (CG), and electrical conductivity (EC). Oleic acid level had no influence on SG but did significantly alter CG and EC of high-oleate lines. Averaged across background genotypes, high-oleate lines had lower seed vigor than their paired lines with normal oleic content. The high-oleate lines of three of the six pairs had significantly lower CG and higher EC. Planting and harvest date affected all the seed quality traits measured. SG of both normal and high-oleate lines was reduced in 2004 when harvest was delayed, but was not affected in 2003. In 2003, CG of the high-oleate lines was significantly lower than that of normal lines in three of the four production environments; EC was significantly higher in the high-oleate lines in all planting date and harvest date combinations. In 2004, there was no statistical difference between the CG of normal and high-oleate lines, but EC was significantly higher in the high-oleate lines for three of the four environments.