C. C. Holbrook

Khon Kaen University, Khon Kaen, Changwat Khon Kaen, Thailand

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Publications (57)41.34 Total impact

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    Dataset: cs-50-1-159
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    ABSTRACT: Drought affects yield of peanut, but its effect on oleic and linoleic acids that influence its oil quality of peanut genotypes with different levels of drought resistance has not been clearly investigated. Therefore, the aims of this research were to determine whether soil water levels could affect oil quality by changes in fatty acid compositions of peanut, and to investigate the changes in oil characters in peanut genotypes with their potential drought resistance under different water regimes. Field experiments were conducted in split-plot designs with four replications during dry season for two years (2003/04 and 2004/05). Three water regimes [field capacity (FC), 2/3 available soil water (2/3 AW) and 1/3 available soil water (1/3 AW)] were assigned as main-plots, and six peanut genotypes were assigned as sub-plots. The data were recorded at maturity for fatty acid compositions and % oil. Seed samples were analyzed for % oil by Soxtec System HT, and fatty acid compositions were analyzed by gas liquid chromatography. Differences among water regimes and peanut genotypes were significant for oleic and linoleic acids content and their ratio (O/L ratio), unsaturated to saturated fatty acid ratio (U/S ratio) and iodine value (IV). Genotype × water regime interactions were also significant for all characters. Drought improved the oil quality by significant increase in oleic acid and O/L ratio, and reduced the linoleic acid, IV and U/S ratio. Peanut genotypes with different levels of drought resistance displayed similar tendency in fatty acid characters under drought conditions.
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    P. Timper, D. M. Wilson, C. C. Holbrook
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    ABSTRACT: Peanut kernels are susceptible to colonization by some species of Aspergillus which, under conditions of drought and high temperatures, can produce aflatoxins prior to harvest. The objective of this research was to determine the mechanism by which the peanut root-knot nematode (Meloidogyne arenaria) increases aflatoxin contamination in peanut. Research determined 1) the role of nematode infection of roots vs. pods in increased aflatoxin contamination and 2) whether increased aflatoxin production in nematode-infected peanut is due to a greater percentage of small or immature kernels. An additional objective was to determine whether a peanut cultivar with resistance to M. arenaria would reduce the risk of preharvest aflatoxin contamination. In the greenhouse, researchers physically separated root growth from pod set and inoculated each location with M. arenaria or a water control in a 2 × 2 factorial design with 12–15 replications. Of the six trials conducted, data indicated that pod and root infection by M. arenaria was associated with elevated aflatoxin concentrations in one and three trials, respectively. This suggests that root infection by the nematode can increase aflatoxin concentrations in the peanut kernel. Another 2 × 2 factorial experiment was conducted with two peanut genotypes (Tifguard and TifGP-2) and two nematode treatments (with and without M. arenaria) with six replications. The cultivar Tifguard is resistant to M. arenaria and TifGP-2 is susceptible. The experiment was carried out in 24 field microplots equipped with a rainout shelter. The experiment was conducted five times from 2006 to 2010. Infection of TifGP-2 by M. arenaria did not lead to greater percentages of small kernels. In only one year (2007), nematodes appeared to increase the percentage of damaged kernels, though aflatoxin concentrations were not affected by nematodes in that year. In the rainout shelter experiment, 2006 was the only year where nematode infection of peanut increased aflatoxin concentrations. In that year, there were lower aflatoxin concentrations in the nematode-resistant cultivar Tifguard than the susceptible germplasm TifGP-2 (12 vs. 136 ng/g).
    Peanut Science 01/2013; 40(1):31-39.
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    ABSTRACT: Breeding for terminal drought resistance in peanut can increase their productivity in drought-prone environments and reduce aflatoxin contamination. To improve selection efficiency for superior drought-resistant genotypes, a study of inheritance of traits is worthy, and provides useful information for planning suitable breeding approaches. The objectives of this study were to estimate the heritability of terminal drought resistance traits, the genotypic and phenotypic correlations between drought resistance traits and agronomic traits, and among physiological traits in peanut. The 140 peanut lines in the F 4:6 and F 4:7 generations were generated from four crosses (ICGV 98348 × Tainan 9, ICGV 98348 × KK60-3, ICGV 98353 × Tainan 9, and ICGV 98353 × KK60-3), and tested under well-watered and terminal drought conditions. Field experiments were conducted under the dry seasons 2006/2007 and 2007/2008 in the Northeast of Thailand. Data were recorded for agronomic traits [biomass, pod yield, number of mature pods per plant, seeds per pod, and seed size] and physiological traits [harvest index (HI), SPAD chlorophyll meter reading (SCMR), and specific leaf area (SLA)]. The heritability estimates for physiological traits were higher than for agronomic traits, and varied among crosses. The heritability for HI, SCMR, and SLA ranged from 0.55 to 0.85, 0.72 to 0.91, and 0.61 to 0.90, respectively. Positive correlation between HI and SCMR were significant, and SLA was also found to be inversely associated with SCMR and HI. Significant and positive correlations between HI and SCMR with most of agronomic traits were found. SLA was also negatively correlated with agronomic traits. These results indicated that HI, SLA, and SCMR are potentially useful as indirect selection traits for terminal drought resistance because of high heritability and good correlation with pod yield. Plant breeding approaches using these traits might be effective and valuable for improving terminal drought tolerance in peanut.
    SABRAO journal of breeding and genetics 01/2012; 44(2):240-262. · 0.23 Impact Factor
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    ABSTRACT: Germin-like proteins (GLPs) play diversified roles in plant development and defense response. Here, we identified 36 expressed sequence tags (ESTs) encoding GLPs from peanut (Arachis hypogaea L.). After assembly, these ESTs were integrated into eight unigenes named AhGLP1 to AhGLP8, of which, three (AhGLP1-3) were comprised 14, ten, and seven EST clones, respectively, whereas the remaining ones were associated with one single clone. The length of the deduced amino acid (AA) residues ranged from 208 to 223 AAs except for AhGLP6 and AhGLP8, which were incomplete at the carboxyl terminus. All of the AhGLPs contained a possible N-terminal signal peptide that was 17 to 24 residues in length excluding AhGLP7, where there is likely a non-cleavable amino terminus. Phylogenetic analysis showed that these AhGLPs were classified into three subfamilies. Southern blot analysis indicated that AhGLP1 and AhGLP2 likely have multiple copies in the peanut genome. The recombinant mature AhGLP1 and AhGLP2 proteins were successfully expressed in Escherichia coli. The purified AhGLP2 has superoxide dismutase (SOD) activity in enzymatic assay, but not oxalate oxidase activity. The SOD activity of AhGLP2 was stable up to 70°C and resistant to hydrogen peroxide, suggesting that AhGLP2 might be a manganese-containing SOD. Furthermore, AhGLP2 could confer E. coli resistance to oxidative damage caused by paraquat, suggesting that the AhGLP2 likely protects peanut plants from reactive oxygen metabolites. Thus, information provided in this study indicates the diverse nature of the peanut GLP family and suggests that some of AhGLPs might be involved in plant defense response. KeywordsPeanut–GLP family–Superoxide dismutase–Oxalate oxidase–Disease resistance
    Plant Molecular Biology Reporter 01/2011; 29(2):389-403. · 5.32 Impact Factor
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    ABSTRACT: With 1 figure and 4 tablesAbstractTerminal drought induces preharvest aflatoxin contamination (PAC) in peanut. Drought resistance traits are promising as indirect selection tools for improving resistance to PAC. The objectives of this study were to determine the effects of terminal drought on PAC and to investigate the associations between surrogate traits for drought tolerance and PAC. Field tests under rainout shelters were conducted in the dry seasons 2004/2005 and 2005/2006. Eleven peanut genotypes were evaluated under irrigated and terminal drought conditions. Data were recorded for physiological traits, total biomass, pod yield, Aspergillus flavus colonization and PAC. ICGV 98305, ICGV 98330, ICGV 98348, ICGV 98353 and Tifton-8 had low aflatoxin contamination in both years. Traits related to drought resistance were associated well with those related to PAC under drought conditions. Specific leaf area, relative water content, chlorophyll density and drought stress ratings are the best traits for use as indirect selection tools for lower PAC. Breeding for drought tolerance using these traits as selection criteria may help to accelerate progress in developing resistance to PAC.
    Plant Breeding 11/2010; 129(6):693 - 699. · 1.18 Impact Factor
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    ABSTRACT: Twelve peanut genotypes were tested under three water regimes in two greenhouses to investigate the effects of drought on biomass production and N<SUB>2</SUB> fixation. Drought reduced biomass production from 36.5 to 56.0% and reduced nitrogen fixation from 26.8 to 68.8%. Most genotypes with high biomass production under Field Capacity (FC) had high reduction in biomass production under drought conditions, but fewer genotypes with high N<SUB>2</SUB> fixed under FC showed high reduction in N<SUB>2</SUB> fixed. Biomass production under FC in general gave more contribution to biomass production under drought conditions than did the reduction. N<SUB>2</SUB> fixed under FC and the reduction in N<SUB>2</SUB> fixed contributed similarly to N<SUB>2</SUB> fixed under drought conditions. Positive and significant correlations between N<SUB>2 </SUB>fixed and biomass production were found at FC and 2/3 available soil water (AW), but the correlation was not significant at 1/3 AW. Tifton-8 was the best genotype for high N<SUB>2</SUB> fixed under FC and KK 60-3 was the best genotype for low reduction. Correlations between N<SUB>2</SUB> fixed and nodule dry weight and shoot dry weight were high and consistent across water regimes. This information is important for breeders to develop peanut cultivars with reasonably high nitrogen fixation under drought conditions.
    Asian Journal of Plant Sciences 07/2010;
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    ABSTRACT: In 2001, entries from the peanut core collection, a subset of the USDA peanut germplasm collection, were planted in non-replicated plots in a field with a history of Sclerotinia blight caused by Sclerotinia minor. Variability existed among entries for reaction to Sclerotinia blight. Of the 744 entries evaluated, 11% had no disease, nearly 30% had <10% disease incidence, and only 21% had 50% disease incidence or more. Most of the resistant entries had an upright growth habit and were in early and mid-maturity groups. Many of the early maturing entries were susceptible to the foliar disease pepper spot which occurred throughout the study. Entries were selected for further evaluation in replicated plots based on a nil to low (<10%) incidence of Sclerotinia blight, adaptation and/or vigor, and other desirable characteristics such as an intermediate to prostrate growth habit and pepper spot resistance. Selected entries were retested in both 2002 and 2003 (n = 62) and compared to resistant (Tamspan 90), moderately resistant (Tamrun 96), and susceptible (Okrun) reference cultivars. Most entries (55 in 2001 and 46 in 2003) had disease incidence less than Tamrun 96 and similar to Tamspan 90. In 2003 when disease incidence was highest, all 46 entries with resistant reactions similar to that of Tamspan 90 had erect plant growth habits except for entries 208 and 582 which were prostrate, and entries 273, 128, and 804 which were intermediate. Resistance to Sclerotinia blight and yield similar to Tamspan 90, plant habit, and/or reactions to pepper spot and web blotch were used to select the best entries. Entries 208, 128, 804, 582, and 273 combined resistance to Sclerotinia blight, pepper spot, and web blotch with less than erect growth habits. Entry 103 had good Sclerotinia blight resistance and yield, but an upright growth habit. Entry 92 had an upright growth habit and low yield, but good Sclerotinia blight resistance. Entries 92 and 103 had upright growth habits but were among the best entries for resistant to pepper spot and web blotch. Entries 426, 184, and 562 were upright and susceptible to pepper spot, but had resistance to web blotch and the best resistance to Sclerotinia blight. These entries appear to be useful sources of resistance to Sclerotinia blight for breeding programs and for increasing the probability of finding additional sources of resistance in clusters of germplasm identified within the entire USDA collection.
    Peanut Science 01/2010; 37(1):1-11.
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    ABSTRACT: A large root system can be an important character for drought tolerance. Measuring root characteristics in soil medium is tedious, time consuming, and labor intensive. The objective of this study was to determine the association between root characteristics of peanut (Arachis hypogaea L.) grown in hydroponics and in pot experiments. Twelve peanut genotypes were planted in a randomized complete block design with four replications in a hydroponics study, a small pot, and a large pot experiment. The studies were conducted in two years during 2004–2005. Shoot dry weight, root dry weight, root-to-shoot ratio, root length, root surface, average diameter of roots, and root volume were measured. Tifton-8 and ‘KK 60-3’ had consistently higher values for all root characteristics compared to the other genotypes. Root characteristics of peanut genotypes grown in hydroponics were positively correlated with those of peanut genotypes grown in pot conditions. The results suggested that assessment of root characteristics of peanut in hydroponics for possible drought tolerance could replace the assessment in soil medium conditions.
    01/2010;
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    ABSTRACT: Peanut is vulnerable to a range of foliar diseases such as spotted wilt caused by Tomato spotted wilt virus (TSWV), early (Cercospora arachidicola) and late (Cercosporidium personatum) leaf spots, southern stem rot (Sclerotium rolfsii), and sclerotinia blight (Sclerotinia minor). In this study, we report the generation of 17,376 peanut expressed sequence tags (ESTs) from leaf tissues of a peanut cultivar (Tifrunner, resistant to TSWV and leaf spots) and a breeding line (GT-C20, susceptible to TSWV and leaf spots). After trimming vector and discarding low quality sequences, a total of 14,432 high-quality ESTs were selected for further analysis and deposition to GenBank. Sequence clustering resulted in 6,888 unique ESTs composed of 1,703 tentative consensus (TCs) sequences and 5185 singletons. A large number of ESTs (5717) representing genes of unknown functions were also identified. Among the unique sequences, there were 856 EST-SSRs identified. A total of 290 new EST-based SSR markers were developed and examined for amplification and polymorphism in cultivated peanut and wild species. Resequencing information of selected amplified alleles revealed that allelic diversity could be attributed mainly to differences in repeat type and length in the SSR regions. In addition, a few additional INDEL mutations and substitutions were observed in the regions flanking the microsatellite regions. In addition, some defense-related transcripts were also identified, such as putative oxalate oxidase (EU024476) and NBS-LRR domains. EST data in this study have provided a new source of information for gene discovery and development of SSR markers in cultivated peanut. A total of 16931 ESTs have been deposited to the NCBI GenBank database with accession numbers ES751523 to ES768453.
    International Journal of Plant Genomics 02/2009; 2009:715605.
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    ABSTRACT: Cylindrocladium black rot (CBR), caused by Cylindrocladium parasiticum, and root-knot nematode, Meloidogyne arenaria, both infect and cause damage to the roots of peanut. Greenhouse and microplot experiments were conducted with the runner type peanut genotypes C724-19-15, C724-19-25 and Georgia-02C with different levels of resistance to nematode and CBR to better understand the interactions between the two pathogens. In the greenhouse, inoculation of 500–3000 eggs per plant of M. arenaria did not affect the level of root rot induced by 1·0 to 5·0 microsclerotia of C. parasiticum per g soil. In microplots, the root rot ratings from Georgia-02C and C724-19-25 were higher in plots infested with M. arenaria (0·4–2·0 eggs per cm3 soil) and C. parasiticum than in plots with C. parasiticum alone; however, M. arenaria did not increase the root rot ratings on the nematode resistant C724-19-15. This was inconsistent with results in the greenhouse. Gall indices were not affected by C. parasiticum inoculations in the greenhouse or microplots. In both 2006 and 2007, a significant interaction between C. parasiticum inoculum densities and nematode level was observed on plant mortality. CBR inoculum greatly increased mortality on C724-19-25 and Georgia-02C, but not on C724-19-15, in the presence of M. arenaria. The mortality increase was more apparent at lower inoculum levels of both pathogens, but on the nematode-susceptible cultivars plant mortality was more with co-inoculations of the two pathogens than from either alone. Simultaneous inoculation with the two pathogens decreased yield of C724-19-25 and Georgia-02C as C. parasiticum inoculum levels increased, but even the largest inoculum of M. arenaria (2·0 eggs per cm3 soil) did not decrease yield of C724-19-15.
    Plant Pathology 01/2009; 58(1):71 - 79. · 2.97 Impact Factor
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    ABSTRACT: Genomic spcience offers new research tools to explore the function of genes and their effects on plants and animals. Arachis hypogaea is a polyploid species of relatively recent origin and molecular analyses with technologies available in the 1980s and 1990s resulted in little progress in the cultivated species because of apparent lack of molecular variation. Large numbers of polymorphisms existing in wild Arachis species led to evolutionary and gene introgression studies. High throughput genomic sequencing technologies have greatly expanded the possibilities for investigating gene function, but techniques are sufficiently expensive that most federal funding has been directed toward model species and major crops. Peanut has lagged behind many other crops, but the number of researchers working on the species in the U.S. and internationally has greatly increased during recent years. In an effort to bring researchers who work with a number of legume crops together to discuss common goals, a national strategic planning workshop was held in 2001 which led to the U.S. Legume Crops Genomics Initiative. A second workshop was held in 2004 to develop a plan with specific objectives for cross-legume genomics research and to outline milestones for accomplishments. Specifically for peanut, a genomics strategic planning workshop was organized at Atlanta in 2004 by the American Peanut Council. A broad view of genomic science was adopted and goals were set by participants to include (a) improving the utility of genetic tools for peanut genomics research, (b) improving the efficacy of technology for gene manipulation in genomics, (c) developing a framework for assembling the peanut genetic blueprint, (d) improving knowledge of gene identification and regulation, and (e) providing bioinformatic management of peanut biological information. Teams of researchers, including molecular biologists, plant breeders, pathologists, and many other disciplines need to be developed to fully utilize the potential of genomics for peanut improvement.
    01/2009;
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    ABSTRACT: Drought is the major abiotic constraint affecting peanut productivity and quality worldwide. There is a pressing need to improve the water use efficiency (WUE) of rain-fed peanut production. Breeding varieties with higher water use efficiency is seen as providing part of the solution. The objectives of this work were to (i) evaluate genetic variation in WUE, harvest index, root dry weight, specific leaf area (SLA) and SPAD chlorophyll meter reading (SCMR) among peanut genotypes in response to different available soil water levels and (ii) assess the relevance of root dry weight, SLA and SCMR to WUE in peanut under receding soil moisture levels. Two greenhouse experiments were conducted in the dry and rainy seasons in 2002/2003. The 11 peanut genotypes (ICGV 98300, ICGV 98303, ICGV 98305, ICGV 98308, ICGV 98324, ICGV 98330, ICGV 98348, ICGV 98353, Tainan 9, KK 60-3 and Tifton-8) and three soil moisture levels [field capacity (FC), 2/3 available soil water (AW) and 1/3 AW] were laid out in a factorial randomized complete block design (RCBD) with six replications. At 37, 67, and 97 day after sowing (DAS), data were recorded for SLA and SCMR. Root dry weight, harvest index (HI) and WUE were recorded at harvest. Drought reduced WUE, root dry weight and HI. Across both seasons, Tifton-8 and ICGV 98300 had high WUE and also had large root systems under drought conditions. ICGV 98324 and Tifton-8 had low SLA and high SCMR under stressed and non-stressed condition. Under drought conditions, ICGV 98324 had high HI and Tifton-8 had low HI. Root dry weight had a greater contribution to WUE under well-watered and mild drought (2/3 AW). Under severe drought (1/3 AW), SLA showed a more important contribution to WUE than the other traits. Traits that were associated to high WUE under drought conditions were different among different peanut genotypes. ICGV 98300 maintained high root dry weight under 2/3 AW and ICGV 98324 maintained low SLA and high SCMR under 1/3 AW. Tifton-8 had both large root systems and low SLA associated with high WUE.
    Agricultural Water Management. 01/2009;
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    ABSTRACT: The aim of this study was to evaluate the responses to difference in available soil water levels for yield and reproductive characters of peanut genotypes and relate these responses to pod yield under drought conditions. Eleven peanut genotypes were tested under three soil moisture levels (Field Capacity (FC), 2/3 available soil water (AW) and 1/3AW). Data were recorded for total number of flowers, pegs (reproductive sinks; RSs), immature pods and mature pods per plant, number of seeds per pod, 100-seed weight and pod yield at harvest. Drought at mild and severe levels significantly reduced Harvest Index (HI), yield components and reproductive developmental characters and the reductions were most substantial for pod yield with increasing moisture stress. High pod yield under drought conditions in ICGV 98300 was caused by high potential yield and low yield reduction, whereas high pod yield in ICGV 98324 was due to low pod yield reduction. High HI and numbers of mature pods are advantageous and necessary for high yield under drought conditions. High conversion of RSs to total pods and conversion of flowers to mature pods were the most important factors contributing to high pod yield under mild drought (2/3 AW) and severe drought (1/3 AW), respectively. Tifton 8 showed the lowest pod yield and poor seed filling under well-watered and droughts conditions.
    Asian Journal of Plant Sciences. 01/2009;
  • Crop Science - CROP SCI. 01/2009; 49(3).
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    ABSTRACT: Three major species of root-knot nematode infect peanut: Meloidogyne arenaria race 1, M. hapla, and M. javanica race 3. Sources of resistance to all three nematodes are needed for developing novel peanut cultivars with broad resistance to Meloidogyne spp. Cultivars and breeding lines of peanut were evaluated for resistance to M. arenaria, M. hapla, and M. javanica in the greenhouse and in the laboratory. Twenty-six genotypes with some resistance to M. arenaria, M. javanica, or M. hapla were identified from 60 accessions based on average eggs per gram of root and gall index relative to a susceptible control. Among these, 14 genotypes were moderately to highly resistant to all three species, 5 genotypes were resistant to M. arenaria and M. javanica, 2 genotypes were resistant to M. javanica and M. hapla, 1 genotype was resistant M. arenaria alone, and 4 genotypes were resistant to M. hapla alone. Reproduction of M. arenaria on lines NR 0817, C724-19-11, and D108 was highly variable, indicating that these genotypes likely were heterogeneous for resistance. COAN, NemaTAM, C724-25-8, and the M. arenaria-resistant plants of C724-19-11 contained the dominant sequence-characterized amplified region marker (197/909) for nematode resistance. Results with the molecular markers indicate that the high resistance to M. arenaria in GP-NC WS 6 may be different from the resistance in COAN, NemaTAM, and C724-25-8. Resistance to M. arenaria was correlated with resistance to M. javanica in peanut, whereas resistance to M. hapla was not correlated with the resistance to either M. arenaria or M. javanica. The resistant selections should be valuable sources for pyramiding resistance genes to develop new cultivars with broad and durable resistance to Meloidogyne spp.
    Plant Disease 01/2008; 92:631-638. · 2.74 Impact Factor
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    ABSTRACT: Inheritance of traits is important for developing effective breeding schemes for improving desired traits. The aims of this study were to estimate the heritabilities (h2) of drought resistance traits and the genotypic (rG) and phenotypic (rP) correlations between drought resistance traits and agronomic traits, and to examine the relationships between drought resistance traits under stressed and nonstressed conditions. The 140 lines in the F4:7 and F4:8 generations from four peanut (Arachis hypogaea L.) crosses were tested under field capacity (FC) and two-thirds available soil water (2/3 AW) in two field experiments. Data were recorded for specific leaf area (SLA), SPAD chlorophyll meter reading (SCMR), and biomass, pod yield, harvest index, number of mature pods per plant, seed per pod, and seed size. The h2 for biomass, pod yield, DTI (drought tolerance index) (pod yield), DTI (biomass), HI, SLA, and SCMR were high for all tested crosses (0.54-0.98). The rG (-0.61 and -0.66) and rP (-0.61 and -0.66) between SLA and SCMR were strong and negative under 2/3 AW and FC. Under 2/3 AW conditions, SCMR was positively correlated with pod yield and seed size. Compared to SLA, SCMR had higher rG and rP with pod yield, biomass, and other agronomics traits. Significant correlations between FC and 2/3 AW conditions were found for pod yield, biomass, SCMR, and SLA, indicating that these traits could be selected under FC or 2/3 AW conditions. SPAD chlorophyll meter reading, which is easy to measure, is potentially useful as a selection trait for drought resistance because of high h2 and positive correlation with pod yield and agronomic traits.
    01/2008;
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    ABSTRACT: Identification and utilization of peanut cultivars with resistance to Cylindrocladium black rot (CBR) is a desirable approach to manage this disease. The objectives of this study were to improve greenhouse and field screening techniques for resistance to CBR, and to evaluate the reaction of selected runner-type peanut genotypes. Georgia-02C (moderately resistant to CBR) and C-99R (CBR-susceptible) were used in comparing the effectiveness of different inoculation methods in the greenhouse. Disease development was affected by both size and density of microsclerotia in soil. Use of microsclerotia at a size of 150 to <250 µm and a density of 1 to 5 microsclerotia/g soil provided the best separation the CBR-resistant cultivar Georgia-02C and the susceptible C-99R based on root rot severity. Genotypes with varying resistance to CBR were evaluated by growth in a naturally infested field, and by inoculating plants in the field and greenhouse. Disease incidence and severity at harvest were the most effective parameters for evaluating CBR resistance in the field and greenhouse, respectively. The cultivars Georgia-02C and Georganic had the lowest disease incidence, whereas C-99R and DP-1 had the highest disease incidence in a naturally infested field in 2005 and 2006. Incidence of CBR was moderate for Georgia-01R in both years, but was inconsistent for C34-24-85. Georgia-02C and Georganic also showed partial resistance to CBR in greenhouse tests. Inoculated plants in the field had similar reaction with Georgia-02C and Georganic showing higher CBR resistance than C-99R and DP-1 in both 2006 and 2007. The root rot severities for genotypes Georgia-02C and Georganic were lower than those for C-99R and DP-1. Incidence of CBR in the naturally infested field was significantly correlated with CBR incidence in the inoculated plants in the field (r = 0.84, P 0.01), but neither was correlated with disease ratings for greenhouse experiments. Peanut genotypes are most reliably screened by inoculating plants in the field or using uniformly infested fields. Further study is needed to improve greenhouse screening procedures.
    01/2008;
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    ABSTRACT: The aims of this study were to evaluate genetic variations in yield and reproductive developmental characters among peanut genotypes in response to drought and relate these responses to pod yield under different soil moisture. Eleven peanut genotypes were tested under three soil moisture levels [field capacity (FC), 2/3 available soil water (AW) and 1/3AW] in field experiments. Data were recorded for number of flowers, pegs (RSs), immature pods and mature pods per plant, seed per pod, 100-seed weight and pod yield at harvest. A drought tolerance index (DTI) for pod yield was calculated as the ratio of pod yield under stress treatment to that under well-watered conditions. The differences among water regimes were significant for pod yield, number of RSs, immature pods and mature pods per plant, seed per pod and 100 seed weight and differences among genotypes were significant for all traits. Drought reduced pod yield, number of RSs, pods and mature pods per plant. Early peak of flowering is important for the formation of mature pods under drought conditions. Two different strategies were used in maintaining high pod yield under drought. High yield potential was important for ICGV 98348 and ICGV 98353, whereas low pod yield reduction was important for ICGV 98305, ICGV 98303 and ICGV 98300. Tifton 8 showed the lowest pod yield and poor seed filling. High RSs and well-filled mature pods were the most important traits contributing to high pod yield in drought resistant genotypes.
    01/2008;
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    ABSTRACT: This study assessed components of resistance for three runner-type peanut cultivars to infection by Cercospora arachidicola (Ca) and Cercosporidium personatum (Cp), the causal organisms of early leaf spot and late leaf spot, respectively. Resistance components were compared to disease resistance observed in the field. A field study monitored the progression of leaf spot incidence and severity in peanut cultivars Georgia Green, Georganic, and DP-1. Time of disease onset (TDO) and temporal epidemic rate (rate) were estimated for incidence with the logistic model, and for severity with the linear model. Early leaf spot was the predominant disease in the field. Estimates of TDO were 9 d later for DP-1 than for Georgia Green, based on incidence models, and 6 and 7 d later for Georganic and DP-1 than for Georgia Green, respectively, based on severity models. Incidence progression rate was highest for Georganic in 2002 and Georgia Green in 2003, while severity progression rate was highest for Georgia Green across years. A detached leaf assay was used to determine components of resistance for these genotypes to infections by Ca and Cp. Infection frequency 30 d after inoculation, lesion diameter, and percent necrotic area were greatest for Georgia Green for both pathogens. Besides a 2-d longer latent period for resistant genotypes, no Ca reproduction differences were detected. For Cp, latent period was shorter for Georganic than DP-1, and sporulation per unit lesion area was greatest for Georganic. Enhanced field resistance to early and late leaf spots reported for DP-1 and Georganic is in part due to lower infection frequencies, smaller lesions, and for DP-1, longer latent periods.
    01/2008;