M.S.

Laboratory experiments were conducted to evaluate Tetranychus urticae Koch dispersal response to fenvalerate and permethrin. A petri dish spider mite trap which allows the partitioning of the dispersal response into spindown and walkoff categories was developed. Fenvalerate induced the more severe dispersal response. A greenhouse experiment was conducted to evaluate Tetranychus urticae oviposition site selection in response to residues of fenvalerate and permethrin, at two levels of residue deposition. When pyrethroid-free leaf surface was available, nearly all eggs were found on the untreated surface, away from the pyrethroid residues. When pyrethroid-free leaf surface was not available, a substantial number of eggs were found suspended by silk threads above the treated surface. The results were found to be in agreement with the general pyrethroid avoidance hypothesis. A series of greenhouse experiments were conducted to evaluate Tetranychus urticae oviposition response following impingement, and contact with residues of fenvalerate and permethrin. In all instances the pyrethroids were found to reduce the number of eggs oviposited. Mites recovered after several days, and oviposition returned to normal. The results did not lend support to the hormoligosis hypothesis of insecticide induced mite outbreaks. Field and greenhouse experiments were conducted to evaluate Tetranychus urticae oviposition response, over time, to soybeans which have received an application of fenvalerate of permethrin. The pyrethroids did not alter the soybeans in a way which improved the quality of the plant as a resource for increased levels of spider mite oviposition. / Master of Science / incomplete_metadata

The possible existence of variability in the reaction of PMV in soybean, which could lead to resistance breaking strains, was investigated in this study. The specific research objectives were to provide evidence of PMV strains in soybean, and to characterize the PMV strains using aphid transmission efficiency and serological relationships. Twelve PMV isolates were placed in five strain groups (P1-P5) based on symptom expression in the soybean cultivars Lee 68, York, and Virginia. Aphid transmission efficiency of the PMV strains by the green peach aphid (Myzus persicae Sulz.) was determined by counting infected test plants following natural feeding by aphids exposed to virus infected source plants. Transmission efficiency from pea (Pisum sativum L.) source plants to pea test plants was 11 to 36% and varied with strains. Pea to soybean and soybean to soybean transmission was 5 to 29% and 20 to 58%, respectively. From these results, the 5 strains could be classified in 2 groups. Two techniques, SDS-immunodiffusion (sodium dodecyl sulfate) and ELISA (enzyme-linked immunosorbent assay), were used to investigate the serological relatedness of the PMV strains. Results indicated that all the strains were serologically closely related. / Ph. D.

This item was digitized as part of the Million Books Project led by Carnegie Mellon University and supported by grants from the National Science Foundation (NSF). Cornell University coordinated the participation of land-grant and agricultural libraries in providing historical agricultural information for the digitization project; the University of Arizona Libraries, the College of Agriculture and Life Sciences, and the Office of Arid Lands Studies collaborated in the selection and provision of material for the digitization project.

Meloidogyne javanica is the most widely spread nematode pest on soybean in South Africa. Only a few registered cultivars have some resistance to this nematode and there is an urgent need for an efficient breeding programme for resistant cultivars of all maturity groups. However, breeding is hampered by laborious screening procedures for selection of resistant lines. The objective of this study was to develop an economically viable molecular marker system for application in selection procedures. Three techniques of marker identification were investigated, i.e. RAPD, RFLP and AFLP analysis. The RAPD technique proved to be applicable in fingerprinting soybean varieties, but was too sensitive for interplant variation to be used as an efficient system for identification of molecular markers linked to nematode resistance. Both RFLP and AFLP screening identified markers linked to gall index variation in a segregating population of 60 F₂ progeny from across between a resistant cultivar, Gazelle, and a highly susceptible variety, Prima. A codominant RFLP marker( B212) was linked significantly to resistance and explained 62% of the variation in gall index. Seven AFLP markers were linked significantly to the resistance trait, of which four were linked in repulsion phase and three in coupling phase. All seven AFLP markers mapped to LG-F on the public soybean molecular map. The QTL for resistance mapped between markers E-ACC/M-CTC2 linked in coupling phase, 8212 and E-AAC/M-CAT1, linked in repulsion phase. These two AFLP markers bracketing the major resistance QTL were successfully converted to SCARs. Marker E-ACC/M-CTC2 was converted to a codominant SCAR marker SOJA6, which acounted for 41% of variation in gall index in the mapping population. Marker E-AAC/M-CAT1 was converted to a dominant SCAR marker (SOJA7) and explained 42% of gall index variation in the mapping population. These two markers mapped approximately 3.8 cM and 2.4 cM respectively from the resistance QTL. This study represents the first report of the development of PCR-based sequence specific markers linked to resistance to M. javanica in soybean. / Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 2000.

Soybean (Glycine max (L.) Merr.) is one the major crops produced worldwide. However, soybean is susceptible to many diseases. Sclerotinia stem rot (SSR) disease caused by Sclerotinia sclerotiorum (Lib.) de Bary is considered one of the most important fungal diseases of soybean. It can be controlled by chemicals (e.g. fungicides), by breeding cultivars with disease resistance and by cultural control (e.g. increasing the width between rows, reducing plant populations). A promising and complementary method of controlling SSR disease in the field is the application of biological control agents. Biological control agents introduced in a soil environment will interact with other soil food web organisms, as do the pathogenic organism and infected plants, which may change the genetic and functional diversity in soil microbial communities. Profiling these changes may lead to an improved understanding of the interactions between these players (biological control agents, pathogens, soil biota and plants) in the biological control phenomenom, permiting us to exploit naturally-occurring ecological relationships and develop more sustainable approaches to control soybean diseases. Fatty acid biomarkers analysis was used to profile microbial communities in soils. Two laboratory studies were conducted to evaluate the methods used for extraction and profiling the fatty acid biomarkers from soil samples with a range of soil properties (clay content, organic matter content), The first study investigated the best solvent mixture for recovering fatty acid biomarkers from soil using an automated pressurized solvent extraction (PSE) system. Solvent mixtures containing chloroform and methanol were more efficient at extracting fatty acids from agricultural soils than hexane:2-propanol and acetone. The second study presented an exploratory pyrolysis-mass spectrometry technique to rapidly fingerprint soil lipids extracted from different agroecosystems. Pyrolysis-mass spectrometry discriminated among soils and crop production systems in the same way as the fatty acid profiling. I also report on the efficicacy of biological control agents to control Sclerotinia stem rot disease in soybean. A two-year study was conducted in soybean fields under conventional or no tillage to determine whether Trichoderma virens (SoilGard(TM)) and arbuscular mycorrhizal fungi (a mixture of Glomus intraradices and G. mosseae ), used alone or in combination, could reduce sclerotinia stem rot (SSR) disease incidence. Generally, SSR disease indicators, as well as the soybean yield, were not affected significantly by the biological control treatments. I then studied whether changes in microbial community composition were related to the inoculation of the biological control agents and the disease incidence in soybean fields. Inoculation of biological control agents changes the expression of many soil fatty acids during both years of the trial. Also, in the plots with severely diseased plants, fatty acids biomarkers of gram positive and actinomycetes bacteria were significantly greater than in plots with healthy plants. I conclude that further improvement in laboratory techniques and procedures will permit researchers to efficiently extract and characterize soil lipids, providing new insight into soil organic matter dynamics and soil microbial ecology. Further study will be needed to verify the efficacy and optimize the application method, dose and timing of biocontrol agents to provide protection against SSR disease in soybean fields.

No description available.

Experiments were conducted at Blacksburg from 1975 to 1978 on soybean, Glycine max (L.) Merrill. The objectives were: (1) to study the inheritance of reaction to peanut mottle virus (PMV); (2) to determine the allelic relationships between genes for resistance from various germplasm sources; and (3) to screen a part of the soybean Plant Introduction germplasm collection and identify PMV-resistant strains. Soybean cultivars, Plant Introduction (PI) strains, and advanced generation progenies derived from selected crosses were artificially inoculated with PMV-S/V74S (a Virginia isolate) and evaluated for their reaction to PMV in the field and greenhouse. Two cultivars, 'Virginia' and 'Pine Dell Perfection', that were previously reported as resistant to a mild PMV strain were found to be susceptible to PMV-S/V74S. Crosses between resistant and susceptible lines and among resistant lines were made in the field in 1976 and 1977. The F₁, F₂, and F₃generation seedlings derived from selected crosses were tested for reaction to PMV in the greenhouse. It was shown that resistance in the cultivar 'Peking' is conditioned by a single recessive gene designated rpv₂. Evidence based on segregation in F₂ populations was also presented that indicates the presence of three other dominant genes for resistance to PMV-S/V74S. The three dominant genes are designated Rpv₁ (first reported by workers in Georgia), Rpv₃, and Rpv₄. Preliminary F₁ and F₂ data were obtained from crosses between 15 resistant PI strains and two resistant "testers," 'York' and PI 89,784. The F₂ data obtained from five crosses indicate the possibility of still other genes for resistance to PMV. The two susceptible lines used in the study, Virginia and PI 229,315, differed markedly in their reactions to PMV. The presence of different alleles or modifying genes controlling the susceptible reactions in the two lines is suggested. A total of 2161 FC and PI strains in Maturity Groups II, III, and IV were inoculated with PMV-S/V74S in the field during 1976 and 1977. Three hundred sixty-six strains that showed 10% or less virus infection were identified. These "resistant" strains provide a pool from which other genes for PMV resistance perhaps can be isolated. Differences in PMV disease reactions of plants from the same strain were noted when plants were tested in both the field and greenhouse. The differences were attributed to the following three factors: (1) differences in stage of plant growth at time of inoculation (field grown plants were generally larger at time of inoculation than plants inoculated in the greenhouse); (2) differences in environmental conditions between the field and greenhouse; and (3) the artist's airbrush inoculation technique was used in the field while the rub inoculation technique (mortar and pestle) was used in the greenhouse. / Ph. D.

The genetic interactions among 9 soybean [<i>Glycine max</i> (L.) Merr.] cultivars and 6 strains of soybean mosaic virus (SMV) were investigated. The objectives were to identify genes and/or alleles conditioning resistant and necrotic reactions to SMV and to determine the genetic relationships among resistance genes from cultivars exhibiting differential responses to the SMV strains. Seven SMV-resistant (R) cultivars (‘PI 486355’, ‘Suweon 97’, ‘PI 96983’, ‘Ogden’, ‘York’, ‘Marshall’, and ‘Kwanggyo’) were crossed in all combinations among each other and with susceptible (S) cultivars ‘Essex’ and ‘Lee 68’. F₂ populations and F₂-derived F₃ lines were inoculated in field with the SMV type strain Gl and in the greenhouse with the virulent strains G4, G5, G6, G7, and G7A. All F₂ populations from R x S and necrotic (N) x S crosses having PI 96983, Ogden, York, Marshall, and Kwanggyo as either resistant or necrotic parents segregated 3R:1S and 3N:1S, respectively. F₂-derived F₃ progenies from R x S crosses exhibited an F₂ genotypic ratio of 1 homogeneous R : 2 segregating (3R:1S) : l homogeneous S. The results indicate that each of these five resistant parents has a single, dominant or partially dominant gene conditioning the resistant and necrotic reactions to SMV. No segregation for SMV reaction was evident in F₂ and F₃ generations from R x R, N x N, and S x S crosses among the five differential cultivars, indicating that the resistance genes in the five cultivars are alleles at a common locus. The alleles in PI 96983 and Ogden were previously labeled <i>Rsy</i> and <i>rsy^t</i>, respectively. Gene symbols, <i>Rsy^y</i>, <i>Rsy^m</i>, and <i>Rsy^k</i> are proposed for the resistance genes in York, Marshall, and Kwanggyo, respectively. It is also proposed that the gene symbol <i>rsy^t</i> be changed to <i>Rsy^t</i> to more accurately reflect its genetic relationship to the susceptible allele. The R x S crosses with PI 486355 and Suweon 97 as resistant parents segregated 15R:1S in the F₂ and 7 (all R) : 4 (3R:1S) : 4 (15R:1S) : 1 (all S) in the F₃, indicating that each has two independent genes for resistance to SMV. The F₂ plants of PI 486355 x Suweon 97 showed no segregation for SMV reaction, suggesting that they have at least one gene in common. The crosses among all 7 resistant parents produced no susceptible segregates when inoculated with strain G1. It is concluded that the 7 resistant cultivars each have a gene or allele at the <i>Rsy</i> locus. Data from the experiments furnished conclusive evidence that the necrotic reaction in segregating populations is highly associated with plants that are heterozygous for the resistance gene. / Ph. D.

M. S.