Dissertations / Theses on the topic 'Soybean – Diseases and pests – Zimbabwe'

Soybean leaf consumption for the most common semilooper species, Trichoplusia orichalcea (F.) was established in the laboratory using a leaf area meter. The total consumption per larva was 120,85 cm$ sp2$. The yield reduction of our soybean cultivars (Glycine max (L.) Merrill) being Duiker, Gazelle, Roan and SCS1 was measured in relation to different levels of defoliation. The defoliation was induced manually singly or sequentially over three different growth stages.
At soybean maturity, counts of pods per plant, number of seeds per plant, weight per 100 seeds and weight of seeds per plant were obtained. Yields were converted to a percentage of yield and linear regressions fitted to the relationship between percentage yield and defoliation. The percentage yield was influenced by the cultivars, the levels of defoliation, and the timing of defoliation.
The most critical growth stage for defoliation was the seed development stage, but the differences between them were small and not significant. SCS1 was the most resistant cultivar to defoliation, while Duiker was the most susceptible to single defoliation.
The economic thresholds were determined for the soybean cultivars demonstrating significant linear relations between percentage yield and single defoliation. Based on three insecticides and two application methods, the economic thresholds were determined at the flowering stage for Duiker, at the pod development stage for Duiker and Roan and at the seed development stage for Duiker, Gazelle, Roan, and SCS1 cultivars. The numbers of semilooper caterpillars per metre of row required to reach the economic thresholds, are generally higher than the natural levels of infestation.

Soybean (Glycine max [L.] Merr.) is one of the most important crops in the world. The average annual yield losses due to soybean diseases and pests are estimated to be around 11% in the United States. Soybean yield losses due to sudden death syndrome (SDS), caused by the fungus Fusarium virguliforme O'Donnell & T. Aoki have been problematic in majority of the soybean producing states. In recent years, reniform nematode (RN, Rotylenchulus reniformis Linford and Oliveira) and frogeye leaf spot (FLS), caused by the fungus Cercospora sojina K. Hara have emerged as a major problem in the southern soybean producing states. Planting resistant cultivars is one of the most cost-efficient methods in managing SDS, RN, and FLS, therefore it would be critical to identify and map the quantitative trait loci (QTL) that underlie their resistances. Two soybean populations were evaluated in this study. The ‘Essex’ × ‘Forrest’ 77 near-isogenic lines were screened in the field to evaluate the disease index of SDS. The Essex × Forrest and ‘Flyer’ × ‘Hartwig’ recombinant inbred lines were screened in the greenhouse to assess the reproduction index of RN and the disease severity of FLS. The BARCSoySNP6k chip was used to genotype the two populations. Four QTL that underlie SDS resistances were mapped in the same region as Rfv06-01, Rfv06-02, Rfv13-01, and Rfv19-01. The Rfv06-02 interval in this study was smaller than the one previously reported. Rrr08-01, Rrr13-01, Rrr15-01, Rrr18-01, and Rrr18-02 were reported to confer resistances to RN. Rrr08-01, Rrr13-01 and Rrr15-01 were novel whereas Rrr18-01, and Rrr18-02 were mapped in previous studies. cqSCN-001 (soybean cyst nematode, Heterodera glycines Ichinohe) was identified in the same region as Rrr18-01, and Rrr18-02 whereas cqSCN-006 was identified in the same region as Rrr15-01. These findings provide further evidence that there are common sources of genetic resistances to RN and SCN. Rcs15-01 and Rcs15-02 were reported to confer resistances to FLS. Rcs15-01 was novel and Rcs15-02 was mapped at the same region as an Rcs mapped in a previous study. This indicated that Rcs15-02 has dual resistances to C. sojina races. Candidate genes were inferred in this study. The QTL mapped in this study could potentially be used in soybean breeding programs that aim to introgress genetic resources that confer resistances to SDS, RN, and FLS.

Soybean cyst nematode (SCN), Heterodera glycines, is a threat to soybean production in North Dakota. Studies on the biology of SCN were conducted to improve my understanding and management of this plant parasitic nematode. The objectives of the research were to; (1) determine if SCN reproduces on crops commercially grown or being tested for production in North Dakota, (2) evaluate the effects of SCN on growth of dry bean, (3) determine if there could be a shift in the SCN population toward greater ability to reproduce on dry bean, and (4) characterize the spatial distribution of SCN in research size field experiments. Canola, clover, lentil, and sunflower were nonhosts while borage, camelina, chickpea, crambe, cuphea, field pea, nyjer, and safflower were poor hosts for SCN with female indices (FI) less than 8. Lupines were susceptible hosts with FI?s of 42 to 57. FI?s of dry bean cultivars varied from 5 to 117. Kidney beans averaged the highest FI at 110 followed by navy, pinto and black at FI?s 41, 39, and 16, respectively. Pod number (PN), pod weight (PW), seed number (SN), and seed weight (SW) of GTS-900 (pinto bean) were significantly less at 5,000 and 10,000 eggs/100 cm3 soil compared with the control by 44 to 56% averaged over the two years. Significant reduction in growth of Montcalm (kidney bean) and Mayflower (navy bean) was observed at 2,500 and 5,000 eggs/100 cm3 soils in 2009, but not in 2008. There was no evidence that SCN was increasing reproduction during two 11 month periods of continual reproduction on roots of dry bean cultivars Premiere and Cirrus (navy), Buster and Othello (pinto), and Eclipse and Jaguar (black). The spatial distribution of SCN in field plots was aggregated in nine of ten field sites with large differences in egg numbers between plots. Lloyd?s index of patchiness ranged from 1.09 to 3.34. Spatial distribution of SCN can be an important factor affecting the results of field experiments.

The genetic interactions among 9 soybean [Glycine max (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 Rsy and rsy^t, respectively. Gene symbols, Rsy^y, Rsy^m, and Rsy^k are proposed for the resistance genes in York, Marshall, and Kwanggyo, respectively. It is also proposed that the gene symbol rsy^t be changed to Rsy^t 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 Rsy 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.

O controle de doenças em plantas geralmente é realizado com o uso de agroquímicos, que podem desencadear efeitos maléficos ao ser humano ou ao ambiente como um todo. Diversos trabalhos mostram o potencial de plantas medicinais no controle de fitopatógenos, tanto por sua ação fungitóxica direta, inibindo o crescimento micelial e a germinação de esporos, quanto pela capacidade de ativar o metabolismo de defesa das plantas, como o acúmulo de fitoalexinas, indicando a presença de moléculas com características elicitoras. Foram desenvolvidos dois experimentos na Universidade Tecnológica Federal do Paraná – Câmpus de Dois Vizinhos, com objetivos de avaliar o pontencial de preparados de Cavalinha (Equisetum sp.) na síntese de metabólitos de defesa em cotilédones de soja (Glycine max L.) e o efeito sobre o crescimento de Rhizoctonia solani, in vitro. O delineamento experimental utilizado para os experimentos foi inteiramente casualizado em esquema fatorial 3x5 resultante da combinação de três formas de extração (extrato alcoólico, infusão e maceração) e cinco concentrações (zero; 1; 10, 20 e 40%), com 4 repetições. No primeiro experimento foi avaliada a indução de fitoalexinas em cotilédones de soja em resposta aos derivados a base de cavalinha. Sementes de soja foram semeadas em areia autoclavada e mantidas em temperatura ambiente por 10 dias. Em seguida os cotilédones das plântulas foram removidos e na face abaxial destes foram aplicados os tratamentos. Após seguir os procedimentos metodológicos da técnica de extração, obteve-se via espectofotometria a quantificação da fitoalexina gliceolina, sendo então utilizados os cotilédones para quantificação da atividade da enzima fenilalanina amônia-liase (FAL) e o teor de fenóis totais. No segundo experimento foi avaliado o potencial fungistático dos preparados sobre Rhizoctonia solani. Em placas de Petri foram adicionados no meio de cultura BDA os preparados nas diferentes concentrações. Após a solidificação foi transferido para o centro de cada placa, um disco de micélio com Rhizoctonia solani. As placas foram incubadas sob alternância de fotoperíodo de 12 horas, em temperatura de 25oC. Os preparados de extrato alcoólico, infusão e maceração de cavalinha apresentam capacidade de indução das fitoalexinas gliceolinas em cotilédones de soja, bem como, ativam o metabolismo de compostos fenólicos. Entre os preparados, o extrato alcoólico e a maceração, se sobressaem sobre a infusão. Os preparados de extrato alcoólico, infusão e maceração de cavalinha em todas as suas concentrações inibem o crescimento do fungo Rhizoctonia solani, in vitro, sendo que o extrato alcoólico apresenta maior capacidade de supressão do crescimento micelial em comparação as demais formas de obtenção dos preparados.
Usually, the plant diseases control is accomplished with the use of chemicals products that can trigger harmful effects for the people or the environment. Several studies showed the potential of medicinal plants in the control of pathogens, either by their direct fungitoxic action, inhibiting the mycelial growth and spore germination, and by ability to activate the metabolism of plant defense, such as phytoalexin accumulation, indicating the presence of molecules with characteristics elicitoras. Two experiments were carried out in the Universidade Tecnológica Federal do Paraná - Câmpus Dois Vizinhos. The aims of these works were to evaluate the potential of "cavalinha" (Equisetum sp.) prepared in the defense metabolites synthesis of soybean cotyledons (Glycine max L.) and the effect on Rhizoctonia solani growth in vitro. The experimental design used for the experiments was completely randomized in factorial 3 x 5 (extract form x concentration), with four replications. The extract forms were alcoholic extract, infusion and maceration and the concentrations tested were zero, 1, 10, 20 and 40%. In the first experiment, the phytoalexins induction in the soybean cotyledons the according with "cavalinha" prepared was evaluated. Soybean seeds were sown in autoclaved sand and kept at room temperature for 10 days. After, the plantlets cotyledons were removed and on the surface of these the treatments were applied. Then, it followed the methodological procedures of extraction technique and it was obtained via spectrophotometry the phytoalexin glyceollin quantification. The cotyledons were then used to quantify of the enzyme phenylalanine ammonia lyase (PAL) activity and total phenols contend. In the second experiment was evaluated the potential fungistatic preparations on Rhizoctonia solani. The prepared with different concentrations was added in Petri dishes with PDA culture medium. After solidification was transferred to the center of each plate, a disc with Rhizoctonia solani mycelia. The plates were incubated alternating the photoperiod in 12 hour, at 25 °C temperature. The alcoholic extract, infusion and maceration "cavalinha" prepared present glyceolin induction of phytoalexins in soybean cotyledons, as well as it to activate the metabolism phenolic compounds. Among the prepared, the alcoholic extract and maceration forms were superior in relation the infusion form. The alcoholic extract, infusion and maceration "cavalinha" prepared using all over the concentrations inhibit Rhizoctonia solani growth in vitro. The alcoholic extract had a higher capacity to suppress mycelial growth compared to other forms of production preparations.

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

Field and greenhouse studies were conducted during 2004 through 2006 at the Rodney R. Foil Plant Science Research Center, Starkville, MS. Six sorghum and soybean rotation treatments were tested to determine their effect on plant pathogen, insect, and nematode diversity and density levels. Treatments included 1) continuous sorghum, 2) continuous soybean, 3) sorghum-soybean-sorghum rotation, 4) soybean-sorghum-soybean rotation, 5) sorghum-soybean-soybean rotation, and 6) soybean-sorghum-sorghum rotation. Several nematode and insect species were identified during the study, but were always below economic thresholds. Six insect species were identified on soybean during each growing season and used as the indicator species for this study. The most prevalent were threecornered alfalfa hopper (Spissistilus festinus (Say) and bean leaf beetle (Cerotoma trifurcata (Forester). Sorghum webworm (Nola sorghiella Riley) and corn earworm (Helicoverpa zea (Boddie) were the most common insects on sorghum panicles. Rotations did not affect the diversity or density levels of the indicator soybean or sorghum insects during the three year study. Plant disease levels during the investigation showed variable results. Three foliar fungal pathogens including Diaporthe phaseolorum (Cooke & Ellis) Sacc. var. meridionalis, Septoria glycines Hemmi, and Cercospora sojina Hara on soybean, and Gloeocercospora sorghi D. Brain & Edgerton ex Deighton on sorghum were observed. The only virus disease on soybean was bean pod mottle, but levels were not affected by the rotations during the study. Zonate spot caused by G. sorghi was the most prevalent foliar sorghum disease, but was not affected by the rotations. Six frequently isolated fungal pathogens from either soybean or sorghum roots included Macrophomina phaseolina (Tassi) G. Goidanich, Rhizoctonia solani Kühn, D. phaseolorum, Aspergillus spp., Trichoderma spp and Fusarium spp. Aflatoxin contamination of sorghum seed was low (<20 ppb) the first two years of the study, but was high (790 ppb) in 2006. Significantly greater soybean and sorghum yields were obtained from rotated systems compared to monoculture systems in 2005. In a greenhouse test, M. phaseolina infection of soybean led to significantly greater root disease ratings, lower plant height and dry weight than the untreated control. Sorghum plant growth was not affected by M. phaseolina and R. solani.

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.

Thesis (MScAgric)--University of Stellenbosch, 2005.
ENGLISH ABSTRACT: Fruit and Leaf Spot Disease (FLSD) of citrus, caused by Phaeoramularia angolensis, is found only in 18 countries in Africa, the Comores Islands in the Indian Ocean and Yemen in the Arabian peninsula. The major citrus export countries in Africa are Morocco, South Africa, Swaziland, and Zimbabwe. Zimbabwe is the only country affected by FLSD. FLSD is a disease of major phytosanitary and economic importance and its devastating effect on citrus is highlighted by the fact that the damage is cosmetic, which renders the fruit unmarketable. Total crop losses are not uncommon in Kenya. The aims of the present study, therefore, was was to determine the occurrence of P. angolensis in Zimbabwe and neighbouring Mozambique, to compare these isolates with the Cercospora Fresen. isolates from Swaziland and South Africa, to determine the epidemiology of the pathogen and to implement an effective control strategy to prevent the spread of FLSD. Leaf samples with citrus canker-like lesions collected in the early 1990’s in Zimbabwe were found to be infected by the fungus, Phaeoramularia angolensis. Surveys were undertaken to determine the spread and intensity of FLSD in Zimbabwe and Mozambique. In Zimbabwe, P. angolensis was limited to an area above the 19° south latitude, predominantly the moist areas and not the low-lying drier parts of the country. In Mozambique, no P. angolensis symptoms were found. Observations during the survey indicated that no proper management systems were implemented by Zimbabwean growers. A cercosporoid fungus causing a new Fruit and Leaf Spot Disease on Citrus in South Africa was identified. From morphological and rDNA sequence data (ITS 1, 5.8S and ITS 2), it was concluded that the new disease was caused by Cercospora penzigii, belonging to the Cercospora apii species complex. The genera Pseudophaeoramularia and Phaeoramularia are regarded as synonyms of Pseudocercospora, and subsequently a new combination was proposed in Pseudocercospora as P. angolensis. Cercospora gigantea was shown to not represent a species of Cercospora, while Mycosphaerella citri was found to be morphologically variable, suggesting that it could represent more than one taxon. A control strategy for the control of FLSD was evaluated in the study. The data showed that P. angolensis in Zimbabwe can be managed successfully by the removal of all old and neglected orchards, and on timely fungicide applications. Trifloxystrobin + mancozeb + mineral spray oil (20 g + 200 g + 500 ml/100 l water) applied in November, January and March was the most effective treatment. Three applications of benomyl + mancozeb + mineral spray oil (25 g + 200 g + 500 ml/100 l water) applied during the same period, was the second most effective treatment, and two applications (November and January) of trifloxystrobin + mineral spray oil (20g + 500 ml/100 l water) and difenoconazole (40 g) per 100 l/water applied twice in November and January, the third most effective treatment. The spore trap and weather data showed that P. angolensis needs high moisture and temperatures in excess of 25°C for disease development. It is concluded that P. angolensis in Zimbabwe can be managed successfully by implementing a holistic approach, which should be supported by the authorities, organised agriculture and all technical personnel involved in citrus production.
AFRIKAANSE OPSOMMING: Blaar- en vrugvleksiekte (BVVS) op sitrus, veroorsaak deur Phaeoramularia angolensis, kom in 18 lande in Afrika voor asook die Comores Eilande in die Indiese Oseaan en Yemen op die Arabiese skiereiland. Marokko, Suid Afrika, Swaziland en Zimbabwe is die belangrikste uitvoerders van sitrus in Afrika. Van dié lande het slegs Zimbabwe blaar en vrugvleksiekte op sitrus. Hierdie siekte is van fitosanitêre en ekonomiese waarde en die nadelige effek van die siekte, wat slegs kosmetiese van aard is, is venietigend aangesien vrugte onbemarkbaar is. Totale opbrengsverliese is nie ongewoon in lande soos Kenya nie. Die doelwitte van die studie was dus om die voorkoms van P. angolensis in Zimbabwe te bepaal, om die Cercospora Fresen. isolate vanaf Swaziland en Suid-Afrika met mekaar te vergelyk, om die epidemiologie van die siekte vas te stel en om ‘n effektiewe beheermaatreël teen die siekte te ondersoek. Blaarmonsters met kankeragtige letsels wat in die vroeë 1990’s in Zimbabwe gevind is, het getoon dat die blare geinfekteer is met die swam, Phaeoramularia angolensis. Ondersoeke is geloots om die verspreiding en intensiteit van BVVS in Zimbabwe en Mosambiek te bepaal. In Zimbabwe was gevind dat P. angolensis beperk was tot gebiede bo die 19° Suid breedtegraad, wat die hoër vogtiger gebiede insluit eerder as die droeër, laagliggende gebiede. Geen P. angolensis simptome kon in Mosambiek gevind word nie. Tydens die opnames was dit duidelik dat geen geskikte beheerstrategieë toegepas word deur Zimbabwe se produsente nie. ‘n Nuwe cercosporoid swam, wat blaar en vrugvleksiekte op sitrus is in Suid Afrika veroorsaak is geidentifiseer. Morfologiese en rDNA volgorde (ITS 1, 5.8S en ITS 2) data het getoon dat die siekte veroorsaak word deur Cercospora penzigii wat tot die Cercospora apii spesie kompleks behoort. Die genus Pseudophaeoramularia kan as sinoniem van Pseudocercospora beskou word en ‘n nuwe kombinasie word voorgestel in Pseudocercospora as P. angolensis. Cercospora gigantea het getoon dat dit nie ‘n spesie van Cercospora kon verteenwoordig nie terwyl Mycosphaerella citri varieërend voorkom en meer as een takson kan verteenwoordig. ‘n Beheerstrategie vir die beheer van BVVS is ondersoek. Die data wys dat P. angolensis in Zimbabwe doeltreffend beheer kan word deur die uitroeiing van ou en verwaarloosde bome, en deur goed beplande fungisied bespuiting. Trifloxystrobin + mancozeb + minerale spuitolie (20 g + 200 g + 500 ml/100 l water), wat in November, Januarie en Maart toegedien is, was die mees effektiefste behandeling. Drie bespuitings van benomyl + mancozeb + minerale spuitolie (25 g + 200 g + 500 ml/100 l water) wat oor dieselfde tydperk toegedien is, was die naas beste behandeling. Trifloxystrobin (20 g) + minerale spuitolie (500 ml) per 100 l/water en difenoconazole (40 g) per 100 l/water, beide as twee bespuitings toegedien in November en Januarie, het die derde beste resultaat opgelewer. Die spoorlokval en klimatologiese data het getoon dat P. angolensis vogtige toestande en temperature hoër as 25°C benodig vir siekteontwikkeling. Die afleiding uit die studie is dat P. angolensis suksesvol beheer kan word indien ‘n holistiese benadering gevolg word en alle rolspelers naamlik die owerheid, georganiseerde landbou en tegniese personeel die proses ondersteun.

Soybeans (Glycine max L. Merr.) are a symbiont of two beneficial associations: biological nitrogen fixation (BNF) with Bradyrhizobium japonicum, and arbuscular mycorrhizal fungi (AMF). Within the Northern Great Plains of the USA, iron deficiency chlorosis (IDC) of soybean is a yield-limiting factor. The effects of IDC on BNF and AMF are not well defined. This study was conducted to determine the effects of IDC on BNF and AMF. A laboratory study was performed to compare three methods of measuring ureide-N, a product of BNF in soybeans. Field studies in soybean were performed at three locations at eastern N011h Dakota. The experimental design was a factorial combination of three cultivars and three treatments. The three cultivars, in order of decreasing chlorosis susceptibility, were NuTech NT-0886, Roughrider Genetics RG 607, and Syngenta S01-C9 RR. The three treatments were control, Sorghum bicolor L. companion crop planted with the soybean seed, and FeEDDHA applied with the soybean seed. Chlorosis severity was the greatest and least for the NuTech and Syngenta cultivars, respectively. The FeEDDHA treatment decreased chlorosis severity. Ureide levels were abnormally high in plants severely stunted by JDC. The excess accumulation of ureides in IDC-stunted plants suggests that plant growth was reduced more than the rate of nitrogen fixation. The AMF population \vas at an adequate level at all locations and not affected by cultivar or treatment, in general. In the laboratory study, the Patterson et al. method had greater ureide concentrations due to the non-specific measuring of ammonium compounds compared to the Vogels and Van der Drift and Goos methods.
North Dakota Soybean Council

Made available in DSpace on 2015-03-26T13:58:39Z (GMT). No. of bitstreams: 1 texto completo.pdf: 486171 bytes, checksum: 6151d75a5f5acc0b6817f61e10bf74cb (MD5) Previous issue date: 2014-02-25
The expansion of soybean cultivation to the Cerrados region in Brazil started in the 1970s, and the state of Mato Grosso has become, since 2000, the largest soybean country producer. Soybean rust was first found on state in 2001/2002 crop season. Since then it has been causing major losses in soybean production. Many methods are indicated to manage the disease. An important disease regulatory method is the fallowing ( vazio sanitário) , and it is unknown whether the method is being effective in reducing rust intensity throughout the years. Therefore, to assess the effect of fallowing in the disease intensity, we analysed data collected from 2008 to 2012 in producing municipalities of Mato Grosso, regarding both the enforcement of soybean following and the data of occurrences of rust. The implementation of fallowing has been effective in reducing rust intensity in all producing regions, as well as to move the first disease ccurrence from November-December to January-February. Rust tis being firstly reported when the plants are in the beginning seed stage (R5), and the earlier plantings are the main source of inoculum for late plantings. Official monitoring is helping maintaining more than 88% growers in the fallowing program to control the soybean rust.
A partir dos anos 1970 ocorreu a expansão da cultura da soja para o cerrado, e o estado de Mato Grosso tornou-se, desde o ano 2000, o maior produtor nacional da oleaginosa. Desde a safra 2001/2002, quando foi detectada pela primeira vez, a ferrugem asiática tem causado grandes perdas. Entre as medidas de controle da doença, inclui-se a adoção do vazio sanitário, mas desconhece-se o efeito da medida em reduzir a intensidade da doença. Assim, analisaram-se dados coletados de 2008 a 2012 nos municípios produtores do Mato Grosso, coletados na fiscalização do vazio sanitário da soja, e os de ocorrências da ferrugem, coletados pelo consórcio antiferrugem, para avaliar o efeito da implementação do vazio sanitário na incidência da ferrugem asiática. A implantação do vazio sanitário da soja reduziu e tem mantido baixas as ocorrências da ferrugem, em todas as regiões produtoras, além de deslocar o período de ocorrências da doença, de novembro e dezembro para janeiro e fevereiro. As primeiras ocorrências acontecem com a planta no estádio de granação (R5), sendo os plantios precoces a principal fonte de inóculo para os plantios tardios. A fiscalização tem auxiliado a manter a adesão dos produtores ao programa de controle da ferrugem asiática da soja em índices superiores a 88%.

Made available in DSpace on 2015-03-26T13:37:55Z (GMT). No. of bitstreams: 1 texto completo.pdf: 1312640 bytes, checksum: f59984d596fc23251bdcc4b712cc574a (MD5) Previous issue date: 2014-07-29
Conselho Nacional de Desenvolvimento Científico e Tecnológico
Rust fungi are obligate parasites that secrete diverse proteins during their interaction with host plants. These proteins promote parasitism by manipulating the host metabolism and interfering with its defense responses. Effector proteins can be recognized by proteins coded by plant resistance genes, in which case they are called avirulence proteins. Six loci containing genes that confer varying degrees of resistance to P. pachyrhizi (Rpp) have been reported, and several genes encoding proteins secreted by the fungus have been identified. However, effector proteins (Avr proteins) recognized by soybean Rpp proteins have not yet been identified. Thus, the objective of this study was to identify proteins secreted by P. pachyrhizi (PPUFV02) able to activate or suppress defense responses in soybean, through the functional analysis of ten candidate effector genes using the TTSS of P. savastanoi pv. glycinea 1462 (EDV system). Among the ten effectors tested in 11 soybean genotypes resistant to the mono-pustule isolate PPUFV02 of P. pachyrhizi, source of candidate genes, PHPA_RSP_71 stood out for contributing to greater disease severity in genotype PI 594538- A and lower in genotype PI 594754. The PHPA_RSP_23 effector was also interesting for additional functional studies on soybean and tobacco, since the results indicated a possible activation of defense response in soybean, in general (all genotypes), and a possible suppression of cell death in tobacco. The effector activity of these candidate genes was not confirmed by analysis of bacterial growth in soybean and tobacco (non-host plant). On the other hand, it is possible that these effects are due to an interference of the effector with the function of the TTSS, making it necessary to conduct protein secretion assays to confirm or reject these hypotheses. Overall, the genotype Williams 82 showed resistance to Psg 1462 and can be used as negative control for disease in functional analysis in soybean. It was not possible to confirm N. benthamiana as an appropriate plant for functional studies of induction/suppression of HR when using Psg 1462, since it is possible that the bacterial isolate is pathogenic to this species. The pEDV6 vector in general was stable in the Psg1462(pEDV6::PHPA_RSP) clones and did not alter the ability of the bacteria to multiply in planta, confirming the EDV as a good system for functional analysis of P. pachyrhizi candidate effector genes in soybean using TTSS of Psg 1462. The inoculation method by dipping was not suitable for functional analysis in soybean because, in general, showed highly variable results. The inoculation method by vacuum was the best suited for this purpose, since it introduced greater uniformity in the severity of symptoms observed.
Os fungos causadores de ferrugens são parasitas obrigatórios que secretam diversas proteínas durante sua interação com plantas hospedeiras. Essas proteínas promovem o parasitismo por meio da manipulação do metabolismo do hospedeiro e interferência em suas respostas de defesa. As proteínas efetoras podem ser reconhecidas pelas proteínas codificadas pelos genes de resistência da planta, sendo, neste caso, denominadas proteínas de avirulência. Seis locos contendo genes que proporcionam diferentes graus de resistência à P. pachyrhizi (Rpp) já foram relatados e vários genes que codificam proteínas secretadas por esse fungo já foram identificados. No entanto, ainda não foram identificadas as proteínas efetoras (proteínas Avr) reconhecidas pelas proteínas Rpp de soja. Desse modo o objetivo deste trabalho foi identificar proteínas secretadas por P. pachyrhizi (PPUFV02), capazes de ativar ou suprimir respostas de defesa em soja, através de uma análise funcional de 10 genes candidatos a efetores, utilizando o SST3 de P. savastanoi pv. glycinea 1462 (sistema EDV). Dos dez efetores testados nos 11 genótipos de soja resistentes ao isolado monopostular de P. pachyrhizi PPUFV02, fonte dos genes candidatos, PHPA_RSP_71 se destacou por contribuir para uma maior severidade da doença no genótipo PI 594538-A e menor no genótipo PI 594754. O efetor PHPA_RSP_23 também se mostrou interessante para novos estudos funcionais em soja e tabaco, uma vez que os resultados indicaram possível ativação de resposta de defesa em soja de forma geral (em todos os genótipos) e possível supressão de morte celular em tabaco. A atividade efetora desses genes candidatos não foi confirmada pela análise de crescimento bacteriano em soja e em tabaco (planta não hospedeira). Por outro lado, há uma possibilidade de esses efeitos serem devido a uma interferência do efetor com a funcionalidade do SST3, necessitando-se de estudos de secreção da proteína para confirmar ou refutar estas hipóteses. De forma geral o genótipo Williams 82 se mostrou resistente à Psg 1462, podendo ser utilizado como controle negativo para doença nas análises funcionais. Não foi possível confirmar N. benthamiana como uma planta adequada para estudos funcionais de indução/supressão de HR ao se utilizar Psg 1462, uma vez que é possível que este isolado bacteriano seja patogênico a esta espécie. O vetor pEDV6 de forma geral apresentou estabilidade nos clones Psg1462(pEDV6::PHPA_RSP) e não alterou a capacidade de multiplicação da bactéria in planta, confirmando o EDV como um bom sistema para análise funcional de genes de P. pachyrhizi candidatos a efetores em soja, utilizando o SST3 de Psg 1462. O método de inoculação por mergulho não se mostrou adequado para as análises funcionais em soja, uma vez que apresentou de forma geral alta variabilidade nos resultados, sendo o método de inoculação a vácuo mais indicado para este fim por ter apresentado maior uniformidade na severidade dos sintomas observados.

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 rust (SBR), caused by the fungus Phakopsora pachyrhizi Syd., is a real threat to soybean crops in South Africa. Its ability to spread rapidly and its potential to severely reduce yields have earned it the reputation as the most destructive foliar disease of soybeans. SBR has been reported in South Africa every year since its arrival in 2001. While extensive research had been done on the epidemiology and fungicide application requirements in South Africa, no work into the long term climatic vulnerability of soybean production areas to SBR had been done. This meant soybean producers do not know whether SBR is a threat in their areas. Through this research a SBR algorithm was developed using readily available climate data, viz. temperature and rainfall, to create a daily index specifying the climatic vulnerability of SBR infection. The algorithm was applied to a 50 year historical climate database, and a series of maps was created illustrating the long term vulnerability of different areas to SBR infection. These maps allow soybean producers to understand the climatic vulnerability of their area to SBR infection. Time series graphs were created for selected key soybean production areas to allow soybean producers to distinguish periods of high and low climatic risk during the season. This may help with decisions regarding the planting times, the maturation rate of different cultivars as well as the timing and application of fungicides. The framework for a near real time forecasting system was created outlining how the system could amalgamate recently recorded and forecasted weather data, run it through the SBR algorithm and provide a near real time, as well as forecasted vulnerability, based on the climatic conductivity for SBR infection. Anticipated limitations and difficulties on developing the forecasting system are also outlined.
Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2009.

Soybean rust (SBR) caused by Phakopsora pachyrhizi Syd. is an aggressive wind dispersed fungal disease which has spread around the world at an alarming rate in the last decade. The disease was first reported in South Africa (SA) in 2001. It has become well established in the province of KwaZulu-Natal. Reports are occasionally made from eastern Mpumalanga, late in the growing season, in years with good rainfall. Yield losses of 10 - 80% have been reported due to SBR infection. Literature was reviewed to better understand the pathogen in an attempt to find suitable disease management strategies. Many strategies involve delaying, rather than preventing, SBR infection. Of the two strategies to prevent infection, the use of fungicides was the only option for disease control in SA, as no resistant cultivars are available. Field trials were conducted to determine which fungicides are effective in controlling SBR. Further research was conducted to determine the timing, frequency and rate of fungicide applications for optimal control of SBR. Trials were evaluated for disease severity, seed yield and the effect of fungicides on seed quality. Fungicides from the triazole class of the sterol biosynthesis inhibiting group of fungicides were found to be the most effective in controlling SBR. A fungicide from the strobilurin group was found to be less effective than the triazoles at the suggested rate, but was found to be as effective when evaluated at higher dosage rates. Triazoles premixed with fungicides from the benzimidazole and strobilurin groups were also effective in controlling SBR. Timing of application was found to be critical for strobilurin fungicides, but not for triazole fungicides, which have a curative ability, unlike strobilurins. Strobilurin fungicides applied preventatively, before the appearance of disease symptoms were as effective as triazole fungicides applied after disease symptoms, but before infection levels had reached 10%. Across both wet and dry seasons two fungicide applications applied at 21d intervals at the R2 growth stage resulted in effective disease control. In wet seasons, a third fungicide application resulted in yields that were higher, albeit not statistically significant, than two fungicide applications. Assessments of individual fungicides for optimal dosage rate found that registered rates were already optimal for some fungicides, but for others it appeared as if alterations were necessary to the rate suggested for registration. This study was one of the first to extensively evaluate the efficacy of the new triazole and strobilurin fungicides on SBR control. The results have been shared globally, but particularly with newly affected countries in South and North America. Although this research has been groundbreaking, there are still many aspects of fungicide control which need to be studied in order to further optimise chemical control of SBR.
Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2005

Phakopsora pachyrhizi H. Syd and P. Syd, the causal organism of soybean rust (SBR) was first reported in Japan in 1902. In 1934 the pathogen was found in several other Asian countries and as far south as Australia. In India, SBR was first reported on soybeans in 1951. There have been several early reports of SBR in equatorial Africa but the first confirmed report of P. pachyrhizi on the African continent was in 1996 from Kenya, Rwanda and Uganda. Since then, the pathogen has spread south with reports from Zambia and Zimbabwe in 1998 and in Mozambique in 2000. In February 2001, P. pachyrhizi was first detected on soybeans near Vryheid, in Northern KwaZulu-Natal, South Africa (SA). As the season progressed, the disease was observed in other parts of the province, and epidemic levels were found in the Cedara, Greytown, Howick and Karkloof production regions. Soybean rust subsequently spread to Amsterdam and Ermelo in the Highveld region of SA. The disease reappeared in SA in March 2002. It is now established that the pathogen is a threat to soybean production in the country with yield losses in the region of 10-80%. A literature review on SBR investigating the taxonomy of the pathogen, its morphology, symptoms, host range, infection process, epidemiology, control options and the economic importance of P. pachyrhizi was complied to provide the necessary background information to conduct research under local conditions and to assist in interpretation of results of experiments. Epidemiological trials were conducted at the University of KwaZulu-Natal under controlled environmental conditions in a dew chamber and conviron. Development of P. pachyrhizi on the susceptible cultivar (LS5995) was quantified in combinations of seven temperatures (15,19,21,24,26,28 and 30°C) and five leaf wetness durations (LWD) (6,9,12,14 and 16hrs) at three relative humidities (RH) (75%, 85% and 95%). Studies indicate that optimum temperature for uredospore infection is 21-24°C with a LWD greater than 12hrs and RH 85-95%. The number of pustules as well as lesion size on the abaxial and adaxial leaf surface increased with increasing LWD at all the RH values tested. Infection did not occur on plants incubated at 15°C and 30°C at 85% or 95%RH whereas at 75%RH infection did not occur on plants incubated at 15°C, 19°C and 30°C regardless of LWD. Number of pustules per lesion produced at 75%, 85% and 95%RH was highest at 24°C and showed a gradual increase with increasing LWD. Lesion size on both leaf surfaces increased after 12hrs LWD at 24°C at 75% and 85%RH whereas at 95%RH lesion size increased after 14hrs LWD at 24°C. Exposure of uredospores to ultraviolet light which is equivalent to ultraviolet C (sunlight) which is < 280nm, shows a decrease in germination (7%). Under continuous darkness, the germination percentage was found to range from 58% after 48 hrs. Germination was found to peak at 16hrs in darkness with a gradual decrease as time increased whereas germination under ultraviolet light was highest after 6hrs with a gradual decrease with increased exposure to light. Germ tube lengths were found to be shorter when exposed to ultraviolet light (107µm) compared to controls kept in the dark (181µm). Results obtained clearly show a negative effect of ultraviolet light on the germination and germ tube length of uredospores. A 0.1 ml suspension of uredospores on 1.25% water agar Petri dishes was exposed to cycles of 14h ultraviolet light and 10h darkness for 48h. Results indicate an increase in germination percentage of uredospores when exposed to 10h of darkness following a 14h period under ultraviolet light. Controlled environmental studies were conducted to determine alternative hosts of P. pachyrhizi in SA. The control used in this experiment was Prima 2000, a susceptible cultivar to soybean rust. Seven legume plants [Cajanus cajan (L.) Huth, Glycine max (L.) Merr, Lablab purpureus (L.) Sweet, Lupinus angustifolius (L.) Finnish, Phaseolus vulgaris (L.), Pueraria lobata (M&S) Wild and Vigna unguiculata (L.) Walp] and three dry bean lines (Bonus; OPS-RS2 and PAN 159) showed typical SBR symptoms when rated after 21 days post inoculation with uredospores for percentage disease severity. Disease severity was significantly different within the alternative hosts, but G. max, P. vulgaris and P. lobata were not significantly different from Prima 2000 (control). A uredospore suspension of 2.5 x 10(5) uredospores ml(-1) from plants that showed typical SBR symptoms was made and inoculated on to Prima 2000, a susceptible soybean cultivar. Uredospores from pustules on G. max, L. purpureus, L. angustifolius, P. vulgaris, P. lobata, V. unguiculata, Bonus and PAN 159 produced viable uredospores on PRIMA 2000. These plants are considered alternative hosts of P. pachyrhizi. Effect of leaf age on susceptibility of soybean to SBR was tested under controlled environmental conditions. Mean number of lesions as well as lesion size were greater on younger leaves than on older leaves of plants at the same physiological age. Plants at the early vegetative and reproductive stages had a significantly lower number of lesions as well as a smaller lesion size. Plants at the V6 and R1 growth stages were significantly more susceptible to P. pachyrhizi than plants at other developmental stages. Trichoderma harzianum Rifai, Eco-77® a commercial biological control product, was evaluated for its efficacy as a biological control agent of P. pachyrhizi. Trichoderma harzianum sprayed at the standard concentration on infected soybean plants was significantly more effective in controlling P. pachyrhizi than plants sprayed at 1/2X and 2x the standard concentration. This was noted in both Trial 1 and 2. Data indicate that spraying the filtrate two days after inoculation produces less disease.
Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2006.

Crop rotation is not commonly practised in the sugarcane industry in South Africa. It has, however, proven to be beneficial to other crops in South Africa. The objective of this study was to determine the impact of soybean-sugarcane crop rotation on selected physiological and phenological indicators of sugarcane performance and its subsequent effect on cane and estimated recoverable crystal (ERC) yields. A field trial was conducted at Mount Edgecombe, where soybean cultivar A5409RG and sugarcane cultivar NCo376 were planted under drip irrigation with different management practices. After the soybean crop, the following sugarcane crop was planted and fertilized with different levels of nitrogen (N) fertilizer (50% and 100% of the recommended N rate). The effects on sugarcane growth were recorded by taking into consideration date of emergence, plant height, tiller population, leaf N, plant performance index and chlorophyll content. Sugarcane yield and quality at harvest were also evaluated. Tiller population in all crop rotation treatments at Mount Edgecombe weresignificantly (P<0.05) higher than the monocrop treatment. There was a trend of increased leaf N in all of the cane-after-soya (crop rotation) crops compared to the cane-after-cane (monocrop) treatment, although this was not significant. A similar pattern was obtained with respect to the chlorophyll content and plant performance index. Sugarcane yields at Mount Edgecombe did not differ significantly between monocrop and crop rotation treatments. Crop rotation with soybean is beneficial for cane production, but its long term impact on soil quality and farm economy requires further investigation.
M.Sc.Agric. University of KwaZulu-Natal, Pietermaritzburg 2013.

Soybean (Glycine max (L.) Merr.) occupies an important position in the world economy of the feedstock of high quality protein and vegetable oils. However, its production is threatened by, Asian soybean rust (ASR), caused by the rust fungus Phakopsora pachyrhizi Syd. & P. Syd. This fungus is highly dependent on environmental conditions, has a wide range of hosts, and evolves rapidly into novel races, making it difficult to control. In addition, most commercial varieties are susceptible to rust, the rust has already developed resistance to triazole fungicides, and most small-scale farmers cannot afford expensive systemic fungicides to control the disease. The use of resistant varieties is the most viable, long-term option to manage ASR, especially in the small-holder soybean farming sector. This study was therefore designed to undertake the following goals: (i) to identify farmers’ preferred varieties and desired traits, their knowledge of ASR, and other key constraints affecting soybean production in Kenya; (ii) to evaluate soybean accessions for rust resistance, and to determine the correlation of rust resistance with other agronomic traits; (iii) to determine the mode of inheritance for ASR resistance and selected agronomic traits; and (iv) to determine yield stability of soybean advanced lines at multiple sites in Central and Eastern Kenya. To understand farmers’ preferred varietal characteristics, knowledge of ASR and other key constraints to soybean production, a survey was conducted using a structured questionnaire in the major soybean growing areas of Kenya. The farmers preferred local varieties because of their desirable characteristics, which included high yields, early maturity, drought tolerance and seed availability. Although the majority of the participating farmers expressed a willingness to grow improved varieties, financial limitations, seed unavailability and lack of information were the major barriers to their use of improved varieties. High yield, early maturity, adaptability and grain quality were the traits that most farmers sought in an ideal soybean variety. Knowledge of the cause of ASR was limited, and its occurrence was largely attributed to environmental factors, poor soil fertility conditions, poor agronomic practices, physiological maturity and specific species of weeds. Their investments in control methods were minimal due to a lack of technical knowledge, poor access to fungicides, and limited resources. Other constraints faced by soybean farmers included: lack of access to grain markets; lack of knowledge in processing and utilization of soybean grain; the unavailability of seeds; losses to pests and diseases; the lack of inputs such as fertilizers; frequent dry spells; and low yielding varieties. A total of 110 soybean accessions were evaluated for their rust reactions and correlations with selected agronomic traits. These included plant introductions possessing single rust resistant genes (Rpp1-4), tolerant lines, gene bank accessions, commercial varieties and advanced lines. Soybean genotypes varied significantly in their reactions to rust severity, sporulation, lesion type and area under disease progress curve (AUDPC) values. Genotypes possessing Rpp4 (G10428) and Rpp2 (G8586) resistant genes, and non-characterized genotypes MAK BLD 11.3, GC 00138-29 and Namsoy 4M, were the most resistant accessions, as indicated by low rust severity scores, low AUDPC values, red brown lesions and low sporulation scores. Other genotypes with known resistant genes including G7955 (Rpp3), G58 and Tainung 4 (Rpp1), a few tolerant lines, and one advanced line (BRS Sambaiba) were moderately resistant. All the other advanced lines, commercial varieties, gene bank accessions and collections from the farmers’ fields were highly susceptible to rust. Rust severity was positively correlated with rust sporulation, indicating that reduction of sporulation made a significant contribution towards rust resistance. An F2 population was generated from a half diallel mating design, involving 4 resistant, 2 moderately resistant and 2 susceptible genotypes selected as parents. The F2 populations along with their parents were evaluated in two environments to determine the type of gene action for rust resistance and other quantitative traits in soybeans. The results revealed that both general combining ability (GCA) and specific combining ability (SCA) were significant for most of the traits studied, indicating that both additive gene action and non-additive gene action played a major role in the inheritance of rust resistance and selected agronomic traits. The GCA/SCA ratio was close to unity for rust severity, rust sporulation, days to flowering, days to maturity and plant height. This indicated that additive gene action played a more significant role in the inheritance of these traits than non-additive gene action. Non-additive gene action was only predominant for soybean grain yield. Parental lines G10428, G8586 and Namsoy 4M were the best general combiners for improving rust resistance across the environments. The most promising parents for early flowering were G7955, G8586 and G58. Parent Maksoy 1N was the best general combiner for early maturity while parents Maksoy 1N, G58, G7955 and Nyala contributed effectively towards reduced plant height. Yield stability analysis was conducted for 30 genotypes in 6 environments, using additive main effects and multiplicative interaction (AMMI), genotype main effect and genotype x environment interaction (GGE) biplot analyses. Genotypes 916/5/19 and G7955 were identified as the high yielding and most stable across the environments. On the other hand, genotypes BRS MG46 and Sable were high yielding but unstable and specifically suitable for the environments EM2 and MW2, respectively (both environments have long rainy seasons). Environment EM2 was identified as the most discriminating and representative among the six environments. Environments IG1 and MW1 (short rainy seasons) were less informative on genotypes tested, as confirmed by short environment vectors. Environment EM1 was better for discriminating genotypes but was a poor representative of the test environments, hence it should only be utilized for developing specifically adapted genotypes. Further analysis using GGE biplot approach grouped the environments into three putative mega-environments in Central and Eastern Kenya. Overall, this study established the need to educate farmers on the cause of ASR, to develop ASR resistant varieties, and to incorporate farmers’ desired traits in the breeding programme, especially by the use of participatory breeding approaches. The resistant and moderately resistant genotypes identified in this study could be used as sources of resistant genes to develop ASR resistant varieties in Kenya. This study also established that genetic improvement for ASR resistance and selected agronomic traits in soybeans is possible based on the use of recurrent selection breeding procedures that result in the accumulation of additive gene effects. Selection of late segregating generations would be effective for soybean grain yield improvement. This study identified potential parents for ASR resistance and selected agronomic traits, but they require further breeding to improve on farmers’ desired traits.
Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2012.