No description available.

No description available.

No description available.

The objective of this investigation was to assign the genes for resistance to leaf rust Puccinia hordei Otth. race 4 in two varieties of barley to the correct chromosome pair by means of crossing with nine translocation stocks. The varieties Sudan (C.I.6489) and Reka 1 (C.I.5051) were crossed with the following translocation stocks: 5051-2 (a-b), 5059-5 (a-e), 5030-2 (b-d), 5034-2 (b-f), 5062-3 (b-g), 5038-2 (c-d), 5056-2 (c-e), 5053-6 (e-f), and 5057-1 (f-g). The F₁ plants from crosses of Sudan with all translocations and Reka with all translocations were tested for rust reaction and found to be resistant. Examination of the mature pollen showed that the F₁’s were partially sterile. In the crosses involving the variety Sudan with translocation stocks having interchanges of chromosomes a with e and c with e, the F₂ data indicated complete linkage. The common chromosome e must carry the gene for resistance. This is substantiated by crosses involving translocations of a with other chromosomes and c with others, which showed independent assortment. In the crosses involving the variety Reka with translocation stocks having interchanges of chromosomes b with f and e with f, the F₂ data indicate that in Reka, the gene for resistance is linked with chromosome f. This is substantiated by crosses involving translocations of b with other chromosomes and e with others, which showed independent assortment. The results obtained from the F₂ data of the crosses studied indicated that in the varieties Sudan and Reka reaction to leaf rust is conditioned by genes that are independent of each other. / Ph. D.

Field studies of a population of Arisaema triphyllum affected by the systemic rust Uromyces ari-triphylli were conducted to examine dynamics of disease transmission and to identify factors influencing plant susceptibility to rust infection. Of undiseased plants that developed foliar infections during the growing season, 55% possessed active resistance to the rust disease, and were able to prevent the foliar infection from becoming a systemic one. Flowering increased the susceptibility of plants to both Mar and systemic infections. A phenotypic selection study was carried out to estimate selection on five plant traits and to determine the role of foliar infections of U. ari-triphylli in mediating selection. Multivariate path analysis suggests that both leaf diameter and a general size factor are under positive directional selection. Pathogen-mediated selection coefficients are small, but comparable in magnitude to overall selection coefficients (15%, on average), indicating the potential importance of diseases as selective agents in nature.

Albugo tragopogonis is responsible for white rust of sunflower. It was first observed in 1929 in South Africa. Recently however, white rust has resulted in lodging exceeding 80% in some sunflower growing areas. Due to the obligate nature of the pathogen, studies of the biology, epidemiology and control of the disease has until now been limited to field trials and observations. Greenhouse trials are needed to understand the infection process, and to examine any resistance mechanisms used by the plant to defend itself against the pathogen. Presently, there is no practical artificial inoculation technique available and effective storage of the fungus is difficult. The purpose of these studies was to find new storage and inoculation techniques. Once the inoculation technique was optimized, the infection process of A. tragopogonis on susceptible and tolerant sunflower genotypes was examined. Infected leaves were collected from sunflower seedlings at the Grain-Crops Institute in Potchefstroom. Infected leaves were covered with plastic bags and freshly cut stems were placed in a cooler box filled with ice water. Some of the infected leaves were also placed in paper bags and allowed to dry for 24 h. Sporangia were collected using a vacuum device and stored in gelatin capsules at -20°C, -70°C or in liquid nitrogen directly after collection or following desiccation for 24 h. Sunflower seedlings at the four-leaf-stage were inoculated with freshly collected sporangia, or sporangia stored for 3, 5, 9, 12 and 15 mo. A zoospore suspension was prepared by allowing 105 sporangia/ml to germinate in distilled water for 3 h at 10°C. The zoospore suspension was then sprayed onto leaves until they were completely wet with a hand held garden spray bottle. Inoculated seedlings were covered with plastic bags to maintain high humidity and placed at 12°C for 16 h and incubated in a greenhouse until symptom development. Infection levels were assessed 10¬14 d after inoculation, using a scale of 1-5, with 1 indicating resistance and 5 indicating severe infection. Infection with fresh sporangia proved to be very consistent. Sporangia stored in capsules immediately after collection at -70°C after desiccation, produced the highest infection. Low levels of infection resulted from storage in liquid nitrogen or directly at -70°C. It is evident that successful storage may be obtained if the sporangia are dried before storage. These techniques to store and inoculate A. tragopogonis have proven to be reliable. Susceptible and tolerant genotypes were inoculated, using the spray bottle inoculation technique described above, to examine the difference in infection of A. tragopogonis. Leaves used for light microscopy were cut into 20 mm2 and those for scanning electron microscopy were cut into 5x5 mm pieces at 2, 4, 6, 8, 10, 12, 24, 36, 48, 72, 96, 120, 144 and 168 h time intervals after inoculation. The epidermis, palisade parenchyma and spongy parenchyma were chronologically stripped using the double-sided tape method. The material for the light microscope was prepared using the whole-leaf clearing and staining technique, the lactophenol-ethanol-analine blue technique and sectioning with freeze microtome. The material for SEM was prepared according to standard procedures and examined with a JEOL 840 SEM at 5 kV. Both the whole-leaf clearing and staining and the lactophenol-ethanol-aniline blue techniques proved to be unsuitable as most of the tissue was damaged by boiling. Sectioning with the freeze microtome was also unsuccessful. The SEM gave the most transparent results. This method gave us the ability to compare results with previous literature and to compare the infection process between of A. tragopogonis in the susceptible (RHA 358) and the tolerant (HYS 33) genotype. / Dissertation (MSc ( Plant Pathology))--University of Pretoria, 2005. / Microbiology and Plant Pathology / unrestricted

No description available.

Thesis (M. Sc. Agriculture (Plant Protection)) -- University of Limpopo, 2021 / Mimusops zeyheri tree groves made up of seventeen trees collected from communities in Southern Africa were used in this study. The trees had high morphological variations in terms of growth rate, fruit (size and taste) and leaf (shapes and sizes) and their identification was made by communal people from where the trees were collected using their morphological characteristics. Generally, this evergreen tree is drought‒tolerant, salt‒tolerant and highly resistant to root-knot (Meloidogyne spp.) nematodes, along with various other pests. This could probably be attributed to high concentration of latex in aboveground organs. Some typical fungal rust symptoms have been observed believed to be the cause of high leaf abscission in certain groves. Currently, there is no report of leaf rust disease on M. zeyheri plants and the mechanism of resistance to other pests is not documented. The objective of this study was to (i) identify the pathogen associated with M. zeyheri leaf rust symptoms using morphological technique and to (ii) determine levels and types of potential defence chemicals and endophytes in M. zeyheri. Samples of M. zeyheri leaves showing rust like symptoms were collected from University of Limpopo, South Africa (23°53”10’S, 29°44”15’E) during summer in September 2018. Light compound microscope and electron microscope were used in the identification of the leaf rust spores. The species identity of the seventeen M. zeyheri trees that form a grove collection at University of Limpopo was confirmed using internal transcribed spacer (ITS) of ribosomal nuclear DNA. DNA extraction and sequencing was done with the help of Inqaba Biotechnologies. Obtained DNA sequences were aligned using CLUSTALX (2.0), with the phylogenetic tree constructed through the neighbour-joining method (NJM) in MEGA v. 5.1 programme. Evolutionary distances were computed using the Juke– Cantor method. Phytochemicals in leaves were identified and quantified using Liquid chromatography–mass spectrometry (LC–MS) at ARC-VOP. The pustules on the collected leaves contained reddish brown spores. The uredospores were oval and ellipsoidal under a light microscope. The size of spores ranged between 35-37 × 24-26 µm. The cell walls showed bilaminate structures with the outer layer hyaline. The warts were rod shaped with one subequatorial germ pore. The most common identified endophytic fungi observed in all M. zeyheri leaves were Teratosphaeria species, Zeloasperium species, Pezizomycotina. In addition, endophytes such as Cladosporium species, Aspergillus species, Phyllosticta species and Epicoccum species were also identified to be associated with some M. zeyheri trees. There were significant differences on the level of tannins, flavonoids, proteins and phenolics among the M. zeyheri trees. The highest level of tannins was 7.2151 mg/g and the lowest being 2.7232 mg/g. The highest level of flavonoids was 1.1537 mg/g the lowest being 0.0123 mg/g. The level of phenolics among the trees ranged from 2.4749 mg/g to 1.5788 mg/g. Protein content ranged from 5.3100% to 2.7967% among the trees. Very high levels of tannins, flavonoids, phenolics among the trees when compared with others studies indicate the potential role of these metabolites in previously reported resistance of M. zeyheri to a number of pests. The morphological characteristics of the identified leaf rust pathogen causing rust symptoms on M. zeyheri in South Africa is more similar to Maravalia species previously isolated from M. caffra. And as such, this finding is paramount, as it is the first report of association between the pathogen and the plant. Teratosphaeria species has been associated with stem cancer in Eucalyptus trees, its presence in M. zeyheri species in asymptomatic plants is a major find since the tree is common in Limpopo and Mpumalanga, Provinces well known for forestry production making M. zeyheri a potential host for pathogens of commercial forestry production. Other endophytes observed such as Aspergillus species, Phyllosticta species and Cladosporium species have been shown to protect plants against some pests and pathogenic organism. Further studies to determine the direct role of identified phytochemicals and endophytes in the resistance of M. zeyheri to pests needs to be conducted

Thesis (MSc (Genetics))--University of Stellenbosch, 2009. / Gene transfer from wild gras species to wheat is complicated by the simultaneous integration of large amounts of alien chromatin. The alien chromatin containing the target gene is inherited as a linkage block and the phenomenon is known as linkage drag. The degree of linkage drag depends on whether, and how readily, recombination occurs between the foreign and wheat chromatin. The S13 translocation line was developed by the department of Genetics, US. A cross was made between Chinese Spring and a leaf rust resistant Aegilops speltoides accession. Resistant backcross F1 was backcrossed to Chinese Spring and W84-17. S13 was selected from the backcross progeny and found to carry three rust resistance genes temporarily named LrS13, SrS13 and YrS13. Unfortunately, the resistance genes were completely linked to gametocidal (Gc) genes that were co-transferred from the wild parent. In wheat Gc genes cause reduced fertility, poor plant phenotype and hybrid necrosis. In order to use employ the rust resistance genes commercially they need to be separated from the Gc genes. At the onset of this study four putative shortened forms of the S13 translocation were provided. The four lines were identified in a homoeologous paring induction experiment (involving the test cross 04M127). This study aimed to achieve the following: (i) characterize the four recombinants with the use of molecular markers, (ii) use the knowledge gained to identify further recombinants in the 04M127 cross, (iii) identify the shortest (most useful) recombinant, and (iv) attempt to shorten the shortest recombinant form still further and thereby remove as many of the Gc genes as possible. In total, seven recombinants of the S13 translocation (04M127-1, -2, -3, -4, -7, -11 and -12; referred to as recombinant group A) were identified and characterised with microsatellite and SCAR markers. These recombinants have exchanged different amounts of foreign chromatin for wheat chromatin, but were still associated with Gc genes, showing hybrid necrosis and seed shrivelling. Some of the recombinants have lost the undesirable „brittle rachis‟ phenotype which occurs in Ae. speltoides and the S13 translocation line. In plants VII having this trait, the rachis spontaneously disarticulates after the third spikelet upon ripening of the ear. Recombinant 3 appeared to be least affected by Gc genes and was therefore used in further attempts to shorten the translocation. Recombinant 3 was crossed with wheat (W84-17) and resistant F1 (heterozygous for the translocation) were test crossed with Chinese Spring nullisomic 3A tetrasomic 3B/D plants. Thirty five resistant testcross F1 plants were identified (named recombinant group B). The resistant group B recombinants as well as nine susceptible test cross F1 (which also appeared to be recombinant) were characterised making use of microsatellites and a SCAR marker. From the results it appeared that each of the 35 resistant plants exchanged substantial amounts of Ae. speltoides chromatin for wheat chromatin. The species chromatin that remained (and which contains LrS13) is probably located either close to the 3AS telomere or within the proximal regions of 3AS and 3AL. A SCAR marker that has been developed specifically for the S13 translocation provided useful confirmation of the presence of Ae. speltoides chromatin in the 35 recombinants. If the SCAR marker proves to be tightly linked to LrS13 it may eventually be used for marker assisted selection of the resistance or it may be employed in continued attempts to reduce the amount of foreign chromatin. Seedling rust resistance tests showed that the recombinants have lost SrS13 and YrS1 during recombination. An attempt was also made to develop additional markers that specifically detect the translocation in order to further characterise the group B recombinants. Published information on Ae. speltoides specific repeated and transposon sequences were obtained and used for primer design. Unfortunately, no suitable markers could be found and the primers that were designed tended to amplify the same fragments in both the wheat and species genomes. DArT markers were also employed in an attempt to characterise the 35 group B recombinants and controls. The DArT results provided an independent verification of the results obtained with the microsatellite markers. The DArT results confirmed that the group B recombinants exchanged large amounts of species chromatin for wheat chromatin. Even though the 35 resistant group B recombinants have undergone extensive recombination they still show signs of residual Gc effects. It is believed these effects can be removed by continued backcrossing to wheat accompanied by selection against Gc symptoms. While the effects of Gc genes per se were not studied, their properties were reminiscent of those of transposable elements. Indications were that complex interactions involving the Gc genes themselves as well as genetic factors in the wheat genome may have a drastic effect on the selective survival of recombinant gametes.

Thesis (MSc (Genetics))--Stellenbosch University, 2008.