Thesis (Ph. D.)--University of Wisconsin--Madison, 1969. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

A foliar pathogen identified as a member of the genus Alternaria was isolated from blighted Sphenoclea zeylanica (gooseweed) collected in 1991 from a rice field near Los Banos, Laguna, Philippines. Inoculum density, dew period, and plant height are factors influencing biocontrol of S. zeylanica with this indigenous pathogen. Significantly higher percent reductions in plant height and dry weight were obtained and all plants were killed at higher inoculum concentrations with 8 h of dew. The number, germination, and virulence of conidia were significantly affected by production techniques, temperature, light condition, and incubation period. Exposure to continuous near-ultraviolet (NUV) light at 28$ sp circ$C stimulated sporulation on agar media and on solid substrates. Overall, the best production technique was the use of sorghum seeds using an equal quantity of sorghum seeds and water (w/v) incubated for four weeks. Another conidia production method using the sporulation medium (S-medium) technique was evaluated with the addition of 20 g L$ sp{-1}$ of calcium carbonate (CaCO$ sb3)$ and 2 ml of sterile distilled water. Primary 1/2 PDA at 18$ sp circ$C in the dark produced the most virulent conidia. This technique produced conidia relatively rapid, but was labour intensive. Host range studies using 49 plant species in 40 genera representing 20 families, selected by using a modified centrifugal phylogenetic and variety strategy indicated that only S. zeylanica was susceptible in the absence and presence of supplemental dew. On the basis of morphological and cultural characteristics, pathogenicity on the host, host specificity, and the absence of a previous record of this fungal pathogen on S. zeylanica, the binomial A. alternata f. sp. sphenocleae is proposed.

TA and TB toxins are host-specific phytotoxins produced by the fungus Alternaria alternata f. sp. Iycopersici, the causative agent of Alternaria stem canker disease in tomato. Both compounds are isolated as an equilibrium mixture of the esters formed by either the C(13) or C(14) hydroxy groups with the Re prochiral carboxy group of tricarballylic acid. The design and execution of syntheses for these toxins is necessary in order to study structure-function relationships for T A and TB toxins and their application as natural herbicides. The aim of the synthetic study presented in this thesis is to develop and implement a methodology for the synthesis of the C(1)-C(9) unit of the C17 amino¬pentol backbone of the TA and TB toxins with the required functional groups and appropriate stereochemistry using a chiral sulfoxide as an auxiliary to control the stereochemistry in key steps of the synthetic route. (2R,4S,5R,6R)-2,6-Dimethyloctane-1 ,4,5-triol, synthon B, and (2S,4R,5R)-1-aminononane-2,4,5,9-tetrol, synthon A were identified by retrosynthetic analysis of the C17 aminopentol backbone of TA toxin as key intermediates for a proposed synthesis. Further analysis of synthon B identified a C5 synthon that can be obtained from (2S)-malic acid by functional group transformations, chiral sulfoxide methodo¬logy and an appropriate protective group strategy. The work presented in the thesis shows that a protected intermediate corresponding to the abovementioned C5 synthon, (2S,4S)-2,4,5-trihydroxy:-pentanal can be prepared from (2S)-malic acid, but that using either Sharpless methodology or chiral sulfoxide methodology for the introduction of the third stereogenic centre and chain extension to a C9 unit, failed as a result of the steric crowding caused by the acetonide protecting group. As a result a different synthetic route is proposed. The results obtained in the work on TA toxin were applied to the synthesis of the C(1)-C(9) aminotetrol unit of the backbone of TB toxin. / Dissertation (MSc (Chemistry))--University of Pretoria, 2007. / Chemistry / unrestricted

No description available.

Thesis (M.Sc.(Chemistry))--University of Pretoria, 2000. / Includes abstract in English. Includes bibliographical references. Also available online.

Cowpea (Vigna unguiculata (L.) Walp is an indigenous food legume in Africa, which has great production potential, especially in areas with low agricultural resources. During surveys of cowpea fields in southern Africa, a new foliar disease was recorded. Alternaria cassiae Juriar&Khan was consistently isolated from diseased plant material. Pathogenicity was confirmed using Koch's Postulates. The effect of different culture media, temperature, light and wounding on the growth and sporulation of the fungus was studied. A. cassiae grew well and produced conidia abundantly when maintained on V8-agar at 25°C in a 12h UV-light/12h dark cycle. Sporulation was further enhanced by wounding the cultures. The pre-penetration and infection process of A. cassiae on cowpea leaves was studied by light and scanning electron microscopy. Conidia germinated within 2-3h post inoculation (hpi), forming multiple germ-tubes randomly that grew in any direction across the leaf surface. By 8hpi terminal or intercalary appressoria were formed above epidermal cells or over stomata. Occasionally germ-tubes entered stomata, without the formation of appressoria. Penetration of the plant surface, whether directly through the epidermis or indirectly through stomata was observed 72hpi. Following penetration bulbous primary hyphae were observed within the sub-stomatal cavities, secondary hyphae developed from the primary hyphae and grew within the intercellular spaces penetrating epidermis and mesophyll cells. A. cassiae is a necrotrophic fungus as the infection process is characterised by a destructive necrotrophic phase where plant cells became necrotic even prior to fungal penetration. Conidial morphology, types and development of the fungus were studied in vitro on different culture media and in vivo on cowpea leaves. A. cassiae produced a mixed population of three conidial types. Conidia were formed singly or in chains of 2-4 conidia. Conidia with long, filiform beaks and conidia with shorter beaks, converted into secondary conidiophores were more frequently produced than mature, beakless conidia on all the media, except on potato dextrose agar. Conidial body and beak sizes were variable when measured in culture and on cowpea leaves. Conidia produced in culture were larger, than those produced in vivo. Conidiophores emerged directly through the epidermis or stomata or were formed when hyphae growing on the leaf surface differentiated into conidiophores. Smooth, bud-like conidial initials were produced at the apex of conidiophores. Conidia matured and became elliptical to obvate and densely verrucose. Once a mature conidium had detached, a small pore was visible at the apex of the conidiophore. A. cassiae was shown to be seed-borne in cowpea. Six fungicides i.e. Benomyl, bitertanol, captab, mancozeb, propiconazole and triforine were evaluated for their efficacy in reducing mycelial growth of A. cassiae in vitro. All fungicides except benomyl proved to be effective. Cowpea seeds were artificially inoculated with A. cassiae and treated with all the fungicides except benomyl. Percentage germination and infection was determined in vitro. Percentage emergence, disease incidence, root and shoot lengths and abnormalities were determined in greenhouse trials. Only bitertanol at l.5x the recommended dosage significantly reduced percentage germination. All treatments except triforine l.0x and l.5x significantly decreased the percentage infection of artificially inoculated seeds. None of the treatments except bitertanol l.5x showed a difference in shoot and root length when compared to the control. Captab l.5x the recommended rate proved to be the best treatment over all. / Dissertation (MSc (Botany))--University of Pretoria, 2006. / Plant Science / unrestricted

The initial phase in the development of a biological control strategy is screening of biological control agents. Secondary to this phase is the establishment of accurate, effective application techniques. However, successful control requires a thorough understanding of all factors affecting the relationship between host plant, pathogen and other microbes. The purpose of this study was to screen and identify potential bacterial antagonists against Alternaria, a fungal citrus pathogen, attachment of the antagonists to bees, and bee dissemination of the antagonist to citrus flowers. A total of 568 bacterial epiphytes were screened on agar plates for antagonism against Alternaria. Only eight of these isolates, which were identified as Bacillus subtilis, B licheniformis, B. melcerons, B. polymyxa, B. thermoglycodasius, B. sphaericus, B. amiloliquefaciens, and B. coagulans, showed inhibitory effects on the growth of Alternaria. The most effective isolates were B. subtilis and B. licheniformis. Further screening was done with B. subtilis and B. subtilis commercial powder (Avogreen). These bacteria were sprayed on citrus flowers for colonisation studies. Mean populations of B. subtilis and the commercial powder recovered from the flowers were 104 and 103 cfu/stamen respectively. The organisms colonised the styler end and ovary of the flowers when observed under scanning electron microscope (SEM). Avogreen was placed in an inoculum dispenser, which was attached to the entrance of the hive. Honeybees emerging from the beehive acquired 104 cfu/bee. The powder attached to the thorax and thoracic appendages, as revealed by SEM. One active beehive was placed in an enclosure with fifteen flowering citrus nursery trees in pots for dissemination trials. Mean populations of commercial B. subtilis recovered from the flowers visited by bees were 104 cfu/stamen. Electron microscope studies revealed that the antagonist was colonising the styler end and ovary of the flowers. Field dissemination studies were unsuccessful due to low yields. / Dissertation (Magister Institutiones Agrariae)--University of Pretoria, 2003. / Plant Production and Soil Science / unrestricted