Thesis (M.S.)--University of Wisconsin--Madison, 1982. / Typescript. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 81-87).

Spongospora subterranea (Wallroth) Lagerheim f. sp. subterranea Tomlinson (Sss) is the causal organism of potato powdery scab. This blemish disease is of economic concern to fresh and seed-tuber producers around the world, as it diminishes both tuber quality and marketability. In this study, techniques for detecting and quantifying Sss were evaluated. Comparison of enzyme linked immunosorbent assay (ELISA), conventional PCR and real-time PCR showed that conventional PCR is more sensitive than ELISA, as conventional PCR detected Sss inoculum from both tuber and soil samples whereas ELISA only detected Sss inoculum from tuber samples. Real-time PCR not only detected Sss DNA in a variety of sample types, but real-time PCR could also be used to quantify Sss DNA. The high sensitivity of real-time PCR gave consistent detection of standard DNA quantities ranging from 10 000 to as few as 1 sporeballs per ml. The benefit of real-time PCR is that it can be used for the study of the different life stages of Sss in a range of sample types. During 2008 to 2010 powdery scab samples were collected from Ceres, KwaZulu Natal, Mpumalanga and Sandveld and herbarium samples dated 1936 from the Sandveld were also obtained to investigate genetic variation in ITS1/2 sequences. Comparisons to known Sss Group Type I/II sequences were made to determine the specific Group Type/s found in South Africa. All these South African samples were identified as belonging to Sss Group Type II A bioassay was developed to investigate the dormancy period of Sss. The results showed that Sss zoospores are released four days after inoculation and infection of tomato roots occurs six days after inoculation. Plasmodia in tomato root hairs develop eight days after inoculation, followed by zoosporangia development by day 12. Several strategies are being used to minimize the risk of powdery scab as there is no single effective method for controlling the disease. One such strategy includes cultivar resistance. Cultivar susceptibility of six South African cultivars (Argos, BP1, Buffelspoort, Caren, Up-to-Date and Valor) was evaluated. Pot trials showed that all the evaluated cultivars are susceptible to infection by Sss and that there is a positive correlation between the severity of root galls and of tuber lesions. Various crops (cabbage, mustard, soybean and tomato) were assessed to determine the host range of Sss in rotation crops in South African potato growing regions and to identify possible trap crops for Sss. Of the crops evaluated, the pathogen was unable to complete its life cycle in wheat and cabbage, whereas soybean is a non-host crop. Both mustard and tomato are hosts of Sss. A severe outbreak of powdery scab occurred during 2006 in a potato mini-tuber production facility in Ceres, Western Cape, South Africa. A study was conducted in the production facility to detect Sss and to identify the source/s of contamination, so that corrective measures could be taken to eradicate the pathogen. Swab samples specified areas in the production facilities that were infested with Sss sporeballs. Following eradication efforts to improve facility hygiene, a second set of swab samples was taken in 2009 to determine the efficacy of the eradication methods. From 2009 onwards, disease-free mini-tubers have been harvested.</p. This comprehensive study will lead to a better understanding of Spongospora subterranea f.sp. subterranea and has emphasised the need for further research which will focus on strategies that will help to reduce the impact that powdery scab has on potato production, not only in South Africa, but globally too. / Dissertation (MSc(Agric))--University of Pretoria, 2012. / Microbiology and Plant Pathology / unrestricted

Barley powdery mildew disease is caused by the highly specialised phytopathogenic fungus, Blumeria graminis f.sp. hordei. Disease is spread prolifically by the production of asexual conidia. Following contact with the barley leaf surface, a short, primary germ tube (PGT) emerges, followed by elaboration of a second-formed germ tube, the appressorial germ tube (AGT). This second-formed germ tube elongates, swells and hooks to form an appressorium, which allows direct penetration of the barley cuticle and infection of the host. Infection structure differentiation in B. graminis is a highly regulated and complex process. It demands the coordinated perception of multiple external signals, but little is known about how these signals are integrated and transduced within the fungus. Protein kinase A (PKA) and cAMP signalling are known to play important, but complex, roles during infection structure development. However, signalling via cAMP alone is not sufficient to promote progression through infection structure differentiation. This study describes the characterization of two B. graminis protein kinase C genes, pkc1 and pkc-like. PKC activity was identified in B. graminis protein extracts. Efforts to find an inhibitor specific for B. graminis PKC were unsuccessful. However, phorbol ester, a PKC agonist, invoked both appressorium formation when applied to spores in vivo and PKC activity in protein extracts. In addition, real-time PCR confirmed the differentially regulated transcript profiles of both pkc1 and pkc-like, revealing a peak in transcript levels just prior to PGT emergence for pkc1, and during PGT differentiation for the pkc-like gene. Two mitogen-activated protein (MAP) kinases, mpk1 and mpk2, were characterized. MAP kinase activity was detected in conidial protein extracts. The MAP kinase kinase (MEK) inhibitor, PD98059, inhibited B. graminis germling morphogenesis. However, a MAP kinase agonist failed to show any effect on germling differentiation. In addition, real-time PCR confirmed the differentially regulated transcript profiles of both mpk1 and mpk2, and revealed a peak in transcript levels during appressorial germ tube elongation and swelling for both genes. The "model" phytopathogenic fungus, Magnaporthe grisea, was employed as a "surrogate host" for the functional analysis of the B. graminis MAP kinase gene, mpk1. Firstly, the mpk1 promoter was sequenced and a plasmid construct made comprising the mpk1 gene under the control of the mpk1 promoter. This, and the control construct, that is the M. grisea PMK1 gene under the control of the PMK1 promoter, were transformed into M. grisea Δpmk1. Southern analysis identified transformants for phenotypic studies. These showed that whereas Δpmk1 was complemented by M. grisea PMK1 in the control experiments, B. graminis mpk1 failed to complement Δpmk1. Expression studies showed that there was no expression of mpk1 in an mpk1 transformant.

Title page, table of contents and abstract only. The complete thesis in print form is available from the University of Adelaide Library. / Grapevine powdery mildew, caused by the fungus Uncinula necator Schweinitz Burrill, is a major disease affecting grape yield and quality worldwide. In conventional vineyards, the disease is controlled mainly by regular applications of sulphur and synthetic fungicides, such as demethylation inhibiting fungicides (DMIs), and in organic agriculture by sulphur and canola-based oils. The impending restrictions on the use of sulphur in organic viticulture, the development of resistance to DMls in Australia and elsewhere, and the demand for residue-free grapes create a need for effective alternatives to sulphur and synthetic chemicals. This research has identified potential replacements for synthetic fungicides and sulphur in the control of powdery mildew, such as milk, whey, bicarbonates and canola oil-based sprays. A series of greenhouse experiments was conducted to evaluate 34 potential novel materials and biological agents for efficacy in controlling powdery mildew. The most effective treatments applied were Bacillus subtilis (which reduced disease by 94% compared to the untreated control), Synertrol Horti-Oil® (a canola oil-based product, 92%), milk (70%), whey (64%) and Ecocarb® (potassium bicarbonate, 58%). Milk and whey provided increased control of powdery mildew as the concentration increased. The efficacy of milk tended to decrease as the fat content of the milk was reduced. The materials that were most promising in the greenhouse were then assessed in field trials in commercial vineyards. Applications of milk, whey and mixtures of a canola oil-based product and potassium bicarbonate, applied at rates of 300 L/ha to 1000 L/ha depending on canopy development, reduced the severity of powdery mildew. The severity of powdery mildew on vines sprayed with a 1:10 dilution of milk, 45 g/L whey powder and mixed programs was not significantly different from that on vines sprayed with sulphur (wettable powder, 3 g/L). However, the relative control of powdery mildew by the test materials in field trials was highly dependent on the degree of coverage of the plant surface achieved. In vineyards where coverage was compromised, the degree of control of powdery mildew was reduced, often to commercially unacceptable levels. Electron spin resonance (ESR) and scanning electron microscopy (SEM) were used to investigate the possible mode or modes of action of milk and whey in the control of powdery mildew. The ESR experiments showed that production of oxygen radicals by various components of milk in natural light was associated with reduced severity of powdery mildew. SEM images showed that milk and whey caused the hyphae of U necator to collapse and damaged conidia within 24 h of treatment. Hydrogen peroxide, applied as a source of free radicals, also caused collapse of the hyphae of U necator within 24 h but did not damage conidia, and appeared to stimulate germination. Lactoferrin (an antimicrobial component of milk) ruptured conidia, but damage to hyphae was not evident in lactoferrin-treated samples until 48 h after treatment. The results suggested that fats, free radical production along with the action of lactoferrin, and possibly other proteins, are associated with the control of powdery mildew by milk. Novel soft fungicides, such as milk and oil plus bicarbonate mixtures, were effective alternatives to sulphur and synthetic fungicides in certain South Australian conditions. Biological agents (including B. subtilis, which was highly effective in greenhouse experiments) did not provide acceptable control of powdery mildew in the vineyard. / http://proxy.library.adelaide.edu.au/login?url= http://library.adelaide.edu.au/cgi-bin/Pwebrecon.cgi?BBID=1116612 / Thesis (Ph.D.) -- University of Adelaide, School of Agriculture and Wine, 2004

Title page, table of contents and abstract only. The complete thesis in print form is available from the University of Adelaide Library. / Grapevine powdery mildew, caused by the fungus Uncinula necator Schweinitz Burrill, is a major disease affecting grape yield and quality worldwide. In conventional vineyards, the disease is controlled mainly by regular applications of sulphur and synthetic fungicides, such as demethylation inhibiting fungicides (DMIs), and in organic agriculture by sulphur and canola-based oils. The impending restrictions on the use of sulphur in organic viticulture, the development of resistance to DMls in Australia and elsewhere, and the demand for residue-free grapes create a need for effective alternatives to sulphur and synthetic chemicals. This research has identified potential replacements for synthetic fungicides and sulphur in the control of powdery mildew, such as milk, whey, bicarbonates and canola oil-based sprays. A series of greenhouse experiments was conducted to evaluate 34 potential novel materials and biological agents for efficacy in controlling powdery mildew. The most effective treatments applied were Bacillus subtilis (which reduced disease by 94% compared to the untreated control), Synertrol Horti-Oil® (a canola oil-based product, 92%), milk (70%), whey (64%) and Ecocarb® (potassium bicarbonate, 58%). Milk and whey provided increased control of powdery mildew as the concentration increased. The efficacy of milk tended to decrease as the fat content of the milk was reduced. The materials that were most promising in the greenhouse were then assessed in field trials in commercial vineyards. Applications of milk, whey and mixtures of a canola oil-based product and potassium bicarbonate, applied at rates of 300 L/ha to 1000 L/ha depending on canopy development, reduced the severity of powdery mildew. The severity of powdery mildew on vines sprayed with a 1:10 dilution of milk, 45 g/L whey powder and mixed programs was not significantly different from that on vines sprayed with sulphur (wettable powder, 3 g/L). However, the relative control of powdery mildew by the test materials in field trials was highly dependent on the degree of coverage of the plant surface achieved. In vineyards where coverage was compromised, the degree of control of powdery mildew was reduced, often to commercially unacceptable levels. Electron spin resonance (ESR) and scanning electron microscopy (SEM) were used to investigate the possible mode or modes of action of milk and whey in the control of powdery mildew. The ESR experiments showed that production of oxygen radicals by various components of milk in natural light was associated with reduced severity of powdery mildew. SEM images showed that milk and whey caused the hyphae of U necator to collapse and damaged conidia within 24 h of treatment. Hydrogen peroxide, applied as a source of free radicals, also caused collapse of the hyphae of U necator within 24 h but did not damage conidia, and appeared to stimulate germination. Lactoferrin (an antimicrobial component of milk) ruptured conidia, but damage to hyphae was not evident in lactoferrin-treated samples until 48 h after treatment. The results suggested that fats, free radical production along with the action of lactoferrin, and possibly other proteins, are associated with the control of powdery mildew by milk. Novel soft fungicides, such as milk and oil plus bicarbonate mixtures, were effective alternatives to sulphur and synthetic fungicides in certain South Australian conditions. Biological agents (including B. subtilis, which was highly effective in greenhouse experiments) did not provide acceptable control of powdery mildew in the vineyard. / http://proxy.library.adelaide.edu.au/login?url= http://library.adelaide.edu.au/cgi-bin/Pwebrecon.cgi?BBID=1116612 / Thesis (Ph.D.) -- University of Adelaide, School of Agriculture and Wine, 2004

Thesis (Ph. D.)--University of Wisconsin--Madison, 1961. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Bibliography: leaves 104-109.

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Thesis (M.S.)--Oregon State University, 2008. / Printout. Includes bibliographical references (leaves 70-78). Also available on the World Wide Web.

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