No description available.

A study involving variation in strains of the potato scab organism as isolated from a soil in British Columbia is reported. Associated with this is a study of soils from scab-free and scab-infested areas. Exchange capacity and related studies of a number of soil samples indicate a trend toward higher average exchange capacity, higher average exchange calcium and higher average calcium-potassium ratios in the samples from scab-infested areas. There is also a trend toward higher organic matter content and lighter soil textures in the scab-producing soils. In spite of these trends no definite correlations can be established under the method of soil sampling adopted. Thirty-four strains of Streptomyces scabies isolated from diseased White Rose tubers grown on the ranch vary markedly in cultural characteristics and in pathogenicity. The effect of hydrogen ion concentration on four of these strains shows a positive correlation between pH, amount of growth and hydrogen ion concentration up to the optimum pH of 7 to 8 and beyond this range a negative correlation exists. The inhibiting acid pH is 3.4 for two strains and between 4.0 and 5.5 for the other strains. The limiting alkaline pH appears to be beyond pH 11.6. Microphotographs of the Streptomyces scabies show typical open spirals and chains of conidia 0.5 to 1.0 microns in diameter. Scab control measures conducted in pots in the greenhouse and using a severely scab-infested soil sample from the ranch indicate no control by mercuric chloride applications. Applications of sulphuric acid, sulphur and calcium hydroxide indicate that a pH of 8.0 or greater is an effective but not complete control. An acid pH of 5.0 gives less control than pH 8.0 and more acidic reactions give even less control. The applications of these necessary to effect control are too high to be of practical value. The fungicidal effect of compound P-162 as determined under laboratory procedures is no indication of its efficacy in the field or greenhouse. Laboratory technique indicates that most strains are completely inhibited at 100 to 230 parts per million of compound P-162 while in the greenhouse tests 40 parts per million proved effective in giving partial control. The practical value of P-162 in the field is as yet to be determined since its commercial value and its usefulness as a control measure in the field is unknown. / Land and Food Systems, Faculty of / Graduate

No description available.

Sheep scab is a highly contagious ectoparasitic disease caused by the mite, Psoroptes ovis, which causes intensely pruritic lesions with severe dermatitis and is a major welfare and production issue in the UK national flock. In an attempt to improve disease control, sheep scab was recently made notifiable in Scotland indicating that early diagnostic tests will be crucial to the success of this legislation. A sensitive and specific antibody based assay has been developed which can diagnose early infestation, but which does not indicate current disease status post-treatment due to residual circulating antibody levels. However, studies into host biology and response to disease have illustrated the potential use of biomarkers (BMs) in diagnostics as indicators of disease progression and the effectiveness of treatment regimes, including a recent microarray analysis which identified over 600 host genes differentially expressed in circulating leukocytes following P. ovis infestation. As many of these genes encoded proteins known to be involved in inflammatory responses, this data was used in the search for potential BMs. Initially the genes were filtered and ranked, using bioinformatic analysis, to identify the most promising BM candidates and then evaluated using Western blot analysis against a range of sera from P. ovis infested and naïve sheep. Promising results were obtained for a complement binding protein, C4BPB, showing it was rapidly up-regulated following infestation and correlated with disease progression as determined by lesion size development. The ovine C4BPB gene was successfully sequenced for the first time and a recombinant form of this protein expressed in E. coli. Antibodies, raised in rabbits against ovine rC4BPB, were used to develop a sandwich ELISA, results from which suggested the potential of C4BPB as a BM for sheep scab as it indicated current disease status postinfestation and post-treatment. The major ruminant acute phase proteins (APPs) serum amyloid A (SAA) and haptoglobin (Hp) were then investigated using commercially available assays, as previous studies indicated they were effective markers of inflammatory disease in ruminants. Results from these analyses indicated that both APPs responded positively to infestation with P. ovis but this was not statistically significant until 4 weeks post-infestation. After treatment, the APPs declined rapidly, as described by their short half life of less than 3 days following successful treatment, compared with 56 days for the estimated half life of the host antibody against the mite antigen Pso o 2. Further statistical analysis of the APP response suggested that SAA was the more discriminatory marker, with lower pre-infestation levels and higher sensitivity at the estimated optimum cut-off values. The possibility of using a signature of BMs, as an alternative to a single BM, was discussed as a method of increasing the sensitivity and specificity of the improved test, along with the potential of combining the BM diagnostic with the existing antibody assay. It was concluded that this would provide a highly sensitive and specific test for sheep scab which would diagnose early infestation as well as indicating current disease status post-treatment, providing a highly beneficial tool to the sheep industry to aid the control of this disease.

Presented as Thesis (Ph. D.)--University of Wisconsin--Madison, 1929. / Cover title. Reprinted from Phytopathology, vol. XXI, no. 5 (May 1931). Includes bibliographical references (p. 520-523).

Thesis (Ph. D.)--University of Wisconsin, 1966. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

Thesis (Ph. D.)--University of Wisconsin--Madison, 1982. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

Common scab, as the name implies, is one of the most common and widespread diseases affecting production of potato in virtually all parts of the world where the crop is grown. In South Africa, the percentage of bags containing scab-infected seed tubers averages 32 %, with a corresponding rejection or decertification of the seed. The disease also reduces the cosmetic value of ware potatoes and, with the growing demand for blemish-free produce, increasingly results in the downgrading of consignments on the ware market. Control is mostly attempted by means of agrochemicals. Streptomyces scabiei is generally considered as the main cause of common scab, also in South Africa. However, various otherStreptomyces species are known to be associated with the disease, either parasitically or saprophytically, but no attempt has yet been made to determine if any of them occur in South Africa. The purpose of this study was to elucidate the etiology of common scab in the country and to evaluate some strategies that could be included in an integrated control programme for managing the disease. The results indicated the following: Isolation and characterisation of streptomycetes from symptomatic seed potato tubers from the five potato-production areas in South Africa worst affected by common scab, showed that the disease is caused by three phenotypically distinct Streptomyces groups, designated 1, 2 and 3, whereas a further three groups were saprophytically associated with scabby tubers. Streptomyces group 1, which corresponded morphologically to the description of S. scabiei, was by far the most common, representing 52 % of all and 82 % of the pathogenic isolates, and occurred in all the regions surveyed. The two other Streptomyces groups containing pathogenic isolates may represent atypical S. scabiei isolates, or could be separate species. Analysis for the production of thaxtomin A, an important pathogenicity factor, confirmed a positive correlation between the pathogenicity of isolates and their ability to produce thaxtomin A. However, 14 % of the pathogenic isolates did not produce thaxtomin A, whereas 6 % of the non-pathogenic isolates tested positively for production of the toxin. Another interesting observation was the ability of 15 % of both all and of the pathogenic isolates to grow at a pH of 4.0, which is atypical of S. scabiei. In the greenhouse, densities of S. scabiei were significantly lower in sand artificially infested with the pathogen and planted to triticale or cabbage than in fallow sand or sand planted to soybean, spinach, maize, pumpkin or sunflower, three months after planting of the crops. Survival in sand planted to rye and pea was also relatively low, albeit not significantly lower than the other crops. Roots of soybean, spinach, rye, sunflower, pea and cabbage were colonised significantly less by S. scabiei than those of pumpkin and maize. Cluster analysis separated the crops into two groups according to their effect on S. scabiei. The first group, comprising pumpkin, maize, soybean, sunflower and spinach, had little impact on survival of the pathogen. The second group, consisting of rye, triticale, pea, and cabbage, showed a distinct suppressive effect and on average supported only 0.23 % of the S. scabiei population sustained by the first group. Incorporation of fresh and dry cabbage residues at rates of 0.25 and 0.1 % (m/v) into soil naturally infested with S. scabiei reduced common scab in the greenhouse by a significant 32 and 41 %, respectively. Amendment of scab-infested soil in the field with dry residues of cabbage, cauliflower, broccoli and Brussels sprouts at 0.33 % (m/v) resulted in reduction in disease of approximately 90 %. / Dissertation (MSc (Agric))--University of Pretoria, 2007. / Microbiology and Plant Pathology / MSc (Agric) / unrestricted

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

[...] Pour les raisons énoncées précédemment, il nous a semblé important d'étudier quelles étaient les réactions du champignon lorsqu'il venait en contact avec les substances naturellement présentes à la surface des feuilles, quelle était lat nature de ces substances et leur rôle dans l'établissement de la maladie et aussi comment une fois la maladie établie, le champignon pouvait affecter la nature et la quantité de ces substances qui sont perdues par les feuilles sous l'action de la pluie. [...]