Thesis (M.S.)--University of Wisconsin--Madison, 1985. / Typescript. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 43-48).

Thesis (M. Sc.(Plant Pathology))--University of Pretoria, 2006. / Includes bibliographical references. Available on the Internet via the World Wide Web.

This item was digitized as part of the Million Books Project led by Carnegie Mellon University and supported by grants from the National Science Foundation (NSF). Cornell University coordinated the participation of land-grant and agricultural libraries in providing historical agricultural information for the digitization project; the University of Arizona Libraries, the College of Agriculture and Life Sciences, and the Office of Arid Lands Studies collaborated in the selection and provision of material for the digitization project.

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Thesis (M.S.)--University of Wisconsin--Madison, 1983. / Typescript. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 48-51).

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

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A protocol is presented for the rapid (28 days) induction of microtubers on micropropagated layered potato plantlets of 'Kennebec', 'Russet Burbank' and 'Superior' in medium devoid of growth regulators. With this method the addition of coumarin, 6-(2-chloroethyl)-trimethylammonium chloride and 6-benzylamino-purine to the microtuberization medium either had no effect or significantly reduced microtuber weight per plantlet. Increasing the incubation period from 28 to 56 days significantly increased the weight of microtubers per plantlet and the proportion of microtubers heavier than 1 gram. Increasing the volume of microtuberization medium from 50 to 100 ml significantly increased the number of microtubers per plantlet. Microtuber dormancy periods were cultivar-specific and microtubers $ le$250 mg had longer dormancy periods as compared to microtubers $>$250 mg. A positive correlation was established between endogenous abscisic acid levels and microtuber dormancy periods. Microtubers $ le$250 mg had lower specific gravity, fewer eyes and produce fewer sprouts than microtubers $>$250 mg. Microtuber-derived plants were generally single-stemmed. Severe physiological ageing treatment ($>$2500 degree-days) had no effect on microtuber sprout development, stem number, tuber number and only minimally influenced tuber weight of microtuber-derived plants. Decreasing field in-row planting density from 30 to 10 cm reduced tuber weights and numbers per plant but increased them on a per hectare basis. Economic analysis indicated that optimum planting density varied depending on plantlet cost. The optimum planting density was 10 cm if the cost of the plantlet was $0.10 or less, 20 if plantlet cost were from $0.10 and $0.20 and 30 cm for plantlet cost greater than $0.20. A potato seed tuber certification program adapted to the needs and constraints of Egypt is presented.

Micropropagated potato (Solanum tuberosum L.) plantlets are routinely used for producing specific pathogen tested minitubers in the Canadian seed tuber certification industry. In vitro hardening methods for micropropagated 'Norland' were investigated, involving full and half strength Murashige and Skoog (1962) based propagation medium supplemented with NaCl, KCl, CaCl2, polyethyleneglycol, or paclobutrazol. Ten different media treatments were compared for their effect on stomatal function and early transplant performance using porometry, microscopy, and direct ex vitro transplanting. PEG, NaCl and 1/2 MS + 3 mM Ca treatments did lead to decreased leaf water losses. However, these treatments did not improve ex vitro transplant performance compared to controls. Minituber production was investigated using ex vitro plantlets in a rockwool-based hydroponic system. Productivity was evaluated for treatments involving photoperiod pretreatment, planting orientation, planting density, and hilling. No difference in total yield was detected when plantlets were exposed to 12 compared with 16 hour photoperiod pretreatments. However, short photoperiod pretreatment increased the number of minitubers in the most desirable size range. Increased planting density reduced yield per plant. However, small increases in yield per m2 occurred with increased planting density. Hilling, pinching, and planting orientation had no effect upon minituber size, number, or overall fresh weight yield. Significant differences in minituber yield occurred in field experiments. Larger minitubers (10--40 g) had larger yields compared with smaller minitubers (1--5 g). These results will contribute to improved minituber production technology for the Canadian certified potato seed tuber industry.

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