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Typescript, etc. / Digitized by Kansas State University Libraries

Thesis (Ph.D.) in Plant Science--University of Maine, 2004. / Includes vita. Includes bibliographical references (leaves 112-123).

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Powdery scab, caused by the obligate Plasmodiophorale fungus <I>Spongospora subterranea</I>, has become an important disease for potato production world-wide. Its success is attributed to the characteristics of its pathogen and features of its life cycle. The release of primary zoospores from sporeball suspensions was studied under laboratory conditions by immunoassay and haemocytometer. Both spontaneous and staggered germination were observed when sporeballs were suspended in deionised water and incubated in the dark at 15°C. The release of primary zoospores was influenced by internal physiological factors and the proportion of dormant and non-dormant spores. Dormancy of sporeballs appeared to be a process of maturation as the time of incubation and age of inoculum affected sporeball germination. The process of maturation and the resultant increase in spontaneous germination were governed by temperature and plant root exudates. Release of primary zoospores was hastened when sporeballs were exposed to warm temperatures (25°C) and in presence of both host and non-host plans of the pathogen. The ability of primary zoospores to infect the roots of solanaceous plants was increased at low temperatures (3, 10 and 15°C). Primary root infection differed between potato cultivars and was not always related to tuber susceptibility. Development of <I>S. subterranea</I> from uniuncleate plasmodia to mature zoosporangia was observed in tomato seedlings grown as a bait plants in culture solution with primary or secondary zoospores as the inoculum. Re-infection of the host roots by secondary zoospores and production of mature zoosporangia occurred within 7 days at 15°C. The release of secondary zoospores was hastened when exposed to warm temperatures (25°C). The prolonged release of primary and secondary zoospores and increased infectivity of primary zoospores at low temperatures may explain the prevalence of powdery scab under cool conditions.

This thesis describes experiments designed to improve microtuberization efficiency, and to evaluate a range of dormancy-breaking agents for microtubers and minitubers. Provision of continuous darkness, agitation to cultures, mechanical resistance to stolons, or lower levels of medium nitrogen did not improve microtuberization. The 16/8 h d/n cycle at step 1 of microtuberization was significantly better than 12/12 or 8/16 h d/n cycles based on microtuber yield. Cultures exposed to prolonged step 2 or 2 successive harvests, rather than a single harvest at 30--35 d, had significantly improved microtuber yield. In a series of chemical and mechanical treatments applied to microtubers and minitubers, with or without variable periods of cold storage, 500 mg l -1 GA3 was the most efficient in breaking-dormancy and inducing precocious sprouting. GA3 was the only agent that was able to break dormancy of minitubers that had not been cold stored. After 2 weeks of cold storage, minitubers treated with GA3 also broke dormancy, while Signal was less effective in promoting sprouting. (Abstract shortened by UMI.)

Thesis (MScAgric)--University of Stellenbosch, 2007. / Bibliography. Also available via the Internet.

Thesis (M.S. in plant pathology)--Washington State University, December 2008. / Title from PDF title page (viewed on Dec. 23, 2008). "Department of Plant Pathology." Includes bibliographical references.

Thesis (Ph. D.)--University of Chicago, 1916. / "A Private Edition Distributed by the University of Chicago Libraries." "Reprinted from the Botanical gazette, vol. LXII, no. 3." "Literature cited": p. 207-209. Also available on the Internet. Also issued online.