A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in partial fulfillment of the requirements for the degree of Master of Science. Johannesburg, 1996. / Tobacco in Southern Africa has been found to exhibit three forms of leaf curl (class I, II and III) based on symptomology. Class I tobacco leaf curl is a disorder of unknown etiology, appearing as bent or twisted leaves exhibiting frilly enations (leafy outgrowths) along the midrib and secondary veins of severely stunted tobacco plants.[Abbreviated Abstract. Open document to view full version] / AC2017

No description available.

No description available.

The black shank (Phytophthora parasitica var. nicotianae) fungus is a very destructive tobacco disease which is responsible for great losses to farmers worldwide. This disease is also a problem in South Africa, as the most popular South African air-cured tobacco cultivar, CDL28, is very susceptible to black shank. This diallel study focussed on finding the most suitable black shank resistance source to include in a resistance breeding programme with CDL28. Four cultivars were crossed in all possible combinations and planted as an F1 field and greenhouse trial. The F1's were selfed to obtain a segregating F2 population, planted in a greenhouse, to be compared with the mean values of the F1 trials. The field trial was exposed to natural infection while the greenhouse trials were root inoculated. The general combining ability effects of the four parent cultivars differed significantly from each other. The specific combining ability effects of the F1 trials were non significant. These experimental results suggest that additive genetic effects were involved in black shank resistance. The Beinhart 1000-1 source of resistance was significantly better than the Florida 301 source. Burley 37, which possesses the Florida 301 as well as another source of resistance performed better than Domkrag with only the Florida 301 source of resistance. In order to incorporate black shank resistance in CDL28, Beinhart 1000-1 and Burley 37 can be used in a backcross breeding programme with CDL28 which can solve the problems encountered in the cultivation of CDL28 in the presence of black shank. / Dissertation (MSc (Genetics))--University of Pretoria, 2006. / Genetics / unrestricted

Tobacco rattle virus (TRY) and trichodorid vector nematodes were found to be widespread in bulb growing areas in The Netherlands. Indications, obtained from pot bait tests, that associations between serotypes of tobacco rattle virus and Paratrichodorus and Trichodorus vector species occurred were confirmed in bait tests using individual trichodorid nematodes. Whereas different Paratrichodorus vector species each transmitted a serologically distinct TRV-isolate, different Trichodorus vector species transmitted serologically indistinguishable TRV-isolates. It remains to be studied whether a particular Trichodorus species is able to transmit all isolates serologically indistinguishable from the one with which it was naturally associated. A TRV-isolate transmitted by T. cylindricus from Scryne, Scotland and isolates transmitted by P. teres from Julianadorp, The Netherlands were considered anomalous TRV-isolates as they serologically resembled pea early-browning virus but had RNA-1 sequences typical of TRV. Virus-free P. pachydermus from Scotland and from The Netherlands acquired and subsequently transmitted TRV-isolates originally obtained from bait-plants infected by P. pachydermus from Scotland or The Netherlands. They failed to transmit a serologically similar isolate obtained from an infected potato tuber. Virus-free P. pachydermus did not acquire and transmit a pseudorecombinant TRV-isolate with the RNA-2 from a non-transmissible isolate, but efficiently transmitted a pseudorecombinant isolate in which the RNA-2 was derived from a efficiently transmitted isolate. This indicated that transmissibility is most likely determined by the virus coat-protein. Transgenic tobacco plants, expressing TRV coat-protein were resistant to mechanical inoculation but susceptible to nematode transmission of the virus. A possible low level of coat-protein expression in the roots could not fully explain this result as the nematodes also transmitted TRV directly to detached leaves of such transgenic plants.

M.Sc. (Biochemistry) / Please refer to full text to view abstract

No description available.

No description available.

[Typescript] Includes bibliography.

Leaf and associated soil samples from 15 locations in Quebec showed that grey tobacco contained less N, P, K, Ca, B, nicotine, total alkaloids, organic acids, fatty acids, but more Fe and Al than non-grey (normal) tobacco. Soils producing grey tobacco had less N, Ca, organic matter, a lower cation exchange capacity but more Cl. / Fe('55) fed to tobacco plants grown in soil known to produce grey tobacco was translocated to vascular and associated tissue in leaves. The grey disorder symptoms were duplicated in hydroponically grown plants. / In a two-year field study, grey tobacco had 20% less dry weight and 30% less leaf area. Relative growth rate and net assimilation rate decreased less rapidly during growth in grey than in non-grey tobacco. N and K content in leaves was lower in grey tobacco but higher in Fe and Mn. / The grey effect was decreased or eliminated in tobacco plants grown in pots with manure and lime. The leaf contents of Fe, Mn and Mg were associated with grade index ((CENTS)/kg) and dollar value/ha of tobacco. Fertilizer and manure reduced leaf Ca. Fertilizer alone increased the leaf content of N, P and K while manure increased leaf dry weight. The ratio of K/Ca and K/(Ca + Mg) increased with increasing levels of manure relative to each level of lime while the percent grey tobacco decreased dramatically.