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Bibliography: leaves 81-100. Entry of inoculum into a crop and disease development in the crop cannot be prevented because spores are airborne and there is a lack of highly resistant varieties. This makes complete control of chocolate spot unlikely. It should however, be possible to improve current levels of disease control through the integration of the factors identified in the study.

Bibliography: leaves 169-185.

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Many parasitic diseases display symptoms beyond the areas invaded by the parasite itself. This suggests that toxic metabolic products of the pathogen diffusing in front of its advancing growth brings about derangements in unoccupied tissues at times far remote from the focus of infection. [...]

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The earliest reference to investigations of "black spot" in pineapples is made by Tryon (1898), who refers to the work of Dr A. A. Brown (1896) of the stock Branch, Victoria (Aus.), who sectioned diseased areas of pineapples and found fungal spores and hyphae in the tissues. The symptoms of this socalled "fruitlet core rot," are described by Tryon as: "well defined dark brown markings immediately beneath the surface, and passing inwards to a depth of 1/4" to 1/2"- the malady commencing in separate fruitlets, the central core of the fruit remaining quite healthy." (This description agrees with the symptoms referred to as "black spot" 1n this country). Subsequent culture of the spores (found by Brown) on slices of healthy fruit resulted in growth of Mucor racemosus. Brown regarded an invasion by this fungus as a primary cause of the disease. Intro. p. 1.

Germinating uredospores of race C17 of Puccinia graminis f.sp. tritici form characteristic infection structures (appressorium, infection peg, vesicle, infection hypha) in response to a 1.5 h heat shock at 29° C administered 2 h after germination at 19° C. The proportion of sporelings forming infection structures was augmented by nutrients, n-nonyl alcohol and, an appropriately timed heat shock. The heat shock temperature required to induce maximum differentiation had a very precise optimum which varied slightly for each spore lot. Variations one degree above or below this optimum reduced the percent differentiation by greater than 40%. The presence of an inhibitor of protein synthesis, puromycin, in the germination medium: (1) prevented uredosporeling differentiation but had no effect on germination, (2) significantly reduced the proportion of germtubes forming appressoria, and (3) in most cases prevented the division of germtube nuclei. It was concluded that essential differentiation-specific proteins are synthesized from the onset of germination, throughout the formation of appressoria and to the completion of differentiation. These results were consistent with the observed effects of heat shock on the rate of protein hydrolysis. During germination there was a net hydrolysis of protein leading to an increase in size of the endogenous pool of free amino acids and to an increased leakage of amino acids to the germination medium. Heat shock effectively reduced the amount of endogenous free amino acids and the extent to which amino acids were lost to the medium. It was concluded that in heat shocked sporelings protein synthesis was increased relative to protein hydrolysis by comparison with the relative rates of these two processes in germinating (non-shocked) uredosporelings. Moreover, there was no net protein synthesis during the formation of infection structures induced by heat shock. The loss of amino acids to the germination medium was selective, particularly in heat shocked sporelings. / Land and Food Systems, Faculty of / Graduate

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