The greenhouse whitefly (Trialeurodes vaporariorum) is one of the most destructive pests of greenhouse tomato (Lycopersicon esculentum) crops. The success of biological or insecticidal control of whiteflies has never been complete. Attention has therefore been directed recently to the possibility of developing tomato cultivars resistant to whiteflies. Whiteflies were caged on leaflets of ten tomato cultivars, two of the woolly mutant lines and four other species of Lycopersicon to observe the effect of host plants within this genus on fecundity and longevity of the insects. The results showed wide variation among hosts. Significant negative correlations were revealed between the mean density of glandular hairs on the upper foliar surface of different cultivars and means for the fecundity of whiteflies caged on these cultivars. A high level of resistance to whitefly, not related to density of glandular hairs, was observed in plants that were either Van Wert's woolly mutant or L. peruvianum var. humifusum. Resistance of the former seemed to be related to a high density of branched non-glandular trichornes whereas that of the latter was not morphologically apparent. There were indications of both antibiosis and nonpreference operating in the humifusum. Further experimentation revealed a high nymphal mortality for whiteflies developing on plants of this line, reduced fecundity of adults which had developed on the humifusum, and a preponderance of male progeny from adult insects which had lived exclusively on these plants. It was concluded that the resistance of L, peruvianum var. humifusum to whitefly is sufficient to justify its use in a breeding program to develop greenhouse tomato cultivars resistant to this pest. / Land and Food Systems, Faculty of / Graduate

Laboratory experiments and some field and greenhouse studies were carried out to determine the mode of action of radiant energy in the development of a post-harvest sunscald disorder of tomato (Lycopersicon esculentum Mill c.v.Vendor) fruits. The effects of the disorder on post-harvest fruit quality and physiology were evaluated by the physical determination of textural characteristics, the analysis of some chemical constituents and the measurement of fruit gas exchange. The ability of various storage temperatures, photoperiods and oxygen concentrations to alleviate injury was also examined. The effectiveness of radiant energy in causing sunscald injury was found to depend on the amount of radiant energy in the infra-red waveband. The action of infra-red radiation was thermal rather than photochemical. The use of high air temperatures to heat fruits produced symptoms similar to sunscald except that the colour of the fruit surface became greyish rather than white. Injury can develop at all stages of ripeness but symptoms vary with stage of ripeness. Injury also develops in light in the absence of oxygen or in darkness. Photooxidation does not appear to be a prerequisite for injury development. Injurious radiation treatments caused a loss of green colour and injured fruits failed to synthesize the red pigment; lycopene, in storage. Injury did not result in any appreciable loss in lycopene content once the pigment had formed. The mode of action of intense electromagnetic radiation on fruit colour therefore appears to be by destruction of the mechanism of carotenoid synthesis rather than by increased carotenoid degradation. Injury prevented protopectin solubilization during subsequent storage. It also led to a large reduction in ascorbic acid content and an increase in pH. The changes in ascorbic acid content and pH occurred progressively during the development of injury. Changes in the textural attributes of tomato fruits were assessed by measurements of six force-deformation characteristics: Deformation, Pericarp strength, Mesocarp resistance, Compliance, Firmness and Toughness. Toughness did not significantly change during ripeness. Although the effect of injury on Mesocarp resistance was not significant, the interaction of injury with storage was significant for the characteristic. Four characteristics: Pericarp strength, Mesocarp resistance, Compliance and Firmness were highly correlated with protopectin content. Of the textural indices, Compliance was the best predictor (R² = 90.4%; SŶ = 1.97) of protopectin content at different levels of injury, irrespective of ripeness or storage. It is therefore recommended that Compliance, rather than Firmness be used for detecting changes in the "softness" of tomato fruits. "Induction" and "incipient" types of injury accelerated the onset of the respiratory climacteric. "Advanced" type of injury reduced rates of respiration during fruit ripening. The failure of injured fruits to ripen normally was not due to lack of ethylene production since rates of ethylene production were high in injured fruits. The relationship between the respiratory climacteric and ethylene production was shifted by injury. Storage treatments involving different temperatures, photoperiods and oxygen concentrations did not alleviate injury. It is suggested that efforts aimed at controlling injury should emphasize prevention of exposure to radiation rather than attempt to cure injury in exposed fruit. / Land and Food Systems, Faculty of / Graduate

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