Browsing by Author "Smit, Renate"

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    Postharvest phytosanitary disinfestation strategies using thermal and atmospheric stress: commodity and insect tolerances
    (Stellenbosch : Stellenbosch University, 2019-12) Smit, Renate; Johnson, Shelley A.; Jooste, Mariana; Addison, Pia; Stellenbosch University. Faculty of Agrisciences. Dept. of Conservation Ecology and Entomology.
    ENGLISH ABSTRACT: South African chill sensitive horticultural products deemed for export cannot be exported using certain phytosanitary cold sterilisation regimes, without negatively affecting fruit quality. Low temperature phytosanitary treatments are required to control a variety of pests, however in some cases, cold temperature treatments are ineffective against insects that display high levels of thermal tolerance. Developing alternative phytosanitary treatments is therefore crucial, and maintaining a balance between desirable fruit quality and effective control of insect pests is an important consideration throughout the process. In the present study, the potential of two postharvest mitigation technologies were investigated to assess their potential in controlling targeted pests while maintaining fruit quality - CATTS (Controlled Atmosphere Temperature Treatment System) and ethyl formate fumigation. CATTS was investigated specifically as a potential postharvest mitigation treatment for chill sensitive plum cultivars. CATTS technology incorporates heat and atmospheric stress to control insect pests. Key phytosanitary pests of South Africa which require control include the grain chinch bug, Macchiademus diplopterus (Distant) (Hemiptera: Lygaeidae), the banded fruit weevil, Phlyctinus callosus (Schöenherr) (Coleoptera: Curculionidae) and the false codling moth, Thaumatotibia leucotreta (Meyrick) (Lepidoptera: Tortricidae). In the first part of this dissertation, different temperature treatments in combination with controlled atmosphere were tested and fruit was cold-stored using two different cold storage regimes, namely standard cold sterilisation and the dual temperature regime, to examine the effectiveness of the CATTS treatments and cold storage for phytosanitary control. Finding a balance to maintain fruit quality and kill both internal and external pests proved challenging. As a pre-conditioning benefit of heat treatments was observed during the first season, treatments were aimed at enhancing this effect during the second season, to enable the fruit to withstand low temperatures for longer periods to control internal pests. The second part of this dissertation is an in-depth investigation into the physiology of Macchiademus diplopterus. This was conducted to provide insight into the thermo-tolerant ability of this pest, as CATTS treatments were found to be ineffective for phytosanitary control. The compositional changes that occur during aestivation were examined through biochemical (macromolecules) and molecular (soluble protein identification) analyses. These were performed on the insects before entering aestivation and during the aestivation period. To examine the biochemical compositional changes the insect undergoes during thermal stresses, insects from early and mid-aestivation were treated with different CATTS treatments and cold storage regimes (cold sterilisation and dual temperature regime). The insect mortality and macromolecule content in each aestivation period provided insight into the different factors that influence its survival. A significant difference was observed in mortality and biochemical composition between early and mid-aestivating insects. Mechanisms identified that initiate defence and survival strategies during unfavourable conditions included heat shock protein and cryoprotectant synthesis. The high thermal tolerance of M. diplopterus therefore requires a different approach for phytosanitary control. The third and final part of this dissertation addresses that need. Fumigation using ethyl formate was investigated as a potential alternative to thermal treatments. The main aims were to examine, firstly, the potential of ethyl formate as a fumigant to control the M. diplopterus, and, secondly, the effect of ethyl formate on the fruit quality of selected stone and pome fruit cultivars. A central composite design (CCD) method was used to treat pome and stone fruit cultivars to assess phytotoxicity after fumigation. A range of ethyl formate concentrations and fumigation durations were tested in conjunction with various other factors such as pulp temperature, harvest maturity, time during the season in which the cultivar ripens and the effect of pre-ripening. No phytotoxic damage was observed on stone fruit. Pome fruit, in contrast, had a phytotoxic response, and the CCD model predicted fumigation limits for treatments. Ethyl formate fumigation is highly effective against M. diplopterus, providing an alternative treatment for this highly thermo-tolerant pest. Both postharvest mitigation technologies tested here provide valuable insight into the response of both the commodity and insect to the various treatments. Challenges for the application of both technologies have been elucidated, and are addressed and discussed. The research presented here represents significant steps taken towards having more effective postharvest disinfestation strategies available for phytosanitary control.