Postharvest phytosanitary disinfestation strategies using thermal and atmospheric stress: commodity and insect tolerances

Smit, Renate (2019-12)

Thesis (PhDAgric)--Stellenbosch University, 2019.

Thesis

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.

AFRIKAANSE OPSOMMING: Suid-Afrikaanse koue-sensitiewe, hortologiese produkte wat bestem is vir uitvoer, kan nie met sekere fitosanitêre koue-sterilisasie metodes uitgevoer word sonder om die vrugkwaliteit te benadeel nie. Lae-temperatuur fitosanitêre behandelings word vereis om ‘n verskeidenheid plae te beheer, maar in sekere gevalle is koue-temperatuurbehandelings oneffektief teen insekte wat hoë vlakke van termiese verdraagsaamheid toon. Die ontwikkeling van alternatiewe fitosanitêre behandelings is dus noodsaaklik en die balans tussen gewenste vrugkwaliteit en effektiewe beheer van insekplae is ‘n belangrike oorweging gedurende die proses. In hierdie studie is die potensiaal van twee na-oesplaagbestuurtegnieke ondersoek om hul potensiaal in die beheer van teikenplae, terwyl vrugkwaliteit behou word, te evalueer – CATTS (Gekontroleerde Atmosfeer Temperatuur behandelingstelsel) en etielformaat beroking. CATTS is spesifiek ondersoek as ‘n potensiële na-oesplaagbestuurbehandeling vir koue-sensitiewe pruimkultivars. CATTS-tegnologie integreer hitte en atmosferiese stres om insekplae te beheer. Belangrike fitosanitêre plae van Suid-Afrika wat beheer vereis, sluit in die graan stinkluis, Macchiademus diplopterus (Distant) (Hemiptera: Lygaeidae), die gebande vrugtekalander, Phlyctinus callosus (Schöenherr) (Coleoptera: Curculionidae) en die valskodlingmot, Thaumatotibia leucotreta (Meyrick) (Lepidoptera: Tortricidae). In die eerste deel van die studie is verskillende temperatuurbehandelings, in kombinasie met beheerde atmosfeer, getoets en vrugte was by lae temperatuur gestoor deur twee verskillende kouestoortegnieke te gebruik, naamlik standaard koue-sterilisasie en die dubbele temperatuur regime, om die effektiwiteit van die CATTS-behandelings en kouestoor vir fitosanitêre beheer te ondersoek. Om ‘n balans te vind om vrugkwaliteit te behou en beide interne en eksterne plae dood te maak, was uitdagend. Omdat ‘n kondisioneringsvoordeel van hittebehandelings waargeneem is tydens die eerste seisoen, was behandelings in die tweede seisoen daarop gemik om hierdie effek te versterk, om die vrugte in staat te stel om vir langer periodes lae temperature te kan weerstaan om interne plae te beheer. Die tweede deel van die studie is ‘n in-diepte ondersoek in die fisiologie van Macchiademus diplopterus. Dit was uitgevoer om insig te verskaf oor die hitte verdraagsaamheidsvermoë van die plaag, omdat daar gevind is dat CATTS-behandelings oneffektief is vir fitosanitêre beheer. Die samestellingsveranderings wat plaasgevind het tydens estivasie, is ondersoek deur biochemiese (makromolekules) en molekulêre (oplosbare proteïen identifikasie) analise. Dit is uitgevoer op die insekte voordat estivasie aanvang geneem het en tydens die estivasieperiode. Om die biochemiese samestellingsveranderinge wat die insek ondergaan tydens termiese stres te bepaal, is insekte van vroeg en mid-estivasie behandel met verskillende CATTS-behandelings en kouestoortegnieke (koue-sterilisasie en dubbele temperatuur regime). Die insekmortaliteit en makromolekuulinhoud in elke estivasieperiode het insig gelewer in die verskillende faktore wat oorlewing beïnvloed. ‘n Betekenisvolle verskil is waargeneem in die mortaliteit en biochemiese samestelling tussen vroeë en mid-estivasie insekte. Meganismes wat geïdentifiseer is wat verdedigings- en oorlewingstrategieë inisiëer tydens ongunstige toestande, sluit in hitteskokproteïen- en kouebeskermingsintese. Die hoë termiese verdraagsaamheid van M. diplopterus benodig dus ‘n ander benadering vir fitosanitêre beheer. Die derde en finale deel van die studie spreek hierdie behoefte aan. Beroking met etielformaat is ondersoek as ‘n potensiële alternatief tot termiese behandelings. Die hoof doelwitte was om, eerstens, die potensiaal van etielformaat as ‘n berokingsmiddel om M. diplopterus te beheer en, tweedens, die effek van etielformaat op die vrugkwaliteit van bepaalde steen- en kernvrugkultivars, te ondersoek. ‘n Sentrale saamgestelde ontwerp (SSO) metode is gebruik om kern- en steenvrugkultivars te behandel om fitotoksisiteit na beroking te evalueer. ‘n Reeks etielformaatkonsentrasies en berokingstye is, in samewerking met verskeie ander faktore soos pulptemperatuur, oesrypheid, tyd gedurende die seisoen waartydens die kultivar ryp word en die effek van rypmaking, getoets. Geen fitotoksiese skade is op steenvrugte waargeneem nie. Kernvrugte, in teenstelling, het ‘n fitotoksiese reaksie getoon en die SSO model het berokingslimiete vir behandelings voorspel. Etielformaat beroking is baie effektief teen M. diplopterus en verskaf dus ‘n alternatiewe behandeling vir die hoogs termies verdraagsame plaag. Beide na-oesbestuurtegnieke wat hier getoets is, verskaf waardevolle insig in die reaksie, van beide die kommoditeit en insek, tot die verskeie behandelings. Uitdagings vir die toediening van beide tegnieke is toegelig en is aangespreek en bespreek. Die navorsing wat hier aangebied word, verteenwoordig beduidende stappe wat geneem is om meer effektiewe na-oes disinfestasie strategieë beskikbaar te stel vir fitosanitêre beheer.

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