Janse van Rensburg, J. B. J.Labuschagne, M. T.Klopper, John2015-10-052015-10-052008-112008-112008-11http://hdl.handle.net/11660/1324English: Since conventional sources of resistance to stem borers are not adapted to South African conditions, resistance has to be introgressed into locally adapted breeding material. Combining ability for resistance possibly differs between heterotic groups, and susceptible elite material do not necessarily respond similarly in crosses with sources of resistance from different genetic backgrounds. From earlier work conducted under greenhouse conditions it was observed that larval development rate possibly differs with difference in time of the year. The objective of this study was to determine which combinations of resistant sources with local elite susceptible material could provide the best expression of resistance. Susceptible and resistant lines were crossed in various combinations to obtain semi-resistant single and double crosses with various resistance levels. These were compared to susceptible and resistant standards in field trials. Plants were artificially infested with neonate larvae. Evaluations of leaf feeding damage, stem damage, ear damage, plant height reduction and grain yield were subjected to factorial analysis, using planting date and infestation as factors. Two resistant lines and one semi-resistant single cross were identified that seemed to provide particularly high levels of resistance. However, the study did not successfully identify which resistance source should be used in combination with which heterotic group, neither did resistance derived from different sources seem to compliment each other. Using principal component analysis of data on double crosses, no pattern could be observed that indicated crosses comprising more than one resistant parent to be more resistant than those in which only one resistant parent featured, whereas variance in resistance did not seem to relate to heterotic patterns. A further aspect of the study involved investigation into the effect of variance in planting date on the expression of resistance. Various lines and hybrid combinations with different resistance levels were evaluated in a field trial comprising five planting date treatments and artificial infestation of plants. Data were subjected to factorial analysis as above. The expression of resistance was significantly affected by planting date. The incidence of leaf feeding damage, internal plant damage, larval survival and plant height reduction decreased with an increase in planting date, whereas larval mass and the incidence of ear damage increased with later planting. It appears that increased ear damage with later planting may result at least in part from an increase in larval development rate as affected by a decreasing photoperiod.Afrikaans: Konvensionele bronne van weerstand teen mieliestamruspers is nie aangepas vir SuidAfrikaanse toestande nie en moet dus geintegreer word in plaaslik aangepaste teelmateriaal. Kombineervermoë vir weerstand verskil moontlik tussen heterotiese groepe, en vatbare elite materiaal reageer nie noodwendig eenders in kruisings met bronne van weerstand van verskillende genetiese agtergronde nie. Uit vroeëre werk wat onder glashuistoestande gedoen is, is bevind dat die larwale ontwikkelingstempo moontlik verskil met verskille in die tyd van die jaar. Die doel van die studie was om te bepaal watter kombinasies van weerstandsbronne met plaaslike elite vatbare materiaal die beste moontlike bestandheid kan lewer. Vatbare en bestande lyne is gekruis in verskillende kombinasies om semiweerstandbiedende enkel- en dubbelkruisings met verskillende weerstandsvlakke te verkry. Die kruisings is in veldproewe vergelyk met vatbare en bestande standaarde. Plante is kunsmatig besmet met pasuitgebroeide larwes. Evaluasies van blaarbeskadiging, stamskade, kopskade, planthoogteverskille en graanopbrengs is aan faktoriaalontledings onderwerp waar plantdatum en infestasie as faktore gebruik is. Twee bestande lyne en een semibestande enkelkruis is geiidentifiseer wat hoë vlakke van weerstand gelewer het. Die studie het egter nie suksesvol geiidentifiseer watter weerstandsbronne in watter kombinasies met verskillende heterotiese groepe gebruik moet word nie. Dit het ook nie kon bevestig dat weerstandsbronne uit veskillende agtergronde mekaar komplimenteer nie. Hoofkomponent analise van die data uit dubbelkruisings het geen weerstandspatrone kon aantoon nie en kruisings waarin meer as een bestande ouer gebruik is, het nie hoër weerstand gehad as kruisings waarin slegs een ouer gebruik is nie. Variasie in weerstand het ook nie met heterotiese patrone verband gehou nie. ‘n Verdere aspek van die studie het gehandel oor die effek van variasie in plantdatum op die uitdrukking van weerstand. Verskeie lyne en basterkombinasies met verskillende weerstandsvlakke is ge-evalueer in ‘n veld-proef bestaande uit vyf plantdatumbehandelings en kunsmatige besmetting van plante. Data is aan faktoriaalanalise soos hierbo onderwerp. Weerstandsuitdrukking was betekenisvol deur plantdatumverskille beiinvloed. Die insidensie van blaarbeskadiging, inwendige plantskade, larwale oorlewing en planthoogteverskille het afgeneem met ‘n toename in plantdatum, terwyl larwale massa en die insidensie van kopbeskadiging toegeneem het met latere plantdatum. Dit wil voorkom asof toenemende kopskade met latere plantdatum minstens ten dele deur ‘n toename in larwale ontwikkelingstempo in reaksie op ‘n afnemende fotoperiode veroorsaak word.enThesis (M.Sc.Agric. (Plant Sciences (Plant Breeding))--University of the Free State, 2008HeterosisCorn -- BreedingCorn -- Disease and pest resistanceStem borersPlant resistanceMaizeBusseola fuscaDissertationUniversity of the Free State