PATTERNS OF HETEROSIS IN CROSSES BETWEEN AFRICAN STALK BORER RESISTANT AND ELITE MAIZE GENOTYPES

• Main Author: Klopper, John
• Other Authors: Prof MT Labuschagne
• Format: Others
• Language: en-uk
• Published: University of the Free State 2010
• Subjects: Plant Sciences
• Online Access: http://etd.uovs.ac.za//theses/available/etd-02152010-141000/restricted/

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.