| Summary: | 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.
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