Genetics of resistance to Stenocarpella maydis ear rot of maize
Rossouw, Johannes Daniël
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The objective of this study was to determine if germplasm available in South Africa and elsewhere, have sufficient variation in resistance to be used for the improvement of resistance to Stenocarpella maydis, and to assess the viability of using recurrent selection as a breeding method under conditions of artificial infection with the pathogen. The study attempted to provide a better understanding of effective methods to screen for resistance without sacrificing other favorable agronomic traits. The inheritance and combining ability of resistance, as well as genetic and phenotypic correlation of resistance parameters with other characteristics, were investigated. The study comprised of three experiments. The first experiment was a complete diallel cross containing 10 commercial and experimental inbred lines, ranging from resistant to susceptible. Five South African developed inbreds, four USA developed and one Brazilian inbred were used. The diallel was evaluated during the 1999/2000 season at three environments across the South African maize growing area. Plants were artificially inoculated twice, two weeks prior to anthesis. Different S. maydis related and agronomic characteristics were measured. The second experiment was a trial containing both South African developed hybrids and hybrids derived from crosses between South African developed germplasm and those of exotic and temperate origin. The trial was evaluated at one environment in the South African maize growing area. Again, plants were artificially inoculated twice, two weeks prior to anthesis. Flowering data were taken along with S. maydis incidence and grain moisture at harvest. The third experiment involved the evaluation of a recurrent selection program, using three previously identified resistant inbreds as donor parents for the development of new genotypes with superior resistance to S. maydis. Selection was done on an ear-to-row basis over three years, using the segregating progenies derived from crosses between the three inbreds. Highly significant differences were obtained between genotypes for all characteristics measured, indicating variation for resistance in germplasm used in local breeding programs. Across all three environments, the general combining ability (GCA) and specific combining ability (SeA) effects were significant. Inbreds D0620Y, MON1 and F2834T had the greatest negative GCA effects for S. maydis related characteristics that contributed towards resistance, with MONI being heterotically unique, combining to all the other inbreds. PI combinations with inbred B37 had the greatest negative SeA effects of all genotype combinations evaluated. GCA:SCA ratio's indicated that resistance is controlled by additive gene effects, with low dominance and interaction effects. Genetic correlations existed between the S. maydis related characteristics, with very high heritability and high correlated response between them. Due to it's simplicity, percentage rotted ears was the best method to measure resistance. Upright ears were found to result from S. maydis infection, rather than to predispose ears to the disease. Significant differences were found between maturity classification and S. maydis incidence for South African developed germplasm crossed to temperate and tropical germplasm. Later flowering hybrids showed less S. maydis infection compared to earlier hybrids. Using resistant parents in breeding populations, superior genotypes could be selected by applying artificial inoculation and recurrent selection on an ear-torow basis.