Potential efficacy of mite resistant genes in corn topcrosses and improvement of resistance in crosses with specific resistance traits
Bynum, Edsel D
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Spider mites significantly reduces yield of maize, Zea mays L., grown in the Great Plains States. Determining the genetic performance of mite-resistant lines is important in developing resistant hybrids. Studies were conducted at either Lubbock, TX or Halfway, TX in 1997, 1998, 2001, and 2002 to evaluate the efficacy of resistance and agronomic performance of testcrosses with B73 and Mo 17. Resistant inbred lines (SI, S2, S3, S4, S5, S7, and S9), testcrosses, check inbred lines (B73 and Mo 17), and check hybrids (B73 X Mo 17 and Pioneer 34K77) were arranged in randomized block design with three replications. Analysis showed that the effects of environment, entry, environment x entry interaction was significant for mite density. Environment and entry effects were significant for damage ratings and mite per damage (M/D). Heritability on a mean basis was 73%, 80%, 86%, and 87% for total mite density, M/D, total damage rating, and seasonal damage ratio, respectively. Testcrosses had less damage than check hybrids. Testcrosses (SI x B73, S2 x B73, S5 x B73, S2 x Mol7, and S3 x Mol7) significantly reduced damage, and S7 x B73 and S7 x Mol7 had highly significant reductions in damage. All testcrosses, except SI x Mol7, S5 x Mol7, and S9 x Mol7, had grain yields statistically similar to check hybrids. A Griffmg's diallel mating design was used to evaluate range of resistance, combining ability and agronomic traits of resistant F1 crosses. Analysis of DR was significant for environments, entry, and environment x entry interaction. Mite density and M/D was significant for entry x environment interaction. Seasonal damage ratios were significant for environment and entry, but not interactions. Heritability showed that > 90% of the damage was associated with genotypic effects. Crosses with SI, S2, and S9 had more mites, but had higher M/D values than S3, S4, S5, and S7, indicating greater tolerance to mite feeding. Crosses between tolerant lines or tolerant x antibiotic lines resulted in better resistance than crosses between antibiotic lines. S1 and S2 had the highest general combining ability for reduce damage. SI x S4, SI x S7, S3 x S5, and S4 X S7, were the best crosses for specific combining ability for resistance to spider mites.