Diallel analysis of spider mite resistant maize inbred lines and F1 crosses

Banks grass mite [BGM; Oligonychus pratensis (Banks)] and the twospotted spider mite (Tetranychus urticae Koch) cause significant yield reductions in maize (Zea mays L.) grown in the Great Plains states. Understanding the inheritance of mite resistance will be useful in developing mite resistant hybrids. A Griffing's Method 2 diallel mating design was used to evaluate the range of resistance and combining ability of parents in F, crosses of seven mite resistant maize lines (T1, U2, A3, A4, A5, A7, and T9). Parent lines, F-1 crosses, and three checks (Mo17, B73 X Mo17, and Pioneer hybrid 34K77) were included in a randomized complete block design with three replications at Halfway, TX, in 2002 and Lubbock, TX, in 2001 and 2002. Following artificial infestations, mite resistance was evaluated for differences in mite infestations and feeding damage. Weekly mite densities and damage samples were used to calculate total mite densities, total damage ratings (TDRs), mite per damage ratio (M/D), and a seasonal damage ratio (SDR). Total damage rating was significant for environments, genotype, and environment X genotype interaction. Total mite density (TMD) and M/D were significant for genotype and environment X genotype interaction. The SDR was significant for environment and genotype, but no interaction. Broad-sense heritability estimates showed that >90% of the TDR and SDR were associated with genotypic effects. Crosses with T1, U2, and T9 consistently supported larger mite populations, but had higher M/D values than those with A3, A4, A5, and A7, indicating greater tolerance to mite feeding. Crosses between tolerant lines or between tolerant lines and antibiotic lines had overall better resistance than crosses between antibiotic lines. Diallel analyses showed that T1 and U2 had the highest general combining ability (GCA) for reducing damage. T1 X A4, T1 X A7, A3 X A5, and A4 X A7 were the best crosses for specific combining ability (SCA) for resistance to spider mites.