Diallel Analysis for Reaction to Exserohilum Turcicum of Maize Cultivars and Crosses

The effectiveness of a diallel cross in initiating recurrent selection as a breeding procedure for concentrating genes for resistance to Exserohilum turcicum (Pass) Leonard and Suggs was studied in maize single and double cross F1's during the first rains of 1993. Resistance was expressed as reduced percentage leaf area blighted, area under disease progress curve of percentage leaf area blighted (AUDPC-S) and lesion numbers (AUDPC-L), and apparent infection rate (r), indicating presence of rate-reducing resistance. General combining ability effects differed with the genotype, cross and disease indices, resistant cultivars giving the highest negative values, and the single cross giving higher negative values than their corresponding double cross gemstypes. High heritability values were obtained for both types of crosses, signifying the highly heritable nature of this polygenic resistance and the low level of resistance present in the parent cultivars.L'efficiencience d'un croisement diallele en selection recurrante comme procedure d'amelioration en vue de concentrer des genes de resistance a l'Exserohilum turcicum etait etudiee en croisement simple et double de mais de F1's durant la premiere saison pluvieuse en 1993. La resistance etait exprimee en fonction de la reduction du pourcentage de feuilles attaquees, de la courbe de progression de la surface malade, du pourcentage de feuilles attaquees (AUDPC-S) et du nombre de lesions (AUDPC-L), ainsi que du rapport d'infection apparante (r) indiquant la presence d'un rapport de reduction de resistance. Les effets generaux d'habilite combinee etaient differents selon le genotype, le croisement et les incidences de la maladie. Les cultivars resistants donnent des hautes valeurs negatives tandis les croisements simples donnent les valeurs negatives plus elevees que leurs correspondants genotypes a croisements doubles. Des valeurs elevees d'heritabilite etaient obtenues pour les deux types de croisement, ce qui explique la nature hautement heritable de cette resistance polygenique et le faible niveau de resistance present chez les parents de cultivars