A diallel experiment (Method 4 of Griffing's model I) was performed among nine corn (Zea mays) lines in order to estimate general and specific combining ability (GCA & SCA) to Mal de Río Cuarto (MRC) disease resistance. The selected environments were: Pergamino, Junín, Ferré (corn region VI); Sampacho and Holmberg (corn region IV). The severity of the disease, evaluated as attack average degree (AAD), was null in Pergamino and Junín, and 0.14 in Ferré, with significant differences between treatments. In Holmberg, it was 0.59 and in Sampacho it was 1.57. The analysis of variance for this region showed significant differences between environments and treatments. It did not show interactions. Average AAD of experimental hybrids did not differentiate from resistant control and it was different from commercial control (more susceptible). Twenty-five experimental materials did not differentiate from the better resistant control. Pedigree selection was effective in order to improve resistance to MRC. The single cross hybrids showed characteristics that would give high competitiveness in the seed market. The most resistant lines to MRC according to their GCA (the average performance of a line in hybrid combination) were 2526 and 2378. According to their SCA (cases in which certain combinations do relatively better, or worse, than would be expected on the basis of the average performance of the lines involved) the most resistant to MRC single cross hybrids were: 2378 x 2600, 2335 x 2600, LP561 x 2568-2 and 2526 x 2600. In Sampacho 42% of yield loss was for MRC, in Holmberg: 8%.
