INHERITANCE OF RESISTANCE TO STAGONOSPORA LEAF-SPOT IN A DIALLEL CROSS OF ORCHARDGRASS

Purple leaf spot of orchardgrass (Dactylis glomerata L.) caused by Stagonospora arenaria (Sacc.) Sacc. is prevalent and damaging in the northeastern USA. Most plants in existing cultivars are moderately to highly susceptible, but an occasional plant is resistant. This study was undertaken to examine the inheritance of resistance in a 10-parent diallel cross involving five susceptible and five resistant parent plants selected from cycles 0 to 3 of a program on phenotypic recurrent selection for resistance. Parents, F1 single-cross progeny, and their reciprocals were rated for lesion size and leaf coverage in two greenhouse inoculation experiments and during two natural epidemics in a field planting. A least squares analysis for a general diallel showed significant effects for general combining ability (GCA) and specific combining ability (SCA) for lesion size and leaf coverage. Sums of squares for GCA were > 10 times larger than those for SCA indicating that additive genes were the major genetic component of resistance, but that nonadditive effects were also involved. Disease scores (high scores indicated susceptibility) of progeny from crosses of susceptible x susceptible, resistant x susceptible, or resistant X resistant parents were often greater than the midparent mean scores suggesting that there was some nonadditive dominance for susceptibility. Small, but significant, maternal effects were detected in the greenhouse experiments, and small reciprocal effects other than maternal effects were detected in the field. Two parents showed high susceptibility in the greenhouse, but intermediate susceptibility in the field. The GCA effects of these two parents were strong in the greenhouse, but weak or nonsignificant in the field indicating that these two parents and their progeny were environmentally unstable for resistance. Recurrent selection procedures that utilize additive effects would be the logical approach to developing resistant lines.