Resistance to Maize Dwarf Mosaic and the Corn Virus Disease Complex in Synthetic Populations of Dent Corn and Sweetcorn

Phenotypic recurrent selection for resistance in corn (Zea mays L.) to maize dwarf mosaic (MDM) and the com virus disease complex was conducted for five cycles in synthetic populations of dent com and sweet com. Selection was carried out concurrently at two locations under a natural epiphytotic of the com virus disease complex of MDMV and maize chlorotic dwarf virus (MCDV) near Waverly, Tennessee and under an artificially induced epiphytotic near Knoxville, Tennessee created by mechanical inoculation with MDMV and transplanting of infected host plants. Resistant plants were selected and interpollinated concurrently and only apparently symptomless plants were harvested. It was of interest to determine the effect of successive cycles of selection as well as the effect of selection under different disease conditions at two locations. Each cycle of selection at both locations in both synthetics was evaluated for virus reaction by determining the number of diseased plants and the severity of infection of the diseased plants. Host reaction to virus infection is largely quantitative, and genotypes with the same percentage of diseased plants may still vary in resistance because of differences in the severity of infection. Evaluation of the dent populations showed no improvement for virus reaction from C0 to C4 at either location. The Waverly selections had significantly fewer diseased plants than the Knoxville selections at an early rating representing MDMV infection. Evaluation of 100 S₁ random selections from the C0 and C3 dent populations showed greater variability for virus reaction in C3. Because hybrid vigor seems to enhance virus tolerance in susceptible and resistant genotypes, it may be that the variability of C3 was due to increasing the number of resistant selections and to greater expression of virus reaction in the susceptible selections due to inbreeding depression. Selection in the sweet com synthetic resulted in reduction in the number and severity of diseased plants at both locations when evaluated at Waverly. The Waverly selections were more resistant than the Knoxville selections. Lack of response to selection in the dent synthetic may have been due to reduction in genetic variability for resistance, inbreeding depression, changes in disease pressure during cycles of selection, or to the inability to identify S₀ plants with high gene frequencies for resistance because of the confounding effects of hybrid vigor and virus resistance in heterozygous plants. Selection among S₀ individuals may have favored heterozygous genotypes and maintained undesirable alleles at higher frequencies than expected.