Genetic Analysis of Resistance to the Corn Virus Disease Complex

Five highly resistant (R) and five highly susceptible (S) inbred lines of corn (Zea mays L.) were evaluated in a diallel set of 45 single crosses to study the inheritance of resistance to the corn virus disease complex. The entries were evaluated under conditions of mechanical inoculation at Knoxville and natural infection at Waverly, Tenn. Evaluations were made prior to flowering and again on fully grown plants. Highly significant differences for mean severity ratings among genotypes were obtained in all experiments. Correlations between ratings in the two locations and for both times of rating were highly significant for both the inbreds and the hybrids. R ✕ R crosses were highly resistant in all evaluations while S ✕ S crosses were highly susceptible. Mean ratings for R ✕ S crosses were significantly lower than the mean of R ✕ R and S ✕ S crosses and the mean of all crosses indicating partial dominance of resistance. General combining ability (gca) and specific combining ability (sea) mean squares were highly significant in all evaluations but gca mean squares were substantially larger than the sea mean squares. Inbred T232 had the largest negative gca effect with both times of rating, followed in order by Tx601, Ky226, Ga209, and Mo18W. By contrast, inbred CI872 had the largest positive gca effect on susceptibility while inbred T105 had the least at both times of rating. Vr, Wr graphs and components of variance analyses of the data provided similar results to those with combining ability analysis. Among the resistant inbreds, T232 indicated the most dominant genes for resistance when tested in both locations and with both times of rating while Mo18W indicated the least number. Inbreds Tx601, Ga209, and Ky226 varied in the two locations and time of rating. As many as four major genes for resistance were indicated to be present in the parental inbreds. Hybrids having high resistance are obtainable from crosses involving these resistant inbreds. Since some degree of dominance was indicated, a backcrossing procedure to transfer the resistance factors into elite lines is suggested. Some form of recurrent selection in populations having a degree of resistance also is suggested. Host reaction and the nature of the genetic variation governing resistance showed that the inbreds used in this study reacted in a similar manner to the virus complex under all conditions of testing. The data also indicate that evaluations for virus resistance can be made prior to flowering and that breeders can make pollinations concurrent with evaluation.