MAIZE RESPONSES TO A SEVERE ISOLATE OF MAIZE CHLOROTIC DWARF VIRUS

The results of incidence ratings to assess the host response of maize (Zea mays L.) to natural infection, or to controlled inoculation in ei ther the field or the greenhouse, with maize chlorotic dwarf virus (MC DV) appear inconsistent for identification of resistant or tolerant re sponses. Our objective was to determine whether assessment of the seve rity of three symptoms on the leaves (veinbanding, chlorosis, and twis ting of the leaf and tearing of the leaf margin) would consistently al low differentiation of the host responses of maize inbreds and hybrids to inoculation with two isolates of MCDV. Symptom severity was assess ed using a 1 to 5 visual rating for each of the three symptoms at thre e to four time intervals after controlled inoculation with a severe is olate of MCDV in both greenhouse and field experiments. Analysis of th e symptom severity assessments grouped genotypes into similar host res ponse categories in both environments. Incidence ratings of genotypes in greenhouse studies were not significantly different, and field inci dence ratings were inconsistent. Height reduction in greenhouse-grown plants was most severe in genotypes with high levels of expression of the other symptoms but was of limited value in differentiating genotyp ic groups. Comparison of the average symptom severity assessments usin g the severe and type isolates indicated that symptom expression in he ld-grown plants was more severe when inoculated with the severe isolat e of MCDV. Regardless of virus isolate, symptoms of plants grown in th e held were not as severe as those on plants grown in the greenhouse. A reduction in symptom severity was associated with hybrid vigor of fi eld-grown plants. In one field study, the level of symptom severity wa s correlated with both yield and plant height reduction of hybrids.