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The Mdm1 Locus and Maize Resistance to Maize dwarf mosaic virus

Previously, Mdm1, a gene controlling resistance to Maize dwarf mosaic virus (MDMV), was identified in the inbred line Pa405. The gene was tightly linked to the restriction fragment length polymorphism marker umc85 on the short arm of chromosome 6. This chromosomal region is also the location of resistance genes to two other viruses in the family Potyviridae, Sugarcane mosaic virus (SCMV) and Wheat streak mosaic virus (WSMV). A diverse collection of 115 maize inbred lines was evaluated for resistance to MDMV and SCMV, and for MDMV resistance loci on chromosome 6S. Forty-six resistant inbred lines were crossed to three MDMVsusceptible inbred lines to develop F₂ populations. The F₂ populations were inoculated with MDMV and scored for infection and symptom type. Environmental factors influenced both the rate and type of symptom development. Bulked segregant analysis of each F₂ population indicated that, in 42 of 43 MDMV-resistant lines, chromosome 6S markers found in the resistant parent also were present in the bulked resistant but not the susceptible tissue. Markers previously associated with resistance to both SCMV and WSMV on chromosome 3 and to WSMV on chromosome 10 were associated with resistance in nine and seven of the F₂ populations, respectively. These data suggest that Mdm1 or closely linked genes on chromosome 6S are associated with MDMV resistance in most germplasm, but that other loci also may affect resistance.