Determination of the Number of Genes for Resistance to Maize Dwarf Mosaic Virus Strain A in Five Corn Inbred Lines

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Abstract
A recently devised method was used to determine the number of genes that condition resistance to maize dwarf mosaic virus strain A (MDMV-A) in corn inbreds Mp71:222, T232, GA203, AR254 and Pa405. The basic assumption of the method is that each allele for resistance delays symptom expression by some length of time. Only 2 classes of plants, diseased and symptomless, are counted in 2 or more segregating generations to determine the number of alleles for resistance that allow symptom expression at any given time after inoculation. In this work, the following 3 generations were used: (resistant [R] .times. suscepitble [S]) F2, (R .times. S) .times. S, and (R .times. S) .times. R. For each evaluation date, the observed ratios of MDM-diseased to total number of plants were compared to the expected ratios for the number of resistance alleles allowing symptom expression by calculating chi-square values for goodness-of-fit. The observed ratios in all generations fit simultaneously the expected ratios for the proposed gene hypothesis at least twice in the course of each experiment. Data obtained when 0 and 1 resistance alleles and again when 0, 1 and 2 resistance alleles allowed symptom expression indicated 2 and 3 genes for resistance to MDMV-A in Mp71:222 and T232, respectively. When 0, 1, 2 and 3 and again when 0, 1, 2, 3 and 4 resistance alleles permitted symptoms to be expressed, disease incidence data suggested 3 genes for MDMV-A resistance in GA203. Data acquired both in 1981 and 1982 provided evidence for the existence of 2 genes for resistance to MDMV-A in AR254. Seven days after inoculation (when plants with 0, 1 and 2 resistance alleles showed symptoms) and 14 days after inoculation (when plants with 0, 1, 2 and 3 resistance alleles showed symptoms), the numbers of diseased plants in all generations best fitted the 5-gene hypothesis for MDMV-A resistance in Pa405 both in 1981 and 1982.