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Determination of the Number of Genes for Resistance to Maize Dwarf Mosaic Virus Strain A in Five Corn Inbred Lines. Eugen Rosenkranz, Plant pathologist, Agricultural Research Service, U.S. Department of Agriculture, and professor, Department of Plant Pathology and Weed Science, Mississippi State University, Mississippi State 39762; Gene E. Scott, agronomist, ARS, USDA, and professor, Department of Agronomy, Mississippi State University, Mississippi State 39762. Phytopathology 74:71-76. Accepted for publication 8 July 1983. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1984. DOI: 10.1094/Phyto-74-71.

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 two classes of plants, diseased and symptomless, are counted in two 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 three generations were used: (Resistant [R] Susceptible [S]) F2, (R S) S, and (R S) 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 zero and one resistance alleles and again when zero, one, and two resistance alleles allowed symptom expression indicated two and three genes for resistance to MDMV-A in Mp71:222 and T232, respectively. When zero, one, two, and three and again when zero, one, two, three, and four resistance alleles permitted symptoms to be expressed, disease incidence data suggested three genes for MDMV-A resistance in GA203. Data acquired both in 1981 and 1982 provided evidence for the existence of two genes for resistance to MDMV-A in AR254. Seven days after inoculation (when plants with zero, one, and two resistance alleles showed symptoms) and 14 days after inoculation (when plants with zero, one, two, and three resistance alleles showed symptoms), the numbers of diseased plants in all generations best fitted the five-gene hypothesis for MDMV-A resistance in Pa405 both in 1981 and 1982.

Additional keywords: genetics of disease resistance, Zea mays.