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Mechanisms of Resistance in Corn to Maize Dwarf Mosaic Virus. J. D. Lei, Graduate assistant, Department of Plant Pathology, University of Massachusetts, Amherst 01003; G. N. Agrios, professor, Department of Plant Pathology, University of Massachusetts, Amherst 01003. Phytopathology 76:1034-1040. Accepted for publication 3 April 1986. Copyright 1986 The American Phytopathological Society. DOI: 10.1094/Phyto-76-1034.

The mechanisms of resistance in corn to maize dwarf mosaic virus strain B (MDMV-B) were studied by using two resistant (Pa405, Bsq) and one susceptible (Ma5125) inbreds and their F1 hybrids. MDMV-B replicated to high titers and spread locally in the inoculated leaves of all genotypes, but spread systemically only in the susceptible genotype. The infectivity of virus extracted from resistant plants was comparable to that from susceptible plants, which suggests that resistance is not due to virus inhibition on inactivation. When uninoculated leaf tissues of previously inoculated resistant plants were challenge-inoculated, high concentrations of MDMV-B were produced in the challenged tissues, suggesting that no induced virus inhibitors were present in these areas. The infection process of MDMV-B was analyzed by immunofluorescent staining. The numbers of infection loci developing in mechanically inoculated leaves were higher in Bsq than in Pa405 and Ma5125, suggesting that resistance is not due to fewer infection sites. The rate of expansion of the localized infected area in Pa405 and Bsq was either higher than or equal to that in Ma5125. Many secondary infection loci developed in the susceptible inbred, whereas the asymptomatically infected areas in inoculated leaves of Pa405 and Bsq were generally continuous. In inoculated leaves of susceptible plants the virus spread faster towards the proximal than the distal end of the leaf, whereas in inoculated leaves of resistant plants the virus spread slowly and in similar rates towards both ends. The pattern of virus spread in the inoculated leaves of resistant plants suggests that the plant inhibits the virus from traveling through the leaf vascular system.