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Incidence of Soilborne Wheat Mosaic Virus and Its Reported Vector, Polymyxa graminis, in Field-Grown Soft Red Winter Wheat. P. T. Himmel, Research Microbiologist, Agricultural Research Service, U.S. Department of Agriculture, Crop Protection Research Unit and Department of Plant Pathology, University of Illinois, Urbana 61801. A. D. Hewings, and D. A. Glawe. Research Plant Pathologist, Agricultural Research Service, U.S. Department of Agriculture, Crop Protection Research Unit and Department of Plant Pathology; and Associate Professor, Department of Plant Pathology, University of Illinois, Urbana 61801. Plant Dis. 75:1008-1012. Accepted for publication 3 April 1991. 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, 1991. DOI: 10.1094/PD-75-1008.

The incidence of soilborne wheat mosaic virus (SBWMV) and Polymyxa graminis in four resistant (Hart, Purdue, IL 87-7394, and IL 85-2655) and four susceptible (Cardinal, Rosette, Michigan Amber, and Maryland 75-266-46) soft red winter wheat cultivars was investigated in field experiments during 19891990. Soilborne wheat mosaic virus antigen in roots and shoots was detected by enzyme-linked immunosorbent assay (ELISA). P. graminis root infections were assessed with bright field microscopy. Both resistant and susceptible cultivars became infected with the virus shortly after planting in the fall. Virus antigen was detected by ELISA in shoots of all resistant and susceptible plants until dormancy. When growth resumed in the spring, detectable viral antigen was significantly (P ? 0.05) lower in the shoots of resistant cultivars. In contrast, the detectable antigen in roots of resistant cultivars was significantly (P ? 0.05) lower than in susceptible cultivars for almost the entire 19891990 field season. The lower incidence of SBWMV in roots was associated with fewer shoot infections after dormancy. Although not statistically different, the number of resting spores of P. graminis per centimeter of root was generally lower in resistant than in susceptible cultivars. The results of this study suggest that mechanisms of resistance to soilborne wheat mosaic may involve a reduction in rates of virus particle assembly, movement, and/or replication.