Variations in Serologically Detectable Antigen of Soybean Dwarf Virus in Soybean Leaflets as a Function of Time After Inoculation and Plant Age. A. D. Hewings, Research Plant Pathologists, U.S. Department of Agriculture, Agricultural Research Service, Foreign Disease-Weed Science Research Unit, Frederick, MD 21701. V. D. Damsteegt, A. E. Sindermann, and S. A. Tolin. Research Plant Pathologists, Biological Laboratory Technician, U.S. Department of Agriculture, Agricultural Research Service, Foreign Disease-Weed Science Research Unit, Frederick, MD 21701; and Professor, Department of Plant Pathology, Physiology and Weed Science, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061. Plant Dis. 74:844-848. Accepted for publication 12 March 1990. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/PD-74-0844.

Two enzyme-linked immunosorbent assay (ELISA) systems were developed for detection of soybean dwarf virus (SDV) and compared for sensitivity. With direct double-antibody sandwich (DAS) ELISA, dilution of conjugated immunoglobulins with uninfected tissue extracts prepared with phosphate buffered saline (PBS) (1:5, w/v) improved absorbance compared with conjugates prepared with PBS alone. The apparent amplification of the positive signal occurred without changing nonspecific (background) absorbance. Indirect antigen-coated ELISA detected the dwarfing strain (SDV-D) but was much less sensitive than DAS-ELISA. DAS-ELISA was used to measure and compare the concentration of SDV antigen in canopy tissues of Wayne soybean harvested 11, 19, 26, 33, and 40 days after initiation of the inoculation access period. At all test dates, aphid-inoculated unifoliolates contained the highest concentration of SDV antigen, and a trend toward higher antigen concentrations in younger tissues was observed. SDV antigen concentration peaked at 19 days and declined thereafter. All soybean test seedlings inoculated 7 and 14 days after emergence became infected. When inoculation occurred at 28 days, only 48% of the plants became infected with the yellowing strain (SDV-Y) and 12% with the SDV-D strain.