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Serologically Specific Electron Microscopy in the Quantitative Measurement of Two Isometric Viruses. Hildburg Beier, Department of Plant Pathology, University of California, Davis, CA 95616, Present address of senior author: Biologisches Institut, Universität Stuttgart, 7 Stuttgart 60, Ulmer Str. 227, West Germany; Robert J. Shepherd, Department of Plant Pathology, University of California, Davis, CA 95616. Phytopathology 68:533-538. Accepted for publication 12 September 1977. Copyright © 1978 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-68-533.

The serologically specific method for the electron microscopic detection and assay of viruses was tested for the quantitative measurement of two isometric viruses, cowpea mosaic (CpMV) and cauliflower mosaic (CaMV). For CpMV it was necessary to coat the grids with highly diluted antiserum and to use a low-salt medium to avoid aggregation and uneven distribution of virus on the grids. Under these conditions the number of CpMV particles adsorbed to the grids decreased linearly with dilution. The lowest detectable concentration was 0.01 μg/ml or 10⁹ virions/ ml, about the same threshold concentration required to produce infections on a local lesion host. In comparative trials for measurement of CpMV during multiplication in cowpea protoplasts, the serologically specific electron microscopic method and bioassays on a local lesion host gave essentially the same growth curves. For CaMV, aggregation and uneven distribution of virions on coated grids was not a serious problem, but the use of a low-salt medium reduced nonspecific adsorption of virus to carbon films. The sensitivity of the method for CaMV was similar to that for CpMV, but a linear relationship was not obtained between the number of CaMV virions adsorbed to serologically specific grids and dilutions of crude extracts of virus-infected plants. The number of CaMV virions adsorbed to the grids at low sap dilutions was less than expected, which indicated that the virus was not released readily from inclusion bodies under those conditions. Hence the method cannot be used for the quantitative measurement of CaMV in crude extracts of infected plants.