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Comparison of Diagnostic Techniques for Determining Incidence of Ratoon Stunting Disease of Sugarcane in Florida. M. J. Davis, Agricultural Research and Education Center, Institute of Food and Agricultural Sciences, University of Florida, 3205 S.W. College Avenue, Fort Lauderdale 33314. J. L. Dean, USDA, ARS, U.S. Sugarcane Field Station, Canal Point, FL 33438.. Plant Dis. 68:896-899. Accepted for publication 21 April 1984. Copyright 1984 The American Phytopathological Society. DOI: 10.1094/PD-68-896.

Fluorescent-antibody staining was more accurate for detecting the RSD bacterium in xylem sap extracts from sugarcane than was phase-contrast microscopy or isolation in culture. Concentration of fluorescent antibody-stained bacteria on the surface of polycarbonate membrane filters (0.2-μm pore-size) by filtration before examination by epifluorescent microscopy resulted in a sevenfold to eightfold increase in sensitivity of detection compared with the usual method of staining dried sap samples on the surface of microscope slides. When phase-contrast microscopy, isolation in culture by the dilution-plate technique, and fluorescent-antibody stains on membranes (FASM) were compared, 95 of 120 (79%) samples from 20 commercial sugarcane fields were determined to be infected with the ratoon stunting disease (RSD) bacterium by at least one of these diagnostic techniques, and about 20% more infections were detected by FASM than by the other two techniques. The frequency of detection of the RSD bacterium in the commercial fields by FASM was 18% greater on 26 October 1982 than on 1 September 1982, with 53 of 60 (88%) samples having bacteria in October. Only plant crops were examined and the RSD bacterium was detected in plants from all fields. Both incidence of RSD and the average population of the RSD bacterium in sap extracts varied among sugarcane clones.