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Serological Differentiation of the Citrus Tristeza Virus Isolates Causing Stem Pitting in Sweet Orange

Citrus tristeza virus (CTV) complex comprises a number of isolates or strains producing several economically important disease syndromes in commercial Citrus spp. The stem pitting syndrome is the most important, and causes substantial losses in many citrus-producing regions of the world. In an attempt to develop a serological tool to rapidly differentiate stem pitting isolates of CTV, we evaluated many combinations of trapping and detecting antibodies in an indirect double-antibody sandwich (I-DAS) enzyme-linked immunosorbent assay (ELISA). Two combinations of trapping and detecting antibodies were found suitable for differentiating stem pitting isolates in extracts of infected sweet orange plants. One used a polyclonal serum raised against bacterially expressed CTV coat protein (CP) for trapping and a conformational monoclonal antibody 3E10 for detection, and the other used two polyclonal antisera generated against bacterially expressed CTV CP. Seventy-six CTV isolates from 20 countries, including 35 that cause stem pitting in sweet orange plants, were analyzed in I-DAS-ELISA using different combinations of polyclonal and monoclonal antibodies for trapping and as intermediate detecting antibodies. The ELISA format developed produces a strong positive signal for CTV isolates that cause stem pitting in sweet orange plants and a negative ELISA signal for CTV isolates that do not cause stem pitting. When combined with data on a universal ELISA format, i.e., reacting with a broad range of CTV isolates, these selective ELISA formats allowed reliable serological differentiation of CTV isolates that caused stem pitting in infected sweet orange plants.