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could be undetected in studies confined to thin sections. This 

 may be attributed to relatively low virus titers in the orchid 

 host . 



The use of light microscopy has special advantages for 

 diagnosing orchid viruses. Many orchid viruses, such as the 

 rhabdoviruses , are poorly characterized, and using alternative 

 techniques for their diagnosis is currently either impractical 

 or unreliable. Light microscopy can be used to detect multiple 

 infections. For example, a Phalaenopsis , which was received 

 for diagnosis, was determined to be triply infected with ORSV, 

 CMV, and a potyvirus. The presence of one virus does not 

 interfere with other viruses with regard to types of inclusions 

 induced. These observations have also been noted for multiple 

 infections of other crops ( Edwardson and Christie, 1979; Russo 

 and Martelli, 1973) . 



Light microscopy proved to be a reliable method for diagnosis 

 of orchid viruses when compared with electron microscopy of 

 negatively stained exudates (Lawson and Brannigan, in press) and 

 immunodiffusion techniques (Wisler et a_l. , 1982). Fifteen of 30 

 samples (Appendix III) gave the same results with all three 

 techniques. Nine of these were infected with CyMV, ORSV or both 

 viruses; the other six were negative. Infections of either a 

 rhabdovirus or a potyvirus were detected by light microscopy alone 

 in 11 of these samples. In 2 samples, ORSV was detected by 

 serology, but not light microscopy. Additional tests indicated 

 this discrepancy was probably due to a sampling error. 



