-3- 

 Electron microscopy of negatively stained samples also has 

 limitations, particularly for isometric viruses such as 

 Cymbidium ringspot and/or cucumber mosaic viruses which have 

 similar particle morphologies. Electron microscopy of thin 

 sections is not also applicable for routine diagnosis due to 

 the time, labor, and expense involved in processing samples. 



The purpose of this study was to explore the potential of 

 light microscopy as a method for detecting and diagnosing 

 orchid viruses. Immunofluorescence microscopy, protein A-gold 

 labelling, VaMo negative staining, thin sectioning, 

 immunodiffusion and slicing bioassay (Appendix I) techniques 

 were used to verify the results of light microscopic diagnoses. 



Christie and Edwardson (1977) have described light 

 microscopic techniques as being very useful for diagnosing 

 plant viruses at the group level and, in some instances, at the 

 specific level. This system has also been shown to be useful 

 for diagnosis of viruses infecting certain crops, such as 

 peppers (Edwardson and Christie, 1979). The low cost of the 

 basic equipment and the relatively short time involved in 

 processing samples are some advantages of light microscopy. 

 However, certain limitations must be overcome before it can be 

 applied to crops such as orchids. Unlike many herbaceous 

 plants, such as peppers, most orchid leaves are thick and 

 leathery, and previously described techniques, such as 

 epidermal peels (Hiebert et al . , 1984), are difficult to 

 obtain. For orchids, therefore, special modifications must be 



