-29- 



Doraiswamy , 1975; Chang et a_l . , 1976). Dispersed virus 

 particles and spokewheel-like virus aggregations were not 

 detected by light microscopy. 



Discussion 



Infected orchid tissues examined by light microscopy 

 contained inclusions characteristic of tobamo-, potex-, cucumo- , 

 rhabdo-, and potyviruses (Table I). The diagnoses based on 

 inclusions were confirmed by electron microscopy and, for ORSV, 

 CyMV, and CMV , by fluorescence microscopy using TRITC- 

 conjugated antisera and immunodiffusion tests. Thus, like 

 other plant viruses (Christie and Edwardson, 1977), orchid 

 viruses can be reliably detected by light microscopy. While 

 other techniques (i.e., serology, bioassay, electron microscopy) 

 have been used previously for diagnosing orchid viruses 

 (Lawson and Ali, 1975; Lawson and Horst, 1984; Lawson and 

 Brannigan, in press; Corbett, 1974; Wisler and Zettler, in press), 

 this is the first systematic study using light microscopy for 

 this purpose. Potential difficulties posed by the exceptional 

 thickness of most orchid leaves in processing samples can be 

 readily overcome by using sandpaper to abrade the leaf surface 

 or by sectioning leaves. The sandpaper technique also proved 

 useful for working with geminiviruses of legumes ( Ko and 

 Christie, unpublished). In some instances, it was necessary to 

 apply mild heating and/or Triton X-100 to assure stain 

 penetration, but neither of these modifications induced 



