Virus Diseases

Viruses and the diseases they produce are the most difficult to tackle , since our knowledge about them is far from complete, and whatever little information we have is often confusing . This is because viruses are not specific or reliable in the reactions they produce in different species of plants, eg. Cymbidium Mosaic Virus produces mosaic mottle on leaves of Cymbidium , whereas in Cattleya it produces necrotic spots (dead spots ), which is entirely different in appearance from the mosaic mottle . The reverse situation may also occur in infected plants , ie. two different viruses producing two different diseases , may induce the same lesions when they occur in the same plant. Another peculiarity of the virus disease is that , when the infection is mild , the virus may remain inside the plant for years together ,w ithout any visible symptoms , but always to be reckoned with as a dangerous sources of fresh infections . At other times the infection may be so virulent that the plants are killed within a few weeks.

Altogether about 32 virus diseases are known to occur in orchids (Jensen , 1959) . But this does not necessarily mean that orchids are susceptible to thirtytwo different kinds of viruses . The same virus may produce more than one diseae in different species of orchids . Among orchid viruses , Cymbidium Mosaic Virus (CyMV) , Odontoglossum Ring Spot Virus ( ORsV) and Dendrobium nobile Mosaic Virus are reported to infect non -orchidaceous plants as well.

CyMV , Tobacco Mosaic Virus -O (TMV-O) and ORsV are the most common among orchid viruses . Most of the cultivated genera are susceptible to these three viruses . CyMV is a flexuous rod 18m in diameter and 500 m in length as shown by Gold (1950), Gold and Jensen (1951) and Jensen and Gold (1955). TMV-O is related to ,but not the same as , the Tobacco Mosaic Virus which infects also the potato plant. It is a straight short road about 300 m in length . According to Kado et al. (1968) it is serologically the same as the TMV has a wide range of hosts whereas TMV-O infects only a limited number of plants. ORsV was identified by Paul and Wetter (1965) as a rigid short rod. According to Kado , this virus is the same as TMV-O (Northen , 1970).