1.13.18
EVIDENCE THAT ORCHID FLECK VIRUS IS EFFICIENTLY TRANSMITTED IN A PERSISTENT MANNER BY THE MITE BREVIPALPUS CALIFORNICUS

T MAEDA, H KONDO, K MITSUHATA and T TAMADA

Research Institute for Bioresources, Okayama University, Kurashiki 710- 0046, Japan

Background and objectives
Orchid fleck virus (OFV) [1] causes serious damage to orchid production. It has been reported in Japan [1] and also occurs in Australia, Brazil, Germany, Korea and USA. OFV is sap-transmissible to several species of orchids and a few dicotyledonous plants, but it is difficult. Partially purified virions are non-enveloped and bacilliform, 150 nm long and 40 nm in diameter. OFV is classified tentatively in the family Rhabdoviridae. Natural infection of OFV in Calanthe plants in the greenhouse has suggested that OFV might be transmitted by the vector. We have obtained evidence that OFV is efficiently transmitted by the mite Brevipalpus californicus.

Materials and methods
The virus isolate of OFV was obtained from a naturally infected Cymbidium plant [2] . The mites, B. californicus, used in this study were reared on infected Cymbidium plants or detached leaves of tea plants. Tetragonia expansa and Phaseolus vulgaris were used as inoculation test plants. Groups of 5-10 mites were usually inoculated to test plants and given an inoculation access period of 2-3 days. OFV infection of inoculated and upper leaves was confirmed by sap-inoculation to T. expansa or by electron microscopy.

Results and conclusions
When B. californicus reared on the infected plants were transferred to leaves of T. expansa or P. vulgaris, clear yellow spots appeared on the mite-inoculated leaves after 2 weeks. Number of yellow spots on inoculated leaves increased with increasing the number of mites or the length of the inoculation access period. The mite Tetranychus urticae, the whitefly Bemisia argentifolii and the aphid Myzus persicae could not transmit the virus. OFV was efficiently transmitted by nymphs (mixture of first and second nymphs) and adult mites, but not by larvae. The mites transmitted the virus after moulting. The transmission ability of the mites was also retained after the viruliferous mites had been maintained on the leaves of non-susceptible plants for 3 weeks, indicating that the transmission pattern is persistent. Viruliferous mites were able to transmit OFV to 10 plant species in 8 families; especially Calanthe, Cymbidium, Dendrobium and Phalaenopsis orchids were susceptible. Most inoculated plants showed local lesion symptoms, but some plants produced systemic vein necrosis or necrotic spots. Immunoelectron microscopy and Western blot analysis revealed that OFV particles and its structural proteins were detected in extract of the viruliferous mites. These results indicate that OFV is efficiently transmitted by the mite B. californicus, and suggest that OFV has a wide host range.

References
1. Doi Y, 1977. CMI/AAB Descriptions of Plant Viruses No. 183.
2.Kondo H, Matsumoto J, Maeda T, Inouye N, 1995. Bulletin of the Research Institute for Bioresources, Okayama University 1, 21-34.