Os vírus estão entre os principais problemas sanitários das orquídeas, no entanto muito pouco se sabe sobre eles em nossas condições. Em função disso, realizou-se o presente trabalho, que teve como objetivos principais identificar quais são os vírus que estão infectando as principais espécies e/ou híbridos de orquídeas no Estado de São Paulo; quantificar a incidência dessas viroses em cultivos comerciais e de coleção do estado e avaliar o efeito do virazole combinado com o cultivo de meristemas "in vitro" para a obtenção de plantas de Cymbidium livres de vírus. Como resultados foram encontrados os vírus do Mosaico do Cymbidium ("Cymbidium Mosaic Virus"-CYMV) da mancha anelar do Odontoglossum ("Odontoglossum Ringspot Virus"- ORSV) e partículas de Rhabdovirus nas amostras testadas. Outros três vírus testados não foram encontrados. As incidências dos principais vírus eram maiores proporcionalmente à idade das plantas em função da quantidade de tratos culturais que elas recebiam. A associação entre as técnicas de cultura de tecidos e quimioterapia foi eficiente na eliminação do ORSV de plantas de Cymbidium infectadas

Viruses are one of the main problems of orchids troughtout the world. Although some of them have already been reported in Brazil, very little is known about which are the prevalent, their incidences and the best strategies for their control. The present work was carried out with the purpose to answer some of these questions. Plants from different species and varieties of orchids from the orchid collection at the Escola Superior de Agricultura ''Luiz de Queiroz"/Universidade de São Paulo, Piracicaba, SP (ESALQ/USP) and from private productions from 16 localities in the State of São Paulo were surveyed. A total of 1773 plants (315 from the ESALQ collection and 1458 from commercial productions), were tested by PTA-ELISA with antisera against the following viruses: Odontoglossum ringspot virus (ORSV), Cymbidium mosaic virus (CyMV), Cucumber mosaic virus (CMV), Cymbidium ringspot virus (CyRSV) and Vanila mosaic virus (VMV). For the last vírus, VMV, it was used the antiserum against Dasheen mosaíc virus (DsMV), since these two viruses are closely related. ORSV and CyMV were found to be the most ímportant viruses infecting orchids in the State with an average incidence around 10%. ln a few plants of Cymbidium, Miltonia, Oncidium, Phaius and Xylobium, there were also found particles of a rhabdovirus, not yet characterized. None of the samples were infected with CMV, CyRSV and DsMV. The high incidence of naturally virus-free plants occurring in the production areas shows that virus-free stock plants are not difficult to obtain, since the plants are properly indexed. However, if any particular high value species or variety is 100% infected by a vírus, its elimination can be obtained by means of the association between meristem tip culture and chemotherapy, as it was demonstrated to Cymbidium Ayako Tanaka 'Hinodezuru' infected with ORSV. The chemotherapeutic agent tested was the virazole and it was added to the meristem culture medium at concentration of 15,20 and 25 ppm or to subcultures media at 20, 25 and 30 ppm. Although almost all treatments were effective, the highest porcentage of vírus-free plants were obtained when the virazole was added to the subculture media. Treatment containing 30 ppm of virazole filtered to the subcultures media gave 100% of virus-free Cymbidium