Many viruses are spread by insect vectors like thrips, aphids, and mites. However, the two most common viral pathogens found in orchids, Cymbidium mosaic virus (CymMV) and Odontoglossum ringspot virus (ORSV), are not transmitted or spread by insects. Instead, the role of the vector is played by orchid caretakers with all their tender, loving care. Viruses can be spread by primping and preening with dirty tools (known as mechanical transmission), reusing dirty pots and potting media, and not cleaning hands or work surfaces. Virus particles live in plant cells and are released when the leaf is cut into or wounded during any kind of orchid maintenance or division. Both CymMV and ORSV are very stable pathogens that can live outside of an orchid for very long periods of time. A closer look at both of these pathogens will now be taken.
© Dr. R. McMillan, Homestead, Florida
Cymbidium mosaic virus (CymMV)
Cymbidium mosaic virus (CymMV) is a Potexvirus and is probably the most common and widely distributed of the orchid viruses. It is generally found only in cultivated orchids, not those in the wild. Mechanical transmission via tools and similar means is how it spreads, not by seed or insects. Symptoms typically include chlorosis or necrosis on the foliage, usually as a mosaic pattern, lesions, or streaks. Sunken necrotic spots may also occur on the leaves. Flower symptoms consist of flower break (irregular discolored spots or streaks on the petals) or necrotic streaks or sunken spots on the petals. Flower symptoms generally do not appear until after the bud has been open for a week or more. Freshly opened flowers rarely have symptoms.
Less obvious symptoms can also occur, such as slowed or stunted growth, poor flowering, or overall poor plant quality. For orchids recently infected with CymMV, it is common for the plant not to exhibit any symptoms until the first time it flowers or becomes stressed after infection. Some orchids never display symptoms. Even without symptoms, the virus is present, infectious, and poses a risk of infecting the rest of the collection.
© Dr. R. McMillan, Homestead, Florida Healthy leaf (left), CymMV-infected leaf (right). Dendrobium species. Dr. R. McMillan, Homestead, Florida
Odontoglossum ringspot virus (ORSV)
Odontoglossum ringspot virus (ORSV) is a Tobamovirus and was once referred to as Tobacco mosaic virus orchid strain (TMV-O). It was determined that these two pathogens are very closely related but distinct enough to be considered separate viral pathogens.
© Dr. R. McMillan, Homestead, FloridaORSV-infected Cattleya species. Dr. R. McMillan, Homestead, Florida
Tobamoviruses other than ORSV are not considered serious pathogens of orchids. Like CymMV, ORSV is spread through mechanical transmission only. Foliar symptoms generally include chlorotic or necrotic rings, spots, or streaks, which may or may not be sunken. In orchid varieties with reddish or violet-pigmented leaves, symptoms could include intense red pigmentation. Flower symptoms are common with ORSV, usually appearing as color break. As with other viruses, ORSV-infected orchids might not exhibit any symptoms while still serving as an infectious host.
These two viruses often occur individually as a single infection. However, if an orchid is exposed to both viruses, they readily occur as a dual infection. A CymMV and ORSV dual infection may cause an enhancement of the symptoms that would otherwise be expected from a single infection.
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