Viruses and plants
What is an Orchid virus?
A virus is a small infective agent that consists of a nucleic acid molecule (genetic material) surrounded by a protein coat in an envelope of lipids. It is able to multiply itself only in the living tissue of a host and may have a detrimental effect. Viruses are microscopic and much smaller than bacteria (commonly 100th the size and 10,000 times smaller than a grain of salt). In addition to multiplying in a host they can also sometimes mutate (and have more serious effect and / or infect other hosts). There is no cure once an orchid is infected.
There are over 30 viruses reported to infect orchids. The two most important are Cymbidium mosaic virus- CyMV, which has been found to infect 56 genera of orchids, and Odontoglossum ringspot virus - ORSV (formerly the orchid strain of Tobacco mosaic virus), which has been reported from 20 orchid genera. Mixed infections of both viruses are common.
Viruses are spread in sap or fluid from infected plants to healthy ones. For this to happen there has to be another agent to assist. Most commonly this is the grower themselves. Viruses are transferred when dividing, repotting, cutting flowers etc. by infected sap on hands, cutting tools, utensils and handling area.
Chewing and sucking insects can also mechanically transmit many viruses very effectively when moving from one plant to another with infected sap. Proven insect transmitters include thrips, aphids and mites. It has been shown pretty dramatically in cut flower nurseries just how fast these can transit a number of viruses. Not monitoring and having effective control measures for such insects will allow for rapid spread.
These viruses are resistant to degradation for long periods on tools and in the soil. These diseases are systemic: virus particles are in roots, bulbs, leaves, and flowers. Once infected, plants cannot be cured. The viruses are not known to be transmitted by seed. The virus, if present, is present in all parts of the plant..
There is no treatment for a virused plant. Destroy the plant to prevent it from infecting other plants. If the plant is valuable, isolate it completely from other plants and follow precautions to prevent infecting other plants.
Cymbidium mosaic virus-on Cymbidiums, symptoms appear first on new growth 6 weeks after infection as small, inconspicuous, elongate, chlorotic areas. Symptoms become more pronounced when light and dark green areas eventually become evident. Brown streaks may appear after 3 to 4 months. On Cattleya leaves, sunken, brown to purple streaks or flecks appear; in severe cases, older leaves drop prematurely. Flowers of Cattleya-type orchids develop brown necrotic streaks or necrosis 5 to 21 days after they open and are most noticeable on white or lavender flower types. Symptoms on Phalaenopsis appear on new leaves as yellow, oval or elongated streaks or spots that may look water soaked. These turn purple-black and sunken with age. Lesions usually are only on the lower leaf surface. Vanda leaves develop chlorotic flecks.
Odontoglossum ringspot virus-causes flower colour-breaking (especially in the flowers of lavender Cattleyas), chlorotic streaking, mosaic, or necrosis. It is symptomless in some cultivars. Oncidium flowers show no symptoms, but mature leaves develop round to irregular, slightly sunken yellow areas on both leaf surfaces. Spots darken with age. Cymbidium leaves show a diamond-shape mottle. Root tip necrosis is also a common symptom.
Symptoms may be more severe when both viruses have infected the same plant. Symptoms of either virus differ greatly from one orchid variety to another and can be confused with fungal disease problems. Vegetatively propagated plants may be infected but symptom less, so relying on symptoms to detect infection is not reliable. Only laboratory testing can make a valid diagnosis.
Cultural control - Focus on preventing infections and good sanitation practices.
- Use plants that have been tested and found free of all known viruses, such as those produced by meristem tip tissue culture. Also, seedlings are less likely to be infected than older plants.
- Disinfect cutting tools after cutting flowers or making divisions. Disinfect tools after each plant. A soak in 10% bleach helps prevent viruses from spreading. Use two knives so a worker can cut with one while the other soaks. When done with a plant switch one knife for the other and allow the used one to soak.
- Discard and destroy infected orchids.
- Avoid reusing pots or trays from a previous crop for propagation. If pots must be reused then wash off all debris and soak in a sanitizing solution or treat with aerated steam for 30 min.
- Some viruses are spread by insects so excluding them by using fine mesh screening in greenhouse openings may be useful.