What are the common orchid viruses and their transmission routes?

I. Common orchid viruses

　　

1. Cymbidium mosaic virus

　　

Abbreviated CyMV (1) Symptoms: Orchid plants infected with Cymbidium mosaic virus frequently tend to develop necrosis symptoms, such as black necrotic spots, necrotic stripes, etc. In addition to necrosis on the leaves and stems, infected plants also develop necrosis on the flowers, which is a symptom unique to Cymbidium mosaic virus. Orchid varieties that are easily susceptible include Cymbidium sinense, Cattleya, Oncidium, Cymbidium hookerianum, Vanda, and Phalaenopsis. Necrotic spots sometimes appear only on the lower surface of the leaves and not on the upper surface; this phenomenon occurs in Oncidium, Dendrobium, Vanda, and Phalaenopsis, and is therefore often mistaken by growers for a fungal infection. Some Cymbidium plants infected with Cymbidium mosaic virus, such as Cymbidium hookerianum, Cymbidium sinense, Oncidium, and Phalaenopsis, also develop yellow-green mottled mosaic or yellow chlorotic streak symptoms. In addition, some orchid species show no recognizable symptoms after infection.

　　

(2) Virus characteristics and transmission method: Cymbidium mosaic virus is a member of the genus Potexvirus. Although the stability of viruses in this genus outside the cell is not as great as that of viruses in the genus Tobamovirus, it is still one of the most stable genera among known viruses. Its particles are slightly flexuous, about 450 nm in length, with an extracellular heat tolerance temperature of 60°C to 70°C, and it can survive for 25 days at room temperature.

　　

At present, no insect vector capable of transmitting Cymbidium mosaic virus has been found. Its transmission route is exactly the same as that of Odontoglossum ringspot virus; the virus enters and infects through mechanical wounds on the root system.

　　

2. Odontoglossum ringspot virus

　　

Abbreviated ORSV (1) Symptoms: According to reports, more than 20 genera of orchids can be infected by Odontoglossum ringspot virus. The symptoms produced vary among different orchid genera or species. Orchid species such as Phalaenopsis, Oncidium, Cattleya, Cymbidium sinense, Cymbidium ensifolium, and Cymbidium goeringii are often easily infected by Odontoglossum ringspot virus, especially Phalaenopsis. The first symptom of Odontoglossum ringspot virus recorded in the literature is ringspot; in addition, mosaic, mottling, chlorotic stripes, flower color streaks, and even necrotic symptoms have all been recorded. Among these symptoms, mosaic and mottling are the most common. In addition, some orchid varieties show no symptoms after infection with Odontoglossum ringspot virus; this phenomenon is especially common in Phalaenopsis and Oncidium seedlings, and infected plants usually only gradually show symptoms after their first flowering. In addition, in artificially cultivated orchid nurseries, mixed infection of the same orchid plant by Cymbidium mosaic virus and Odontoglossum ringspot virus is extremely common, and the effect on the orchid plant is additive, with symptoms becoming far more severe than with infection by a single virus.

　　

(2) Virus characteristics and transmission method: Odontoglossum ringspot virus is a member of the genus Tobamovirus. Its particles are short rigid rods, about 300 nm in length. The common characteristic of viruses in this genus is that they are extremely stable, with strong heat tolerance outside the host cell; they can still survive for a considerable time at temperatures as high as 95°C. According to reports by Japanese scholars, Odontoglossum ringspot virus can survive outside the cell for more than 10 years at 20°C, making it an infectious virus with extremely high stability. This virus can only enter the plant through mechanical wounds; therefore, during tissue culture or greenhouse cultivation management, all operations that may cause surface wounds, including cutting flower stalks, pruning, and even friction between plant leaves, can become routes for virus entry and infection. To date, it has not been proven that Odontoglossum ringspot virus can be transmitted by any specific insect vector. However, any insect or small animal that may cause mechanical damage on orchid plants, such as cockroaches, snails, etc., will increase the possibility of Odontoglossum ringspot virus infection, and precautions should be taken.

　　

3. Cucumber mosaic virus

　　

CucumberMosaicVirus, abbreviated CMV (1) Symptoms: The earliest record of Cucumber mosaic virus infecting orchids was found in Phalaenopsis. The symptom recorded at that time was the appearance of yellow chlorotic stripes parallel to the leaf veins on the plant leaves. Cucumber mosaic virus can also cause fading or streak symptoms on the petals of red-flowered Phalaenopsis lines. In addition, Cucumber mosaic virus infection has been found in Oncidium specimens. The specimen leaves showed inconspicuous mottling symptoms, with faintly visible yellow stripes parallel to the leaf veins, but the plant size was no different from that of a normal plant.

　　

(2) Virus characteristics and transmission method: Cucumber mosaic virus is a member of the genus Cucumovirus. Its particles are spherical, with a diameter of 28 nm. This virus has an extremely broad host range and can infect more than 800 plant species in 365 genera and 85 families. Most major economic crops in many regions of our country have records of Cucumber mosaic virus infection, and its population distribution in the field is extremely common. This virus can be transmitted through wounds caused by mechanical friction. In the field, it is mainly transmitted by aphids in a nonpersistent manner; because there are many species of aphids that can transmit Cucumber mosaic virus, the population distribution of this virus is very broad.

　　

The best way to control orchid viral diseases is to prevent their introduction before the disease occurs, that is, to thoroughly understand all possible transmission routes and sources of the virus, and then completely block them, thereby eliminating opportunities for virus introduction.

　　

1. Routes of virus infection during orchid production.

　　

(1) If virus-infected plants are used as parents for pollination, and seed pods are collected for sterile sowing to propagate seedlings, the infected pod tissue and hairs contain high concentrations of virus, which will contaminate the seeds and culture medium during sowing. Because Odontoglossum ringspot virus and Cymbidium mosaic virus are both extremely stable viruses, they can survive outside the cell for a long time. After orchid seeds germinate, the virus enters through wounds caused by manual transplanting operations and infects some seedlings. These seedlings then become a source of virus infection, continuing to contaminate other plants during subculture, and even during the process of seedlings maturing and being removed from bottles for planting, they again become a source of virus contamination, infecting other orchid seedlings.

　　

(2) Because the variety of a plant already infected with the virus meets market demand, the operator conducts meristem tissue culture without virus testing, mass-propagating mericlone seedlings, resulting in all propagated seedlings carrying the virus.

　　

(3) Although the operator selects parents with excellent traits for virus testing, these parents happen to be in the infection period, so the virus concentration in the tissue is low or unevenly distributed, and the test results are mistakenly interpreted as indicating that these parents are not infected with the virus. Using these parents for meristem cloning results in seedlings carrying the virus.

　　

(4) The parents with excellent traits are not infected with the virus, but they become contaminated by the virus during the tissue culture process. If mericlone seedlings are contaminated by the virus at the mother bottle stage, then all subsequently propagated seedlings will carry the virus. According to investigations, the procedures and techniques commonly used in operations cannot completely avoid virus contamination, which may lead to mutual infection between different batches of orchid seedlings.

　　

2. Operating methods that may cause virus transmission.

　　

(1) Incorrect tool disinfection methods: Operating tools used in tissue culture, such as tweezers, scalpels, etc., must undergo sterilization procedures to avoid bacterial and fungal infection; therefore, operators may use different sterilization methods according to their own habits. Traditionally, most use high-temperature sterilization methods, such as dry heat sterilization using a high-temperature oven, moist heat sterilization using an autoclave, and flame disinfection used on a sterile workbench.

　　

Because Odontoglossum ringspot virus and Cymbidium mosaic virus are extremely stable, if appropriate and effective elimination methods are lacking, these viruses are very likely to persist long-term and contaminate the surface of the sterile workbench or tools, and even contaminate the hands and clothing of operators, thereby having the opportunity to indirectly infect tissue-cultured seedlings.

　　

(2) Contamination of transfer petri dishes: In addition to using a sterile workbench for transferring tissue-cultured seedlings, workers also mostly use petri dishes or supports during operation to isolate tissue-cultured seedlings from direct contact with the surface of the sterile workbench. However, if used improperly, this can cause contamination between different batches of seedlings. Strictly speaking, petri dishes must be changed with each different batch of seedlings. This is because if the previous batch of seedlings was infected with the virus, it may contaminate the surface of the petri dish, and reusing the contaminated petri dish will cause the next batch of seedlings to also be infected by the virus.

　　

(3) Negligence and bad habits of workers: Negligence or bad habits of workers during operation may increase the probability of tissue-cultured seedlings becoming infected with the virus. For example, careless and incomplete use of flame disinfection tools, or failure to replace petri dishes when they should be replaced, are key factors that make it difficult for tissue-cultured seedlings to avoid virus infection.

　　

Managers can take measures such as training employees, cultivating their professional dedication, and strengthening quality monitoring to reduce the probability of virus infection in seedlings.

　　

3. Tissue-cultured seedlings are not infected with the virus, but become infected during the process of being removed from bottles and planted.

　　

According to investigations, even if mericlone seedlings are not infected with the virus in the bottle, if strict measures to prevent virus infection are not taken during the process of removal from the bottle and cultivation, virus infection can still occur.

　　

These commonly made mistakes include:

　　

(1) After removing the seedlings from the bottle, the operator places them in a container to wash off residual culture medium or treat them with protective agents. If the same container is used to handle different batches of seedlings at this time, and some batches of seedlings carry the virus, the residual virus will contaminate the surface of the container, causing virus infection when the next batch of seedlings is handled.

　　

If some individuals in the same batch of seedlings are virus-infected, when all the seedlings are placed in the same container for handling, it will also cause other individuals to become infected.

　　

(2) The workbench surface, tools, or workers' hands used for seedling planting operations may all come into contact with virus-carrying seedlings during the transplanting process and become contaminated with the virus, thereby becoming a vehicle for spreading the virus.