Fertilization Methods for Edible Mushrooms and Common Orchid Viruses and Their Transmission Routes
I. Common Orchid Viruses
1. Cymbidium Mosaic Virus
Cymbidium Mosaic Virus, abbreviated as CyMV (1) Symptoms: Orchid plants infected by Cymbidium Mosaic Virus often tend to develop necrotic symptoms, such as black necrotic spots and necrotic streaks. In infected plants, in addition to necrosis on the leaves and stems, necrosis also occurs on the flowers, which is a symptom unique to Cymbidium Mosaic Virus. Orchid varieties susceptible to infection include Cymbidium sinense, Cattleya, Oncidium, Cymbidium hookerianum, Vanda, and Phalaenopsis. Necrotic spots sometimes appear only on the lower surface of leaves and not on the upper surface; this phenomenon occurs in Oncidium, Dendrobium, Vanda, and Phalaenopsis, and is therefore often mistaken by growers as being caused by 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 yellowed stripe symptoms. In addition, some orchid varieties show no identifiable symptoms after infection.
(2) Virus characteristics and transmission methods: 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 flexible, with a length of approximately 450 nanometers, an extracellular heat tolerance temperature of 60°C to 70°C, and it can survive for 25 days at room temperature.
At present, no vector insect 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
OdontoglossumRingSpotVirus
Abbreviated as ORSV (1) Symptoms: According to reports, more than 20 genera of orchids can be infected by Odontoglossum Ringspot Virus. The symptoms produced by different orchid genera or species may vary. Orchid species such as Phalaenopsis, Oncidium, Cattleya, Cymbidium sinense, Cymbidium ensifolium, and Cymbidium goeringii are often susceptible to infection by Odontoglossum Ringspot Virus, especially Phalaenopsis. The earliest symptom of Odontoglossum Ringspot Virus recorded in the literature is ringspot; in addition, mosaic, mottling, yellowed stripes, color-breaking streaks on flowers, 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 particularly 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, the phenomenon of combined infection of Cymbidium Mosaic Virus and Odontoglossum Ringspot Virus in the same orchid plant is extremely common, and the effect on the orchid plant is additive; the symptoms are far more severe than with infection by a single virus.
(2) Virus characteristics and transmission methods: Odontoglossum Ringspot Virus is a member of the genus Tobamovirus. Its particles are short, rigid rods, approximately 300 nanometers in length. The common characteristic of viruses in this genus is that they are extremely stable and highly heat-tolerant outside the host cell; they can still survive for a considerable period 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 body through mechanical wounds. Therefore, during tissue culture or greenhouse cultivation management, all operations that may cause external 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 confirmed that Odontoglossum Ringspot Virus can be transmitted by any specific vector insect. However, any insect or small animal that may cause mechanical damage to orchid plants, such as cockroaches and snails, will increase the possibility of Odontoglossum Ringspot Virus infection, and attention should be paid to prevention.
3. Cucumber Mosaic Virus
Cucumber Mosaic Virus, abbreviated as 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 yellowed stripes parallel to the leaf veins on the plant leaves. Cucumber Mosaic Virus can also cause fading or stripe 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 methods: Cucumber Mosaic Virus is a member of the genus Cucumovirus. Its particles are spherical, with a diameter of 28 nanometers. The host range of this virus is extremely broad, capable of infecting more than 800 species of plants in 365 genera across 85 families. Most important economic crops in many regions of China have records of Cucumber Mosaic Virus infection, and its population is extremely widespread in the field. This virus can be transmitted through wounds caused by mechanical friction. In the field, it is mainly transmitted by aphids in a non-persistent manner; because there are many species of aphids capable of transmitting Cucumber Mosaic Virus, the population of this virus is very widely distributed.
The best method for controlling orchid viral diseases is to prevent their introduction before disease occurs, that is, to thoroughly understand all possible transmission routes and sources of the virus, and then comprehensively block them, thereby eliminating opportunities for virus introduction.
1. Routes of viral 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 seed pod tissues and hairs contain high concentrations of virus, which will contaminate the seeds and culture medium during the sowing process. Because Odontoglossum Ringspot Virus and Cymbidium Mosaic Virus are both extremely stable viruses, they can survive outside the cell for long periods. After orchid seeds germinate, the virus enters through wounds caused during manual transplanting operations and infects some seedlings. These seedlings then become a source of viral infection, continuing to contaminate other plants during subculture, and even during the process of seedlings maturing and being removed from bottles for planting, they once again become a source of viral contamination, infecting other orchid seedlings.
(2) Because the variety of a plant already infected with virus meets market demand, the operator conducts meristem tissue culture without virus testing and propagates mericlones in large quantities, resulting in all propagated seedlings carrying the virus.
(3) Although the operator selects parents with superior traits for virus testing, these parents happen to be in the infection period, so the virus concentration in the tissues is low or unevenly distributed, and the test results mistakenly indicate that these parents are free of viral infection. Using these parents for meristem cloning results in virus-carrying seedlings.
(4) The parents with superior traits are not infected with virus, but they become contaminated by virus during the tissue culture process. If meristem seedlings are contaminated by virus during 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 viral contamination, and this may lead to mutual infection among different batches of orchid seedlings.
2. Operational methods that may cause viral infection.
(1) Inaccurate tool sterilization methods: The operating tools used in tissue culture, such as tweezers and scalpels, must undergo sterilization procedures to prevent bacterial and fungal infection; therefore, operators may adopt 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 sterilization 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 highly 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 culture seedlings.
(2) Contamination of transfer culture dishes: In addition to using a sterile workbench for transferring tissue culture seedlings, workers also mostly use culture dishes or supports during operation to isolate tissue culture seedlings from direct contact with the sterile workbench surface. However, improper use can cause contamination between different batches of seedlings. Strictly speaking, culture dishes must be changed with each different batch of seedlings. This is because if the previous batch of seedlings was infected with virus, it may contaminate the surface of the culture dish, and reusing the contaminated culture dish will cause the next batch of seedlings to also become infected with virus.
(3) Negligence and bad habits of workers: Negligence or bad habits of workers during operation may increase the probability of tissue culture seedlings becoming infected with virus. For example, careless and incomplete use of flame sterilization tools, or failure to change culture dishes in time when they should be changed, are key factors that make it difficult for tissue culture seedlings to avoid viral infection.
Managers can reduce the probability of viral infection in seedlings by training employees, cultivating their professional dedication, and strengthening quality monitoring.
3. Tissue culture seedlings are not infected with virus, but become infected during the process of being removed from bottles and planted.
According to investigations, even if meristem seedlings are free of viral infection in the bottle, if strict measures to prevent viral infection are not taken during the process of removal from bottles and cultivation, viral infection can still occur.
These commonly made mistakes include: (1) After removing seedlings from the bottle, the operator places them in a container to wash off residual medium or treat them with protective agents. At this time, if the same container is used to handle different batches of seedlings, and some batches carry virus, the residual virus will contaminate the container surface, causing viral infection when the next batch of seedlings is handled.
If some individuals within the same batch of seedlings are virus-infected, when all seedlings are placed in the same container for handling, it will also cause the remaining 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 virus, thereby becoming a medium for spreading the virus.