Cucumber Mosaic Virus: The Unknown Virus on Bedding Plants and Perennials By Dr. S.G.P. Nameth Department of Plant Pathology The Ohio State University Much has been written about the destructive potential of impatiens necrotic spot virus (INSV) and tomato spotted wilt virus (TSWV) on bedding plants and herbaceous perennials. This would give one the false impression that these are the only two viruses that cause diseases of floral crops. That is definitely not the case. There are many other plant viruses that have the potential to cause damage on these crops. One of the most common of these is cucumber mosaic virus (CMV). Cucumber mosaic virus has the widest host range of any plant virus in the world and is a common problem in a wide variety of ornamental plants including greenhouse-grown bedding and perennial plants. The virus infects plants in hundreds of families and in some cases has the ability to go undetected unless one has a keen eye as to it's sometimes subtle symptoms. How does CMV get into my greenhouse? CMV can enter a greenhouse in a variety of ways. Aphid-transmission is its primary method of moving the virus from plant to plant. These pesky insects can feed on weeds and other virus-infected hosts outside the greenhouse and then move indoors where they spread the virus form plant to plant very efficiently. CMV can also be brought into the greenhouse via virus-infected seed (in selected hosts) and cuttings. Mechanical transmission (plant to plant contact or propagation tool to plant contact) is another method of entry. However, seeds, cuttings and mechanical transmission are negligible when compared to aphid-transmission. What Do I Look For? The most common symptoms associated with CMV infection are mild to severe leaf mosaic, leaf distortion, flower color-break, plant stunting, and yellowing (Figure 1) , (Figure 2) , (Figure 3) , (Figure 4) , (Figure 5) , (Figure 6) . In some cases plants infected with CMV can appear to look "healthy" and may, depending on the environmental conditions "grow-in and grow-out" of symptoms. CMV-infected plants tend to express symptoms during the cooler times of the growing season and as the greenhouse gets hotter the symptoms may be suppressed. However, once a plant is infected, its infected for life, symptoms or not. Symptomless plants can still serve as a source of the virus if fed-on by aphids or if cuttings are taken from these plants. Damage caused by CMV is dependent on the host species and the age of the plant at which infection takes place. In some hosts CMV infection can cause severe stunting and deformation, while in a host such as geranium the virus has little to no effect on the outward appearance of the plant. The further along in the grow stage of the host, the less likely CMV infection will be serious problem. Plants infected in the early stages of growth and development can be severely stunted, may not set flowers and in extreme cases the plants will die. It should be noted that once a plant is infected with any plant virus there is NOTHING that can be applied to such plant that will "cure" it of the virus. There are products that will "kill" viruses on bench tops and floors, however, these products have no affect on a virus-infected plant. Therefore, the best way to control CMV (as it is with all plant viruses) is to avoid virus infection to begin with (prevention). This means making sure that the seed or the propagation stock that you purchase is virus-free. Most major producers of seed and propagation material go to great lengths to make sure that the product they sell you is free from viruses. Problems arise when growers try to cut corners and purchase second-rate propagation material or cheap seed. This philosophy will only cost you more money in the long run. Since aphid transmission is the number one way CMV moves in, out and about the production facility, it is of utmost importance the population of these insects be keep to a minimum. Aphids populations should be monitored within the greenhouse using yellow or blue sticky cards and when aphid populations reach a critical level the appropriate insecticides should be applied. Also, weeds growing outside of the greenhouse can harbor CMV and other viruses. Aphids can acquire these viruses when feeding on infected weeds and move the viruses into the greenhouse. Weeds should be eliminated via mowing or herbicide applications. Caution should be taken when using herbicides outside the greenhouse. Be aware that some herbicides can drift into the greenhouse and cause problems on the greenhouse crop. If a plant is suspected to have CMV the plant should be set aside or moved to another part of the production facility until it can be tested to confirm virus infection. Plants should not be destroyed based on symptoms alone. Most university, state or private testing labs can test for CMV. Once infection is confirmed the infected plant(s) should be removed from the greenhouse and destroyed. If your greenhouse has a confirmed case of CMV it is important to not carryover plants from one season to the next. Prior to the introduction of new plant material, the suspect greenhouse should be cleared of all plant material and surface disinfestants such as GreenShield, Physan or ZeroTol should be applied to the benches and floors. This will insure a clean, virus-free greenhouse for the start of the new growing season.