The second edition of our blog series Plant virus snapshots focuses on a plant virus from the Bromoviridae virus family, the Cucumber mosaic virus (CMV). Similar to tobamoviruses like ToBRFV, plants serve as natural hosts for this plant virus family. The Cucumber mosaic virus has the reputation of having the widest host range of any known plant virus.
What is the Cucumber mosaic virus?
CMV is a cucumovirus found worldwide in both temperate and tropical climates. It affects over 1200 plant species. The scientists Doolittle and Jagger first described CMV in detail in cucumbers in the US states of Michigan and New York. Considering its name, it could be assumed that CMV only infects cucurbits, but this is actually not the case. This plant virus infects a wide range of fruits, vegetables, herbaceous and woody ornamentals, and weeds. It affects e.g. tomatoes, eggplants, peppers, beans, carrots, spinach, lettuce and beets. CMV is so pervasive, with scientists having even detected it in Antarctica. As CMV is very similar to Tomato mosaic virus (ToMV) and Tobacco mosaic virus (TMV), only an expert horticulturalist or laboratory testing can distinguish these plant viruses. One key difference however is that contrary to ToMV e.g., CMV is not a very stable plant virus. This means that mechanical transmission is not very common, for example by workers touching infected plants during normal field operations.
Virus structure
The Cucumber mosaic virus is a linear positive-sense, tripartite single-stranded RNA virus. The CMV genome consists of three single-stranded, messenger-sense RNA molecules, designated RNA 1 (~3,350 nucleotides), RNA 2 (~3,050 nucleotides) and RNA 3 (~2,200 nucleotides). Each RNA molecule is enclosed within a protective protein coat, with each being a distinct single spherical-shaped particle. These three RNAs encode five proteins – proteins 1a, 2a, 2b, movement protein (MP) and coat protein (CP). While proteins 1a and 2a are responsible for the replication of the virus, protein 2b is the host-silencing suppressor. Some CMV strains support a satellite RNA (designated RNA 5 or satRNA). The satRNA is a single-stranded molecule of approximately 332 to 342 nucleotides in length, and is completely dependent on CMV for its replication.
Transmission
CMV has varied means of transmission. It can be transmitted from plant to plant both mechanically by sap and by aphids. It can also be transmitted in seeds or pollen, as well as parasitic weeds such as Cuscuta sp.. The most common means of transmission are through apids, which can pick up the virus from infected plants and transmit it to healthy plants as they feed. Over 80 species of aphids can potentially transmit CMV. Aphids usually acquire the virus in short probes and then can only retain it for a few minutes in their normal feeding activities. These insects would have to reacquire the virus again to be able to transmit again. After a few hours, most aphids completely lose the ability to transmit CMV. Of course, transmission efficiency varies with the aphid species, virus strains, host plant species, environmental conditions and time of the year. It is also important to note that CMV can overwinter in perennial plants and weeds. This plant virus can survive the winter in the roots of the plant and then move to the aerial parts in spring, where aphids can transmit it to other plants.
Symptoms
The symptoms of CMV typically vary from host plant species, the age of the plant and different environmental conditions. CMV causes a systemic infection in most host plants, especially in cucurbits, but may even remain symptomless in some crops. Generally, CMV causes the following symptoms in infected plants:
- Stunting;
- Mosaic pattern of light and dark green (or yellow and green) on the leaves;
- Malformation of leaves or growing points;
- Yellow streaking of leaves;
- Yellow spotting on leaves;
- Ring-spots or line patterns on leaves or fruit;
- Flower color breaking;
- Distinct yellowing only of veins.
Furthermore, the severity of CMV depends on many different factors. This includes e.g. the size and activity of aphid populations in an area, as well as the number infected plants in an area serving as reservoirs for the virus. Symptoms could also be more severe due to high temperatures, herbicides or mineral deficiencies. CMV also induces a type of symptom on leaves known as the “shoestring” effect on many host species. This effect causes young leaves to appear narrow and the entire plant to be stunted. The below images show the varied symptoms caused by ToMV in tomato and cucurbit crops:
Virus management and control
There are no chemicals that cure a CMV-infected plant, nor any that can protect plants from becoming infected. Furthermore, using insecticides to reduce aphid transmission is neither practical nor environmentally friendly. However, a range of mitigation methods exist to keep CMV in check:
- Use only certified, virus-free seeds and plants;
- Maintain strict aphid control;
- Remove all weeds since these may harbor both CMV and aphids;
- Immediately set aside plants with symptoms and obtain a diagnosis through e.g. ELISA/PCR testing;
- Discard virus infected plants;
- Disinfest tools used for vegetative propagation frequently;
- Propagate plants via seed rather than vegetatively;
- Finally, choose where possible CMV-resistant varieties, which is possible e.g. especially for cucumber crops.
Current status
CMV still remains a very important viral disease across the globe. It often induces severe damage, which can lead to economical losses ranging typically between 10-20% loss of field yield. In some instances, CMV-infected crops may still be harvested, but are of poorer quality and appearance. While the value chain has made substantial progress in managing and controlling CMV, this plant virus still causes important crop losses every year worldwide, including for both cucurbits and tomatoes. For a long time, no tomato varieties with CMV resistance have been made available to growers. But this is changing, with CMV-resistant varieties for tomatoes now also on the horizon.
