Cucumber mosaic virus is a plant pathogenic virus in the family Bromoviridae. It is the type member of the plant virus genus, Cucumovirus. This virus has a worldwide distribution and a very wide host range. In fact it has the reputation of having the widest host range of any known plant virus. It can be transmitted from plant to plant both mechanically by sap and by aphids in a stylet-borne fashion. It can also be transmitted in seeds and by the parasitic weeds, Cuscuta sp..
Hosts and symptoms
In plant tissue this virus makes characteristic viral inclusion bodies which can be diagnostic. They are hexagonal in shape and stain both in a protein stain and a nucleic acid stain. The inclusions can also be rhomboidal, may appear hollow and can form larger aggregates. The inclusions are not uniformly distributed and can be found in epidermal, mesophyll, and stomatal cells. These inclusions are made up of virus particles. This virus was first found in cucumbers showing mosaic symptoms in 1934, hence the name Cucumber mosaic. Since it was first recognized, it has been found to infect a great variety of other plants. These include other vegetables such as squash, melons, peppers, beans, tomatoes, carrots, celery, lettuce, spinach and beets, various weeds and many ornamentals and bedding plants, such as Narcissus. Symptoms seen with this virus include leaf mosaic or mottling, yellowing, ringspots, stunting, and leaf, flower and fruit distortion. CMV shows symptoms on leaves known as the "shoestring" effect for most host species. This effect causes young leaves to appear narrow and the entire plant to be stunted. Specifically CMV can cause cucumbers to turn pale and bumpy. The leaves of these plants turn mosaic and their rugosity is often changed, making leaves wrinkled and misshapen. Growth of these plants is usually stunted and produces few flowers. Often cucumber fruits are oddly shaped and appear gray. This appearance often leads to cucumbers being referred to as "white pickles". Often infected cucumbers are bitter. In celery, CMV can cause streaking and spotting and can be often confused with symptoms of the celery mosaic virus. Symptoms of CMV in lettuce can be similar to those of lettuce mosaic virus. Infected plants show symptoms of chlorosis, stunting and often do not properly head. Some of the most important fruits and vegetables affected by CMV are peppers, bananas, tomatoes and cucurbits. CMV in peppers causes slightly different symptoms than the previously mentioned. Pepper plants often have severe foliar damage, shown as mosaic and necrotic rings. Often the peppers themselves are misshapen and contain chlorotic rings and spots. Tomato plants are usually stunted and have poorly shaped leaves, or "fernleaf", when infected by CMV. Also certain strains of CMV can cause partial or total crop loss. The cucumber mosaic virus has been found on American beautyberry, an important wildlife and pollinator food source plant native to North America.
Importance
As CMV is easily spread, it can be found worldwide. It is transmitted by more than 60 different aphid species, among other vectors, and it can infect over 1200 plant species, including important crops and ornamental species. In its plant host, CMV can cause severe damage, which can lead to economical losses, as it can lead to 10-20% loss of field yield.
Disease cycle
CMV is mainly transmitted by aphids, but it can also be mechanically spread by humans in some cases. However, the mechanically spreading of this virus is not as common as the case of other virus, because CMV is not a very stable virus. When it is transmitted by aphids, this virus has an acquisition period of five to ten seconds and an inoculation period of about a minute. Nevertheless, after two minutes, the probability of inoculation largely decreases, and within two hours it is practically impossible to transmit it. Moreover, CMV can overwinter in perennial plants and weeds, as it can survive the winter in the roots of the plant and move to the aerial parts in spring, where it can be transmitted by aphids to other plants. Once the virus penetrates into the host cell, it releases its RNAs into the host cytoplasm. Then, proteins 1a and 2a are produce to enable the virus replication, which takes place in , which are subcellular compartments which increase the efficiency of this process. There, a dsRNA genome is synthetized from the ssRNA and transcribed in order to obtain viral mRNAs as well as new ssRNAs. Afterwards, the capsid proteins are produced and the new viral particles are assembled. Finally, the virus is ready to move to a new cell by triggering the formation of tubular structures which mediate the movement of the virions . The short-distance movement of the virus is achieved via plasmodesmata, while the long-distance one occurs via the phloem.
Properties
Genome
CMV is a linear positive-sense, tripartite single-stranded RNA virus . Its genome size is 8.623 kb and it is divided among RNA1 , RNA2 and RNA3 , all of which has a tRNA-like structure . These three RNAs encode five proteins, proteins 1a, 2a, 2b, movement protein and coat protein. While proteins 1a and 2a are responsible for the replication of the virus, protein 2b is the host-silencing suppressor. The RNA is surrounded by a protein coat consisting of 32 copies of a single structural protein which form isometric particles.
Virion
This virus presents non-enveloped, icosahedral or bacilliform virions of 26-35 nm in diameter. The different RNAs are encapsidated in distinct particles, which results in a variety of virions .
Environment
CMV is naturally found in temperate areas, where aphids, one of its main vectors, are also found.
Diagnosis
The presence of this virus in a plant can be confirmed by serological, genetic or host range tests.
Management
Currently there is no chemical capable of removing this virus from an infected plant, and therefore the best control is prevention of the infection and eradication. To achieve this, it is crucial to remove weeds and diseased plants from the field, as well as use clean and sanitized tools. Another option consists of the use of resistant varieties or the so-called "trap crops".
