By: Jackie Rhoades
Cucumber mosaic disease was first reported in North America around 1900 and has since spread worldwide. Cucumber mosaic disease isn’t limited to cucumbers. While these and other cucurbits can be stricken, Cucumber Mosaic Virus (CMV) regularly attacks a wide variety of garden vegetables and ornamentals as well as common weeds. It’s so similar to the Tobacco and Tomato Mosaic Viruses only an expert horticulturalist or laboratory testing can distinguish one from the other.
What causes Cucumber Mosaic disease is the transfer of the virus from one infected plant to another through the bite of an aphid. The infection is acquired by the aphid in just one minute after ingestion and is gone within hours. Great for the aphid, but really unfortunate for the hundreds of plants it can bite during those few hours. If there’s any good news here it’s that unlike some other mosaics, Cucumber Mosaic Virus can’t be passed along through seeds and won’t persist in plant debris or soil.
Cucumber Mosaic Virus symptoms are rarely seen in cucumber seedlings. Signs become visible at about six weeks during vigorous growth. The leaves become mottled and wrinkled and the edges curl downward. Growth becomes stunted with few runners and little in the way of flowers or fruit. Cucumbers produced after infection with cucumber mosaic disease often turn grey-white and are called “white pickle.” The fruit are often bitter and make mushy pickles.
Cucumber Mosaic Virus in tomatoes is evidenced by stunted, yet bushy, growth. Leaves may appear as a mottled mixture of dark green, light green, and yellow with a distorted shape. Sometimes only part of the plant is affected with normal fruit maturing on the uninfected branches. Early infection is usually more severe and will produce plants with low yield and small fruit.
Peppers are also susceptible to Cucumber Mosaic Virus. Symptoms include the mottled leaves and stunted growth of other mosaics with the fruit showing yellow or brown spots.
Even though botanists can tell us what causes cucumber mosaic disease, they have yet to discover a cure. Prevention is difficult because of the short time between when the aphid contracts the virus and its passing it along. Early season aphid control may help, but there is no known Cucumber Mosaic Virus treatment at the present time. It’s recommended that if your cucumber plants are affected by Cucumber Mosaic Virus, they should be removed immediately from the garden.
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Usually my attempts get better over the years, but for several years now I’ve been attempting to grow cucumbers on my balcony. Unlike my other attempts, every year my efforts go unrewarded. Normally I would have some idea of what had happened, but with the cukes, I could never figure out what was going wrong.
Each year, sometime into the new year I would start the seeds indoors. I would use two separate containers and plant a couple of seeds in each. These would be thinned out as they grew, allowing only the strongest-looking ones to survive. Since I don’t have a lot of space, I would aim to have two plants to move outdoors.
Once the danger of frost had passed, they’d be transplanted to larger containers outside on my balcony. Each cucumber plant had its own container, sized about 14″ x 14”.
Everything would start out really well, but usually within just a week I would notice the inevitable decline. This would start out as a bit of leaf discoloration. The leaves would slowly lighten and get patchy, browning and withering at the edges.
Is very common and also worldwide in distribution. A discussion of TMV is included because of its wide host range and high potential for spread in production facilities. It infects more than 150 plant genera that include many ornamentals. The most common symptom of TMV is a mosaic pattern of dark green and light green areas on infected plant leaves. Infected sap, grafting, dodder and seed spread TMV. Insects do not spread TMV. The most common means of transmission is by people handling infected and healthy plants simultaneously.
In plant tissue this virus makes characteristic viral inclusion bodies which can be diagnostic. They are hexagonal in shape (Fig.1) and stain both in a protein stain and a nucleic acid stain.  The inclusions can also be rhomboidal (Fig. 2b), may appear hollow (Fig. 1) and can form larger aggregates (Fig. 3b). The inclusions are not uniformly distributed and can be found in epidermal (Figs. 1 and 2b), mesophyll (Fig. 3b), and stomatal cells (Fig. 4b). These inclusions  are made up of virus particles.
This virus was first found in cucumbers (Cucumis sativus) 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 (Fig. 2a), yellowing (Fig. 3a), ringspots (Fig. 4a), 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. 
Fig.4a. Symptoms in Commelina diffusa
Cucumber mosaic virus inclusion bodies
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. 
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 (such as Tobacco Mosaic Virus, TMV), 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 produced to enable the virus replication, which takes place in viral factories, 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 (cell-to-cell) movement of the virus is achieved via plasmodesmata, while the long-distance one (within the plant) occurs via the phloem. 
CMV  is a linear positive-sense, tripartite single-stranded RNA virus . Its genome size is 8.623 kb and it is divided among RNA1 (3357 pb) , RNA2 (3050 pb)  and RNA3 (2216 pb) , all of which has a tRNA-like structure . 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.  The RNA is surrounded by a protein coat consisting of 32 copies of a single structural protein which form isometric particles. 
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 .
CMV is naturally found in temperate areas, where aphids, one of its main vectors, are also found. 
The presence of this virus in a plant can be confirmed by serological (ELISA), genetic (PCR) or host range tests.
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".
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