Cucumber mosaic virus: A difficult viral illness

If you've noticed a dark green mosaic on your cucumbers or squashes, your plants may have contracted cucumber mosaic virus (CMV). CMV affects a wide variety of plants and there is no cure for cucumber mosaic virus infection. The sad truth about this disease is that it cannot be treated.

But that doesn't mean you can't prevent CMV infection. You have the power to avoid bringing CMV strains into your garden. CMV infects healthy plants, but there are plants with resistance to the virus. And there are ways to control the variables commonly attributed to CMV.

What are the host species for CMV? What do symptoms look like – especially the worst symptoms? Read on as we cover everything we can know about CMV infection and what to do if you catch one. We will also discuss the prevention of CMV overall.

What is cucumber mosaic virus?

Cucumber mosaic virus is one of the most prevalent mosaic virus types. Source: Dieter O

Cucumber Mosaic Virus (CMV) is one of the worst viral diseases to hit a garden. Potentially infected plants should be removed immediately, set aside, and diagnosed by agents at an Agricultural Advisory Bureau. This is because there has recently been extensive viral research on CMV isolates and products with antiviral properties that promote gene silencing. In this process, antiviral compounds stop the removal of a plant cell's protein coat and prevent replication.

life cycle and emergence

CMV overwinters in weeds. It is transmitted by insects or mechanically. It also spreads via infected seeds. Insects and mechanical vectors can spread randomly and with secondary infection. Most vine crops acquire CMV through secondary infections caused by insects, infected gardening tools, and hands that have been in contact with CMV-infected plants and plant debris. The primary cycle of CMV is generally slow and gradual, while secondary infection is more rapid.

Virus strains invade the plant cell. The virus coat protein is removed and its viral genomic RNA is released into the plant cytoplasm. The virus then expresses two types of proteins that produce replication proteins. Viral replication occurs in "factories" made up of membrane vesicles. Another RNA genome is being engineered and transcribed within to provide even more RNA genomes. Subgenomic RNA develops that produces more coat protein, and more virus transmission and generation occurs via the virus moving protein in the plant cell.

Aphid populations (at least 75 different species) are the main cause of the spread of virus particles. While feeding on infected host plants, they will ingest the virus within 1 minute and spread it while it remains in their style. Cucumber beetles are also a vector of cucumber mosaic virus. After about 2 minutes, the ability to transmit the virus decreases significantly. Within two hours it is almost impossible to spread CMV that has been ingested by an insect that has been feeding on an infected plant.

The most susceptible plants are young host plants. Because ants tend to harvest honeydew from aphids, they are also one of the main insect vectors of CMV. Controlling these insects in your garden can help you build CMV resistance from the start.

Another vector of CMV spread is by mechanical means, via infected plant sap taken up by the wind, or through infected cutting tools. The latter is why it is so important to disinfect garden tools between uses. Because CMV is not a stable virus, it is not easily spread by mechanical means.

The infected seed also transmits CMV strains to plants. Growers plant the seed and learn they have an infected plant after doing the work to start it. Finally, certain infected weeds can transmit the virus to other plants, particularly those that climb over vines around a garden bed. For example, foxtail and wild amaranth are hosts for the virus. Keeping weeds out of the garden will also help you prevent them from spreading.

It is particularly important to be vigilant for CMV in environmental conditions where it can multiply. The virus prefers temperatures between 70 and 85 degrees, which typically fall in spring and summer. That's because these temperatures coincide with the peak of aphid reproductive cycles. If you want to understand CMV resistance, think of plant stress. The times that are more stressful for plants are more stressful for CMV. In particular, high heat causes a decrease in CMV incidence. Environmental conditions in temperate and tropical climates promote virus replication. Another condition affecting replication occurs when nights are much longer than days. At this point, the CMV symptoms are more evident.

Symptoms of cucumber mosaic virus

Cucumber Mosaic on Climbing DayflowerCucumber mosaic virus on climbing dayflower, Commelina diffusa. Source: Sam Fraser Smith

Understanding the CMV disease cycle is the basis for identifying systemic infection. While symptoms vary by plant species, a general understanding of systemic symptoms is required. Within minutes, cucumber mosaic disease gets to the plant species using the methods listed in the last section. While young plants are most susceptible, symptoms do not show up until a few days after infection begins. Sometimes it takes 2 weeks for symptoms to manifest. Early stage mosaic symptoms in a seedling include yellowed and wilted cotyledons. The leaves become mottled and wrinkled in appearance. Plant growth is dramatically slowed.

There are many symptoms that infected plants show when they contract CM virus infection. Symptoms of the disease vary in mature plants. The leaves take on a yellowish green to deep green mottled or mosaic pattern. Vines become clustered in appearance as stem growth between the leaves is stunted. The vines are yellow in the middle and fruit production is slowing down. All parts of the plant that grow after infection become dwarfed, while parts of the plant that were previously not susceptible to infection grow normally.

Look for the distinctive chlorotic mosaic pattern on fruit. Cucumbers become overly warty and soft. They are often too bitter to enjoy. More serious symptoms manifest themselves in different ways in different vegetable crops. Other cucurbits like muskmelon and watermelon also take serious damage in the form of warts. Melon warts are usually lighter in color than cucumber warts. Occasionally, the watermelon will develop necrotic local lesions that render the fruit inedible. Squash also develops warts that are light in color. How the exact symptoms appear depends heavily on the particular CMV strain.

In pepper plants, CMV infection looks like stunted growth, mottle on the leaves, and fallen foliage. Sometimes necrotic lines also affect the leaves. The pepper itself sometimes looks pale with sunken lesions. Some pepper varieties have ring spots. CMV-infected tomato plants turn yellow and stunted in the early stages. Sometimes this is confused with tomato aspermia virus, another of the many viruses that infect tomatoes. The most noticeable symptom in a tomato plant is lacing of tomato leaf blades. Similarly, potato mosaic virus is often confused with CMV. The most noticeable symptom in potato plants is ruffled leaf ends and a bushy overall appearance.

CMV also affects legumes and is often confused with Bean Common Mosaic Virus or Peanut Stunt Virus. To tell them apart, look for an agricultural advisory office that can help you diagnose. Characteristically, CMV-infected beans have mottled leaves and zipper-like roughness along the leaf veins. Bean pod production is greatly reduced and flowers are aborted before they can pollinate.

In general, broadleaf plants exhibit typical mottle, leaf curl, and defoliation associated with CMV. Some develop rings, some have spots, and some develop necrotic lesions shaped like oak leaves. Mottle occurs in the longitudinal veins of blades of grass. In later stages, grasses develop round, yellow-edged lesions with a dark center.

Vulnerable Species

Because CMV affects over 1200 plant species, we've compiled a list of some of the most well-known hosts. This allows you to protect nearby plants from infection.

This list is extensive but not complete. Study cucumber mosaic viruses to determine whether or not a plant species you are growing has CMV resistance.

Edible hosts

ornamental plants

  • aconite
  • aster
  • astilbe
  • bluebell
  • Commelina
  • Coreopsis
  • Datura
  • delphinium
  • Echinacea
  • gypsophila
  • Helianthus
  • Heuchera
  • hosta
  • Ligularia
  • lily
  • lysimachy
  • Oenothera
  • penstemon
  • phlox
  • primrose
  • scabiosa
  • sedum
  • viola

weed hosts

  • Bittersweet
  • Black nightshade
  • Caltrop
  • Common pokeweed
  • crowfoot grass
  • Chickweed
  • figwort
  • Great mullein
  • mallow
  • Swamp Yellow Cress
  • foxtail
  • plantain
  • primrose
  • purslane
  • Sabi grass
  • Smartweed
  • Threesome Joe Pye
  • Wild Gooseberry
  • wood sorrel

Control of cucumber mosaic virus

Cucumber mosaic virus on cucumber fruitOn cucumbers, CMV-infected fruit becomes mottled, warty, and bitter. Source: William M. Brown Jr.

Once a species is infected with CMV, you must remove the entire plant and either quarantine it in a sealed plastic bag while awaiting diagnosis, or discard it. Do not compost infected plants as they could spread the virus to other materials in the pile. There are no chemicals that treat or cure the disease, and with such a broad host range, the best way to control CMV is prevention through culture methods, seed treatments, and planting virus-resistant cultivars.

Prevent cucumber mosaic virus

With integrated pest management (IPM) practices, you can prevent CMV from taking root in your garden. Control aphids and aphid vectors like ants and you're on your way. Neem oil is a great organic option for aphid infestations in cooler to temperate weather. Spray a diluted commercial formula on your plants as a prevention or treatment for aphids. Insecticidal soap is another useful organic way to combat pesky aphid infestations.

Avoid spraying during the flowering phase and in high heat. Heat will either burn the plant covered with neem or simply evaporate the treatment altogether. Lacewings, ladybugs and birds like to eat aphids. Release them in your garden or encourage them with certain plants and foods. You can also remove aphids by hand or by wiping them off plants with a rag dipped in soapy water. Or use companion plantings to attract beneficial insects.

There are cultural practices that help prevent CMV. The basis of this is visiting your garden regularly to make sure there are no diseases present. Remove weeds as soon as they appear in your garden beds. Clean up dirt that falls on the floor.

Because CMV is so widespread, much research has been done to engineer plants with partial to full resistance. Planting resistant varieties is a great way to prevent the disease. When it comes to tomatoes, finding a variety that is CMV resistant can be difficult. However, some cultivars have been developed for resistance to other viral diseases such as tobacco mosaic virus. These cultivars also sometimes exhibit disease resistance to CMV.

Over-fertilizing plants can attract aphid species. This allows soil to dry out. Keep the plants well watered and you will prevent the viral protein coat associated with CMV from opening. Since seeds can carry and transmit the disease into a garden, always buy from reputable sources. If you bought seeds from a smaller company that doesn't have the resources, a large dealer would treat the seeds to prevent possible infection. The same goes for seeds you received from a seed swap or a friend. Simply soak the seeds in a mason jar solution of water and 10% bleach solution for 15 minutes. If bleach isn't your style, use 20% apple cider vinegar. Then rinse the seeds thoroughly with cold water to prepare them for planting.

A hot water seed treatment is also an effective way to prevent CMV. Wrap your seeds in cheesecloth or a coffee filter. Soak the pack in water heated to 120 degrees Fahrenheit for 20 to 30 minutes. Then soak the wrap in cold water for 5 minutes. A good way to avoid infection is to never plant seed from an infected plant.

frequently asked Questions

Q: How do I get rid of cucumber mosaic virus?

A: Unfortunately, there is no cure. The best control is to prevent aphids spreading CMV on your plants.

Q: Can you eat cucumbers with mosaic virus?

A: You can, but you probably don't want to consume them. At this point they are intensely bitter and sometimes mushy.

Q: Can tomatoes get cucumber mosaic virus?

A: Yes, they can. All nightshades can contract CMV.

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