There are several viruses that affect cucumbers, melons, pumpkins, squash, and other members of the cucurbit family. Cucumber mosaic virus (CMV), zucchini yellow mosaic virus (ZYMV), watermelon mosaic virus (WMV) and papaya ringspot-W (PRSV-W) are found in Connecticut. All are transmitted from diseased plants to healthy plants by aphids. They are transmitted from plant to plant in a non-persistant manner. This means they acquire the virus from an infected plant almost immediately but are able to infect healthy plants for only a short time, usually a few days to a week. Only a small number of aphids are needed to spread the virus throughout the field. CMV is also spread by spotted and striped cucumber beetles.

Cucumber mosaic, caused by the cucumber mosaic virus, is one of the most widespread and destructive diseases on cucumber and muskmelon. The disease has been known since the early 1900’s, and is now found worldwide. The virus can infect cucumber, squash, muskmelon, and numerous other hosts in 34 plant families, including tomato, lima bean, beet, sweet corn, and sweet potato. Most often, actively growing and mature plants are affected. It rarely infects plants in the seedling stage, but will kill them quickly when it does. It causes a decrease in the number and the quality of the fruit.

Symptoms. When plants are vigorously growing, symptoms appear on the youngest leaves. Leaves have small yellowish areas, are curled slightly downward at the edges, and become puckered or crinkled with the tissue between the small veins becoming raised. As the leaf expands, it becomes distinctly mottled yellow and green. All future leaves will grow this way. As the leaves age, the puckering becomes more distinct and the mottling less distinct. Leaves are smaller than normal. Plants are severely stunted. The plant will produce little fruit.

When older, less actively growing plants are infected, the symptoms are less distinctive. Older leaves may look healthy for some time. A few leaves near the growing tip may turn yellow and wilt, and have brown, withered edges. Occasionally, all the leaves of a shoot may appear this way. The oldest leaves may gradually die. Usually, portions of the leaf turn yellow, often starting at a lobe of the leaf or a V-shaped section of the leaf, at the edge. The yellow portions, and usually the entire leaf, quickly turn brown and die. The leaves usually remain attached to the vine.

Fruit symptoms vary greatly by species:

• In cucumbers, a yellowish-green mottling starts at the stem end and gradually turns the entire fruit yellowish-green mottled with dark green. The dark green areas may be raised, giving the fruit a warty appearance. Fruit may also be russetted or whitish-green and blunt.
• In summer squash, warty raised yellow areas appear on the fruit, surrounded by dark green areas.
• For other cucurbits, fruit symptoms are often absent, or the fruit may be a bit mottled or warty.

Watermelon mosaic is caused by the watermelon mosaic virus type 2. It affects cucurbits and peas, alfalfa, vetch, clover and a few other plants. The virus is widespread, and is found on all continents. It has been known since the 1930’s.

Symptoms. Symptoms can vary depending on the plant species.

Watermelon and muskmelon:

• Plants are stunted; severely if infected young.
• Leaves are malformed, mottled yellow and green, and blistered.
• Watermelon fruit can be misshapen, dwarfed, mottled, or spotted.
• Muskmelon plants may produce few fruit.

Cucumbers:

• Uniform green to dark green mosaic or mottling occurs on leaves.
• Fruit are shortened, gnarled and sometimes knobby.

Winter squash:

• Leaves have a faint green to severe yellow mottle, and may be malformed puckered or blistered.
• Veins sometimes extend beyond the edges of the leaves.
• Plants may be distorted or very bushy (no branch growth).
• Knobby overgrowths cover the fruit.

Summer squash:

• Green overgrowths on infected yellow fruit.
• Plants may become vine-like.
• Leaf symptoms are similar to those on winter squash

Papaya Ringspot-W (PRSV-W) is caused by papaya ringspot virus type W. It was formerly known as watermelon mosaic virus-1. It can infect papaya and most of the cucurbits. Although it is most common in the tropics and sub-tropics, it occasionally causes big problems in temperate regions.

Symptoms. This virus can cause severe plant stunting. On the leaves, a green mosaic or mottled pattern is usually accompanied by malformations, including puckering, blisters, leaf distortions, and narrow leaf blades. The youngest leaves are often reduced to just the main veins. Fruit is often malformed, and can have a color break pattern, where the normal color of the fruit is ‘invaded’ by an unusual color, generally in sharply defined sections.

Zucchini Yellow Mosaic is caused by the zucchini yellow mosaic virus (ZYMV). It is known to infect pumpkin, squash, summer squash, melon, watermelon, other cucurbits, and possibly many other plants. It was first recognized in 1981, and is found in 22 countries on 5 continents. This is a relatively new virus and very little is known about it. Although the symptoms are similar to PRSV-W, these viruses are not related. However, it is related to WMV.

Symptoms. This disease causes plant stunting. On the leaves, it causes a yellow mosaic or mottling pattern, leaf malformation and blisters, and dead patches. Malformation and severe seed set problems can occur on the fruit. Knobs can be formed on squash and pumpkin. On melon and watermelon, cracks can occur, which are usually deep, and longitudinal and radial.

Prevention of all cucurbit viruses.

• Although seed transmission is rare for most of these viruses, use disease-free seed.
• Plant away from or upwind of other infected fields.
• Plant early to escape peak aphid season.
• Control weeds, as many are reservoirs where the virus spends the winter.
• Handle the plants as little as possible to avoid mechanical transmission.
• Planting wheat in and around cucurbits can serve as a protection crop, allowing the aphids to wipe the viruses off of their mouthparts before they reach the cucurbit plants. Also, aphids are unable to reproduce on wheat.
• Reflective mulches may help deflect insect vectors.
• See current recommendations for control of insects. Mineral oil sprays have been used to prevent transmission of the virus when insects are feeding.
• Resistance is available for CMV, but nor for ZYMV. There are a few varieties that are resistant to WMV-2 and PRSV.

References.

• Provvidenti, R. 1996. Zucchini Yellow Mosaic in Compendium of Cucurbit Diseases, p. 44. APS Press, St. Paul, MN., T. A. Zitter, D. L. Hopkins, and C. E. Thomas, eds.
• Provvidenti, R. 1996. Cucumber Mosaic in Compendium of Cucurbit Diseases, pp. 38-39. APS Press, St. Paul, MN. T. A. Zitter, D. L. Hopkins, and C. E. Thomas, eds.
• Provvidenti, R. 1996. Watermelon Mosaic in Compendium of Cucurbit Diseases, . pp. 43-44. APS Press, St. Paul, MN. T. A. Zitter, D. L. Hopkins, and C. E. Thomas, eds.
• Provvidenti, R. 1996. Papaya Ringspot-W in Compendium of Cucurbit Diseases, p. 40APS Press, St. Paul, MN. T. A. Zitter, D. L. Hopkins, and C. E. Thomas, eds.
• Sherf, A.F. and A. A. MacNab. 1986. Vegetable Disease and Their Control. John Wiley and Sons, New York.
• Wick, R. L. 1997. Proceedings of the New England Vegetable and Berry Conference. p. 184. Cooperative Extension System.

By Pamela S. Mercure, IPM Program Assistant, University of Connecticut, 1998

Reviewed by: T. Jude Boucher, IPM, University of Connecticut. 2012

This information was developed for conditions in the Northeast. Use in other geographical areas may be inappropriate.

The information in this document is for educational purposes only.  The recommendations contained are based on the best available knowledge at the time of publication.  Any reference to commercial products, trade or brand names is for information only, and no endorsement or approval is intended. The Cooperative Extension System does not guarantee or warrant the standard of any product referenced or imply approval of the product to the exclusion of others which also may be available.  The University of Connecticut, Cooperative Extension System, College of Agriculture, Health and Natural Resources is an equal opportunity program provider and employer.