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:

Cucumbers:

Winter squash:

Summer 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 reserviors 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.

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

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.

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

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