Mosaic Diseases of Tomatoes

Common Mosaic and Cucumber Mosaic

Pathogens. Tobacco mosaic virus (TMV) and the closely related tomato mosaic virus (ToMV) cause common mosaic; cucumber mosaic virus (CMV) causes cucumber mosaic. Both diseases cause stunting of the plants and a lowering of yield. For both diseases, symptoms can vary widely, depending on the age of the plant, the variety of tomato, the strain of the virus causing the disease, and the environmental conditions.

TMV is a worldwide pathogen and one of the first plant viruses that scientists described. It has been important in Europe since the mid-1800s. In the United States, it was first reported in Connecticut on tobacco in 1899 and on tomato in 1909. It has a very wide range of hosts, including tomato and the related plants of eggplant, nightshade weeds, pepper, potato, and tobacco. TMV is seen on apple, beet, sugar beet, buckwheat, currant, grape, pear, spinach and turnip, as well. In addition, ornamentals, foxglove, phlox, snapdragon and zinnia, and weeds of the amaranth and goosefoot families are affected.

CMV is another widespread virus. It was first reported in the 1900's in several places in North America. It is now considered to be worldwide. It has an very wide host range, which includes tomato, carrot, celery, cucurbits, legumes, lettuce, spinach, pepper, dahlia, delphinium, columbine, geranium, petunia, phlox, zinnia and viola, and many weeds, such as chickweed, pokeweed and milkweed.

Symptoms. Common mosaic (TMV/ToMV) often causes leaves to be stunted or elongated, in a condition called "fernleaf." This name is due to the strong resemblance of these leaves to leaves of many kinds of ferns. The youngest leaves may be curled. Leaves may be mottled yellow and dark green. This is the symptom which gives the disease the name "mosaic." The dark green areas may be raised. Mottling usually occurs most severely on plants grown under low light and low temperature, conditions which may exist in a greenhouse during the winter. Leaf stunting and distortion are usually worse under these conditions, as well.

Stem streaking occasionally occurs with dark streaks that are either sparse and short or prevalent and long. Such stems are easily broken and have brown areas inside. Fruit is rarely affected. It may be mottled or a brownish bronze color inside, which can be seen through the thin skin of the fruit. Fruit may show uneven ripening or yellow rings, as well.

Severe strains of TMV/ToMV cause the lower leaves to turn downward at the petiole, become rough and crinkled or corrugated, and possibly cause the leaflets to curl downward at the edges. Younger leaves may have extensive yellow to white areas with dark green blisters.

When some tomato varieties are infected with TMV/ToMV and kept at high temperature conditions (80o to 85o F) for a prolonged time, they develop dead areas on leaves, stems and roots.

Cucumber mosaic virus (CMV) causes plants to become yellow, bushy and very stunted. Leaves may be extremely distorted and malformed. Leaflets are often very narrowed; this is called "shoestring". Often the leaves on one portion of the plant (e.g., the top or the bottom) show severe symptoms, while those higher or lower in the plant are less affected. Other leaf symptoms include a yellow and green mottling similar to tobacco mosaic symptoms. Severely affected plants produce few fruit.

Identification of the Diseases. It is difficult to diagnose which virus is present without the assistance of an experienced diagnostician. The fernleaf and the shoestring symptoms are very similar, and the mosaic symptoms are indistinguishable. Control and prevention measures are very different for the two diseases, so accurate diagnosis is important.

Prevention. TMV is spread readily by touch. The virus can survive on clothing in bits of plant debris for about two years, and can easily enter a new plant from a brief contact with a worker's contaminated hands or clothing. Tobacco products can carry the virus, and it can survive on the hands for hours after touching the tobacco product. Ensure that workers do not carry or use tobacco products near the plants, and wash well (with soap to kill the virus) after using tobacco products. Ensure that workers wear clothing not contaminated with tomato, tobacco or other host-plant material. Exclude non-essential people from greenhouses and growing areas.

Choose resistant varieties. Use disease-free seed and transplants, preferably certified ones. Avoid the use of freshly harvested seed (2 years old is best if non-certified seed is used). Seed treatment with heat (2 to 4 days at 158o F using dry seed) or trisodium phosphate (10% solution for 15 minutes) has been shown to kill the virus on the outside of the seed and, often, most of the virus inside the seed as well. Care must be taken to not kill the seeds, though. Use a two-year rotation away from susceptible species. In greenhouses, it is best to use fresh soil, as steaming soil is not 100% effective in killing the virus. If soil is to be steamed, remove all parts of the plant from the soil, including roots. Carefully clean all plant growing equipment and all greenhouse structures that come into contact with plants.

When working with plants, especially when picking out seedlings or transplanting, spray larger plants with a skim milk solution or a solution made of reconstituted powdered or condensed milk. Frequently dip hands, but not seedlings, into the milk. Wash hands frequently with soap while working with plants, using special care to clean out under nails. Rinse well after washing. Tools should be washed thoroughly, soaked for 30 minutes in 3% trisodium phosphate and not rinsed..

Another method for control of this disease is to artificially inoculate plants with a weak strain of the virus. This will not cause symptoms on the plants but protect them against disease-causing strains of the virus. This is used commonly in Europe, but strains of the virus are not yet available in the United States due to concerns about the possibility of the weak virus strains causing disease on the plants.

Cucumber mosaic is spread in a nonpersistant manner by aphids. It is not spread by seed. Control weeds, many of which are host species. Surrounding tomato fields with a taller, non-susceptible plant, such as corn, may help shield the plants from aphids blowing in from other areas. See current recommendations for control of aphids, although it is generally considered that insecticides will not control this disease. The aphids pick the virus up from the plants in about a minute and are able to spread it immediately. Insecticides take longer than this to kill the aphids. Mineral oil sprays can be used to prevent the virus from being transmitted.

At this time there are no tomato varieties resistant to CMV.

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

References.

Sherf, A.F. and A. A. MacNab. 1986. Vegetable Diseases and Their Control. John Wiley and Sons, New York.

Zitter, T.A. 1991. Cucumber Mosaic in Compendium of Tomato Diseases. APS Press. St. Paul, MN. pp. 35-36. J.B. Jones; J. P. Jones; R.E. Stall; T. A. Zitter, eds.

Zitter, T.A. 1991. Tomato Mosaic and Tobacco Mosaic in Compendium of Tomato Diseases. APS Press. St. Paul, MN. p39. J.B. Jones; J. P. Jones; R.E. Stall; T. A. Zitter, eds.

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