VIRUS DISEASES OF GREENHOUSE TOMATO AND THEIR MANAGEMENT

Virus Diseases of Greenhouse Tomato and Their Managment
Vegetable Disease Information Note 15 (VDIN-0015)
Charles W Averre, Extension Plant Pathologist
Guy V. Gooding, Research Plant Pathologist
Virus diseases of greenhouse and field tomatoes in North Carolina occasionally cause serious damage and large economic loss. The amount of loss can vary from 5% to 90% depending on the virus disease involved, the strain of the virus, the variety of tomato, the age of the plant at infection time, the temperature during disease development, the presence of other diseases, and the extent that viruses have spread in the planting. The symptoms, means of spread, hosts, sources, and control procedures for virus diseases growers are likely to encounter are summarized in table 1. In order to control a virus problem it must be properly identified. Terminal shoots may be sent to the Plant Disease and Insect Clinic at NCSU by the county agent for an accurate diagnosis. With the exception of TMV, most viruses attacking tomatoes survive only in living plants or briefly in insects. They are spread primarily by aphids. Very few viruses are seed-borne. Usually the initial field distribution of diseased plants tend to be spotty and concentrated along the edges of the field. Some varieties are more susceptible than others. (See Vegetable Disease Information Note No. 6).

The most important virus diseases on tomatoes in North Carolina is tobacco mosaic virus (TMV), but other viruses may cause significant losses. There are many strains of TMV and a few which occur in North Carolina are very damaging. The common ones only cause a mild mosaic. TMV can survive many months outside a living plant or insect, on tools, greenhouse frames, sawdust, and in the soil; in dried leaves such as in cigarettes it can survive for many years. It is rarely transmitted by insects. It is easily spread by touch from diseased plants or from contaminated objects. TMV can be seed-borne; however, the magnitude of seed with virus in contaminated lots is usually under two percent, but with subsequent handling of plants all plants usually become infected before picking.

Management

Control of viruses on tomato is based on a total program all year. By following these procedures you will reduce virus diseases and losses in a tomato crop.

Obtain seed from reputable sources that have been fermented or treated with acid or bleach by the seedsman. If seed has not been treated, proceed as follows: Mix 2 pints of 5.25% sodium hypochlorite solution (household bleach eg. Clorox) in 8 pints of water. Wash one pound of cleaned, fresh or dry, tomato seed in one gallon of the mixture for 40 minutes. Provide gentle and continuous agitation. Remove seed and spread on paper to air dry promptly. Prepare fresh bleach solution for each batch of seed. Caution and remarks: (1) Beginners should try this seed treatment on inexpensive seed prior to treating large lots. (2) Seed germination may be reduced with some lots of seed. (3) Research has shown that this seed treatment is enhanced by prewashing the seed for 15 minutes in a solution of trisodium phosphate (one ounce of TSP in two quarts of water). (4) Do not recontaminate the seed by placing in used containers etc.

Control aphids early in the season to reduce initial infection and spread. It may be a good idea to spray weeds bordering the field with an aphicide prior to seeding or planting the field, and all annual weeds in the field should be destroyed. Avoid planting tomatoes close to established cucumber, squash, potato, tobacco, and pepper fields. Greenhouses containing tomatoes should be bordered by at least 150 feet of turf or pavement.

Decontaminate stakes, tools, tables, etc. by: (1) heating or steaming at 300 degrees F for 30 minutes; (2) soaking 10 minutes in 1% formaldehyde or a 1:10 dilution of a 5.25% sodium hypochlorite (10% Clorox) solution, do not rinse; or (3) by washing (enough to clean) in detergent at the concentrations recommended for washing clothes or dishes. Keep all solutions fresh.

Spray the plant bed prior to pulling or handling plants (24 hours) with whole or skim milk at the rate of 0.5 gallon per 100 square feet of beds; thorough coverage of plants is important.

For tomato production, seed in individual pots (peat pots, etc.) and do not touch or handle plants prior to setting in the field or greenhouse. Discard pots with seedlings that show leaf twisting, mosaic, or unusual growth. Do not touch other seedlings while discarding them.

Dip hands in milk while handling plants every 5 minutes (more often if different lots of plants are handled). Rubber gloves will protect hands.

Remove and destroy diseased plants early in the season. Do not touch healthy plants with the diseased plants when removing them.

Disinfest equipment, tools, and hands on a regular basis while pulling, pruning, trellising, harvesting, and spraying plants, and when moving from one row or area to another.

Destroy tomato plants in the bed, field, or greenhouse as soon as possible after harvest.

Rotate tomato crops with small grain, corn, or pasture. Avoid following tomato crops after crops of tobacco, pepper, eggplant, or cucurbits.

Table 1. Summary of virus diseases and their control. Symptoms may vary greatly depending on the type of virus, time of infection, and growing season (those with asterisk are commonly found in North Carolina).

Disease	Common Symptoms	Usual Means of Spread	Principal Hosts	Virus Source	Control Summary
Tobacco mosaic* (TMV, tobacco and tomato strains)	Variable depending on strain. Leaves light green or yellow mottling, rough edges turn down. Plant Dwarfed. Fruit small, poor set, brown streaks. Shoestring of leaves may occur on young plants.	Contact, hands, tools, etc.	Pepper, tobacco, petunia, horsenettle, jimsonweed, Jerusalem cherry, nightshade, ground cerry, plantain, others	Dried tobacco products, tools, seed, perennial weeds	Sanitation, disease-free plants, milk, bleach, detergent, steam, head, soil fumigation. Avoid tobacco products and plants.
Cucumber mosaic* (CMV)	Similar to TMV, upper leaf may be very narrow and twisted giving them a shoestring appearance that may be confused with 2, 4-D type injury. Later excessive number of lateral leaflets are produced. Plants may be dwarfed or stunted.	Aphids	Weeds, flowers, vegetables, milkweed, ground cherry, horsenettle, pokeweed, jimsonweed, marigold, petunia, zinnia, cucumber, melons, celery, pepper, others	Weeds, flowers	Do no plant closer than 100 ft. from weeds or flowers. Control aphids.
Double streak TMV + PVX (Potato virus X or CMV)	Usually affects large plants, dwarfed, spindly appearance, rolling and withering of leaves. Long brown streaks on petioles and stems. Irregular ripening of fruit, light brown sunken spots on green fruit.	Same as for TMV and aphids if CMV involved.	Same as for TMV and CMV	Same as for TMV and CMV Irish potato	Same as for TMV. Avoid potato or potato plantings near the planting. Control aphids. Temperatures above 100 F for a few days will attenuate PVX and CMV.
Potato virus Y* (PVY)	Dark brown dead areas between veins in leaflets near maturity, yellowing along veins, faint mottling on leaves; petioles curved downward; stems with purple streaking; symptoms usually become mild, but severe symptoms develop if TMV is present.	Aphids	Solanaceous and leguminous plants	Potato and pepper. Usually does not overwinter in North Carolina.	Do not plant close to potato, tobacco or pepper fields. Do not obtain plants from south Florida. Control aphids.
Tobacco etch* (TEV)	Terminal leaflets cupped, petioles bent downward. Stunting, similar to PVY except not as severe and no stem or fruit mottling.	Aphids	Many similar to PVY	Weeds, horsenettle, ground cherry	Same as for PVY.
Tobacco and tomato ringspot (TRSV)	Ringspot or zigzag lines on foliage, occasionally on stems, petioles, and fruit. On fruit, rings are often concentric.	Dagger nematode, thrips, mites, grasshopper, flea beetles	Very extensive; annuals, weeds, vegetables, ornamentals, tobacco, soybeans	Weeds, petunia seed.	Do not plant closer than 100 ft. from weeds or flowers. control all insects.
Spotted wilt (SWV)	Orange or bronze colored flecks on middle or lower leaves and calyx or stems and petioles. Stunting, black depressed spots.	Thrips	Very extensive on annuals, bulb crops, and weeds.	Thrips, bulbs	Do not plant closer than 100 ft. from weeds or flowers. Control thrips.
Tomato aspermy (TAV)	Death of terminal tips, much branching, bushy appearance of plant. Fruit seedless, distorted, and production reduced.	Aphids	Solanceous plants, lettuce, spinach, weeds and many others.	Weeds	Do not plant closer than 100 ft. from weeds or flowers. Control aphids.

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Recommendations of specific chemicals are based upon information on the manufacturer's label and performance in a limited number of trials. Because environmental conditions and methods of application by growers may vary widely, performance of the chemical will not always conform to the safety and pest control standards indicated by experimental data. All recommendations for pesticide use were legal at the time of publication, but the status of registration and use patterns are subject to change by actions of state and federal regulatory agencies. 08/91

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Reformatted Dec. 2000 by A.V. Lemay