This virus, abbreviated TSWV, has become a serious problem for some Iowa tomato growers in 1994. While most fields we have seen have only a few symptomatic plants, some fields have more than one-third symptomatic plants. Because plants showing symptoms this early in the season seldom produce marketable fruit, this means a yield loss of at least one-third. According to unconfirmed reports, some TSWV-stricken fields have been plowed down. Some questions and answers about this situation:

What are the symptoms of TSWV?

Symptoms vary depending on plant age, environmental conditions, and other factors. Plants infected early in the season tend to be stunted, often dramatically, and may fail to set any fruit. These symptoms are often the first ones that catch a grower's eye. Typical foliar symptoms include yellowing and a bronze or purplish cast, especially in the upper leaves. Eventually, the discolored leaves wilt and die, at which point TSWV infection is often mistaken for bacterial or fungal diseases. If plants become symptomatic after fruit set, immature fruit sometimes show green or brown, concentric rings with raised centers. More often, though, typical fruit symptoms include slight malformation or brown, sunken spots that can be mistaken for symptoms of bacterial spot.

Because of the ambiguities invloved in diagnosing TSWV on the basis of field symptoms, it's a good idea to send plants to ISU for a confirmation. Send entire plants, including fruit to the Plant Disease Clinic, 323 Bessey Hall, ISU, Ames 50011. We can usually make a rapid diagnosis at no cost to the grower.

Why is TSWV more severe than usual in 1994?

The likely answer is that populations of thrips, the insect vector of TSWV, were unusually high this spring and summer. In many cases, the problem probably began in the greenhouse. TSWV can survive the winter in hundreds of plant species, including scores of common bedding plants and weeds. Several growers recall seeing populations of insects on their tomato transplants; it's likely that thrips were present in at least some of these cases. Thrips can pick up the virus from an infected plant and spread it repeatedly, every time it feeds on a new plant. Because there is usually a time lag of several weeks between TSWV infection and symptom development, growers can unsuspectingly transplant infected plants.

Thrips can continue to spread the virus in the field. If infected plants are present early in the season, the extent of the problem can continue to snowball. It's a tough problem to control in the field for several reasons:

1. Insecticide control of thrips may be erratic due to unpredictable timing of arrival of more thrips from nearby fields, and to resistance by thrips to many insecticides.

2. Thrips tend to hide in crevices in flowers and young foliage, where spray penetration is often poor.

3. Roguing out (removing) symptomatic plants is seldom effective as a control, because TSWV has often spread before symptoms develop.

What should be done to prevent TSWV in 1995?

In the greenhouse:

1. Practice good sanitation. Clean up all weeds in and surrounding the houses. Remove any symptomatic plants.

2. Do not accept any shipments of transplants with thrips populations or symptoms.

3. Monitor thrips in the greenhouse, using yellow or blue sticky cards placed near vents and slightly above crop level. Thrips become stuck in the sticky material on the cards and should be counted at least once per week. Numbers as low as five thrips per card per week may be enough to justify insecticide use, especially if the numbers are increasing.

4. Spray recommended insecticides at a 5-day interval if sticky cards indicate sufficient numbers of thrips present. Rotate classes of insecticides (pythrethroids, carbamates, chlorinated hydrocarbons, organophosphates, and soaps) to help prevent the buildup of resistant populations. Apply insecticides in the early morning, when thrips are most active and when the potential for phytotoxic reactions is lowest.

5. If possible, grow tomato transplants in a separate house, isolated from other possible sources of TSWV and thrips.

In the field:

1. If possible, buy transplants that have been certified disease- free. Inspect transplants and reject any shipments with bronze or purple discoloration on leaves.

2. As already mentioned, roguing and insecticide sprays in the field may be ineffective, but worth a try in the absence of any other strategy. Infected plants cannot be cured.

To repeat: Once symptoms start developing in the field, it is often too late to head off an epidemic. The bottom line for TSWV control is this: focus on starting with disease-free plants. If you grow your own transplants, take the recommended actions to assure that you won't start the season with a problem. If you buy transplants, make sure that a rigorous virus-control program has been practiced by your supplier.

This article originally appeared in the July 15, 1994 issue, pp. 1994 issue, pp. 112-113.