Disease Information

History
Damage
Crops
Vector
Hosts

History

What about this name, Topsovirus?…..

Tospoviruses are a group of plant viruses now widespread in the greenhouse industry. The genus name is derived from the first well-known member in the group -tomato spotted wilt virus. Many people are familiar with tobacco mosaic virus - the type member of the genus tobamovirus, named in a similar fashion.

Tomato spotted wilt virus (TSWV) has long been known to infect ornamentals, vegetable crops and field crops. The name tomato spotted wilt virus specifically describes a wilt and spotting disease of tomatoes, but many plant species react to this particular tospovirus with a range of other symptoms. Compared to other plant viruses, TSWV infects an extremely wide range of plants, including vegetables, floral crops, bedding plants and a large number of weeds. In general, TSWV is found in field crops and vegetables, although greenhouse tomato and pepper varieties can also be infected. For field tomato, tobacco and peanut growers in the southeastern United States, TSWV is a major problem with a significant economic impact on these industries.

In the last 10 years ornamental greenhouse crops have also been plagued with tospovirus problems. Researchers noted that the viruses isolated from greenhouse crops usually did not react in ELISA tests for TSWV. In 1989, an ELISA test was developed specifically for TSWV-Impatiens, the greenhouse strain of TSWV. By 1991 TSWV-impatiens was declared a separate species of tospovirus and named impatiens necrotic spot virus (INSV) descriptive of the effect on garden impatiens. INSV has had an enormous impact on floriculture but so far remains primarily a greenhouse and florist crop problem.

While the name of the virus commonly found destroying impatiens, cyclamen and other greenhouse crops changed from TSWV (1988 and before) to TSWV-Impatiens isolate (1989) and finally to INSV (1991) the impact of this virus continues to be enormous.

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Damage

Why is INSV suddenly so damaging in greenhouses?…..

Tospoviruses are transmitted by various thrips species. Disease specialists report that the epidemic of INSV coincided directly with the rise in western flower thrips (Frankliniella occidentalis Pergande) populations in the greenhouse industry in the late 1980's. Western flower thrips is the primary vector of INSV and as thrips spread through the industry, losses due to INSV quickly followed. Thrips are difficult to control and may spread the virus throughout a crop before a grower even realizes they are present.

INSV is also menacing because of the large number of plant species it can infect. Most plant viruses cause disease in a narrow group of plants. For example, carnation mottle virus is restricted mostly to Caryophyllaceae members such as Dianthus. INSV is unusual, as it is a generalist virus with many hosts, and its casualty list of florist crops, annual, perennial and herb species gets longer each year. This is disastrous for those greenhouse businesses growing a large selection of flowering crops because thrips will spread the virus to almost every major florist crop as well as many minor crops. Roses and poinsettias, both prone to many insect and disease problems, remain two popular crops that so far are not known to be susceptible to tospoviruses (INSV and TSWV).

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Crops

What greenhouse crops top the list for losses due to INSV?…..

While the list of plants infected by INSV keeps growing, and many growers may wonder if there anything that is not infected, there are some crops that are extremely susceptible. At the top of the list are cineraria, cyclamen, gloxinia, impatiens and New Guinea impatiens. Also devastated by the disease are exacum, begonia, ranunculus and primroses. Gloxinia is very sensitive to INSV and young plants are usually killed by the virus. If INSV infects older gloxinias, the result is usually brown leaf spots and crown rot, and sometimes necrotic lines in the leaves.

Impatiens, of course a popular host for INSV and part of its namesake, usually do not die - but they do not thrive either. Infected impatiens still flower, becoming attractive to western flower thrips as well as a source of virus to be spread to other crops. Since impatiens may be infected as very small transplants, growers need to guard against bringing infected plugs into their operations.

Cyclamen infected with INSV often do not show obvious symptoms until they are just about to flower. Flowers emerge with color break and entire plants may wilt and collapse, after a nearly full investment has been made into the crop. Exacum shows a few papery spots, then suddenly entire sections of plants wilt and collapse. Within days, plants are dying throughout the crop.

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Vector

So how do thrips actually infect the crop?.…..

Thrips that attack plants are insects about 1/16th of an inch long. They have larval, pupal and adult life stages. Depending on the weather, a thrips life cycle may take a little over a week (under ideal greenhouse conditions) to as long as 5 weeks. While thrips spread easily by crawling from plant to plant, flying (as adults) or by being blown in wind currents, they are also moved on transplants, cut flowers and potted plants from business to business in the industry.

Thrips feed with piercing-sucking mouthparts and extract plant sap out of cells. Thrips injury is often seen as a silvering of leaves and flower petals. If thrips feed on leaf and flower buds, they cause distorted growth. If during their feeding they extract sap from an infected plant, move to a new plant, and resume feeding, they can introduce tospovirus and spread the infection to another plant. Thrips can move INSV through a crop with results that are much more serious than the thrips damage alone. Since thrips can also maintain continuous populations in greenhouses with various crop species and alternate hosts (e.g. weed species under benches), control can be very difficult.

The western flower thrips, Frankliniella occidentalis, is the vector of INSV. Several other thrips, including the western flower thrips, transmit TSWV. The western flower thrips has become very common in greenhouses throughout the United States and feeds on most flowering plants. Some western flower thrips populations have developed resistance to commonly used pesticides making control a challenge. A pest in its own right, it is infinitely more damaging when vectoring tospoviruses. Tiny larval thrips acquire tospovirus when feeding and then spread it to new plants as adults. Combine a thrips vector that feeds on almost any flowering plant and a virus with a very wide host range and the result is epidemic INSV.

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Common Hosts

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