Greenhouse - INSV and TSWV Viruses

INSV and TSWV are two common viruses that attack greenhouse plants in Delaware. The following is an article on this subject.

Greenhouse growers are gearing up for production of bedding plants and perennials for spring sale. Production of these transplants may involve propagation of possibly infected overwintering stock or movement of plugs, transplants and sometimes plant diseases long distances. Diseases caused by impatiens necrotic spot virus and tomato spotted wilt virus are important causes of losses in greenhouse ornamentals. Although the virus and thrips (the disease vector) are common, growers can avoid crop losses by aggressively controlling thrips and the viruses it spreads. Impatiens Necrotic Spot Virus (INSV) and Tomato Spotted Wilt Virus (TSWV) are two different but closely related viruses causing similar symptoms.

The most common and dramatic problems of greenhouse ornamentals in Delaware have been due to INSV. This virus is usually the one causing disease symptoms on impatiens, New Guinea impatiens, begonias, petunias snapdragons, cyclamen, cineraria and gloxinia. Both viruses are transmitted from plant to plant by western flower thrips. An adult thrips can infect a plant with virus after feeding for only 30 minutes. TSWV is very damaging to tobacco, tomatoes, and peppers, but it also attacks some ornamentals, most often dahlias imported from overseas, and chrysanthemums and (rarely) ivy geraniums. Both viruses have a very wide host range and both are vectored by western flower thrips.

Virus Disease Symptoms. TSWV/INSV causes a wide variety of symptoms including wilting, stem death, stunting, yellowing, poor flowering; and sunken spots, etches, or ring spots on leaves. Symptoms are not very specific or consistent, and merely tell the grower that there is something wrong with the plant. Many other diseases and plant problems can cause symptoms that resemble TSWV/INSV. Virus symptoms may depend on time of year, type of plant, age of plant, plant physiological state, growing conditions at the time of infection, and strain of virus. In the UD Plant Disease Diagnostic Laboratory, positive diagnosis is made by testing infected plant tissue with an ELISA test based on a chemical reaction to the virus proteins. Separate tests are used to look for both TSWV and INSV because a plant may have either or both viruses.

Managing INSV and TSWV in greenhouse ornamentals.

Inspect incoming plant material for signs of thrips feeding injury, or for symptoms indicative of TSWV or INSV infection. Most plant materials coming from suppliers are not guaranteed to be disease free; thus your inspections are most important. Insist on good thrips control from your plant suppliers.

Isolate incoming plants from all other plants in the greenhouse until certain they are free of the viruses.

Separate cutting crops from seedlings. The disease frequently enters the greenhouse within vegetatively propagated plant material. Hanging baskets of infected cutting crops over seedlings can lead to bedding plant losses, since the young seedlings are highly susceptible.

Immediately discard plants showing distinctive TSWV/INSV symptoms. Early destruction of a few infected plants may prevent an epidemic through all the susceptible plants in the greenhouse. If in doubt, throw them out. Infected plants cannot be cured.

Do not vegetatively propagate infected plants. The virus can still be maintained in a crop through vegetative propagation even in the absence of western flower thrips.

Plants may act as reservoirs of the virus. Flowering pot plant crops such as cyclamen can serve to carry the disease over from the fall to the following bedding plant season, as might weeds left under the benches. Eliminate weeds in and near the greenhouse which may harbor thrips and/or the virus.

Consider using petunia plants as indicators to monitor for TSWV/INSV and thrips feeding injuries; 'Calypso', 'Super Blue Magic' or Summer Madness' petunias may all be used as indicators of TSWV/INSV. Use a yellow (NON-sticky) card to help attract the thrips to the petunias.

Losses have been greatest with gloxinia, double flowered impatiens, New Guinea impatiens, begonia and cyclamen crops. Be particularly careful to keep these crops isolated from potential sources of virus.

Be aware that vegetable and tobacco transplants are also susceptible to these viruses and can serve as reservoirs of infection.

Perennial plant growers need to aggressively attack TSWV and western flower thrips in both greenhouse and outdoor plantings, and must be aware that plants originating from greenhouse production but now planted outdoors may carry the virus. Even if the thrips do not overwinter in DE, vegetatively propagating infected plants will maintain and spread the virus. Garden center operators must also be aware of the biology of TSWV and western flower thrips, especially if they keep herbaceous plants all year. Many perennials are susceptible to the virus and attractive to thrips. An infected perennial will retain the virus until that plant dies.

Adapted from "INSV AND TSWV ARE THREATS TO GREENHOUSE ORNAMENTAL PRODUCTION" By John Hartman in the Feb 12, 2007 edition of the Kentucky Pest News from the College of Agriculture at the University of Kentucky.