Tospoviruses

Aster TSWV Bottom: Impatiens INSV donut

In the early 1990s, a newly important virus identification was made. It was closely related to tomato spotted wilt virus (TSWV), long known in vegetables and other crops, but it was identified as impatiens necrotic spot virus (INSV). Both viruses were found to be vectored by thrips, making insect control an integral part of any virus management program. Many types of thrips are known to vector viruses from plant to plant including Western flower thrips, onion thrips and chili thrips, initially discovered in 2005.

In the ornamentals world, the most important of the thrips-borne viruses are INSV and TSWV, but tobacco streak virus (TSV) is seen occasionally. TSV, which has a large host range, is transmitted by Frankliniella occidentalis and Thrips tabaci. An additional tospovirus, chrysanthemum stem necrosis (CSNV), has been reported from Brazil, the Netherlands, and the U.K.; it is thought that F. occidentalis and F. schultzei are vectors for this virus, known to affect chrysanthemum and tomato.
 

Symptoms of thrips infestation
The most common symptoms are tiny white or tan feeding scars of flowers and leaves. Distortion of flowers or leaves and lower leaf drop can also occur. Another sign is the presence of dark droplets of frass on the scars and, in some cases, pollen may be spilled out onto petals.
 

Symptoms of Tospovirus infection
The most troubling aspect of tospovirus infection is the extreme range of variability of the symptoms. Sometimes white, yellow or tan ring spots. New leaves may be distorted, mottled and crinkled. One of the most telltale symptoms is the presence of black patches, sunken wavy lines or spots. When infection is severe, the plants may be stunted.

Plants attacked by Tospoviruses
Hosts of INSV and TSWV are in the hundreds and include ornamentals, vegetables and weeds. Some of the most commonly attacked ornamentals include: African violet, Aphelandra, Aster, Begonia, Calla lily, Campanula, Chrysanthemum, Cineraria, Coleus, Cosmos, Cyclamen, Dahlia, Diascia, Exacum, Gloxinia, Hoya, Impatiens, Lipstick vine, Lisianthus, Lobelia, Maranta, Marigold, Monarda, Nemesia, Peony, Poppy, Primrose, Ranunculus, Salvia, Snapdragon, Stephanotis and Verbena.

Cultural control of thrips and Tospoviruses
Monitor crops for thrips using yellow or blue sticky cards, and check every seven days. You can check for thrips in foliage or flowers by tapping over a white surface, or you can blow lightly into buds or open flowers to draw thrips out for detection.

A good idea is to remove or exclude older stock and pet plants from the production area. Collect unneeded buds and flowers that may contain thrips, if practical, and bag, seal and discard. If you can, allow a fallow time in the greenhouse. Keep the greenhouse free of weeds inside and around the outside, since both thrips and tospoviruses are easily harbored in greenhouse weeds. When new cuttings or plugs arrive, check them for thrips or virus symptoms immediately.

You can also employ virus indicator plants in the greenhouse. Fava beans (e.g. Little Toto) and petunias (e.g. Calypso, Super Blue Magic and Summer Madness) show small tan or brown spots where thrips that are infected with INSV have fed. Confirming virus infection can be easily made by submitting samples. Submit suspect samples to a diagnostic lab, or you can make use of tests strips readily available from Agdia.

On very high value crops or under severe thrips and tospovirus pressure, you may need to employ insect screening to prevent thrips from entering the greenhouse. It is best to avoid or prevent disease from starting. It is never more true than when combating viruses, as they cannot be controlled after they have infected a plant. Prevention is the only real control available at this time.
 

You can write A.R. Chase at archase@chaseresearch.net.