Fact Sheets
Pest Management
Impatiens Necrotic Spot Virus and Tomato Spotted Wilt Virus
Introduction
Until recently, tomato spotted wilt virus was considered to have two strains; the impatiens strain and the lettuce strain (also known as the common strain). The impatiens strain was recently found to be different from tomato spotted wilt virus and has been renamed Impatiens Necrotic Spot Virus (INSV). The impatiens strain is more commonly found in greenhouses than the lettuce strain. Both strains may occur in greenhouses as well as in vegetables but it is usually the lettuce strain which is found in vegetables. Tuberous dahlias also appear to be a common host of the lettuce strain. The two different viruses can cause different symptoms in tobacco but it is not yet clear if they cause different symptoms in other plants. The host ranges of the two viruses overlap but it is not known to what extent.
Impatiens necrotic spot virus is becoming one of the most important problems in the floriculture industry today. The virus is widespread due to the distribution of infected plant material and the increased prevalence of the insect vector which transmits the disease.
The host ranges for the viruses are extensive. More than 300 plant species are known to be susceptible (see below for a partial listing). It is likely that many more new hosts will be discovered.
Symptoms on Greenhouse Crops
The original name of the virus was derived from the symptoms (spotting followed by wilt) that develop on tomato. On other hosts, ringspots, purple to black lesions on leaves and stems, flower color breaking, stunting, death of terminals, and wilt may occur. The stage of growth can also influence symptom development.
For many hosts, symptoms are not well known or are not described. However, in some commonly infected crops, symptoms can be diagnostic. Gloxinias infected as young plants develop necrosis of the central leaves, resulting in collapse. Older gloxinias develop necrotic ringspots on the foliage, as well as necrosis along the veins.
vein lesions on underside of leaf
Cineraria develops chlorotic ringspots and/or mottling on the upper leaf surface, and purple to black lesions on the veins underneath. On exacum, the virus causes tan to dark brown cankers on stems. Reiger begonias develop necrotic ringspots, mosaic, and necrosis of the leaf veins. Symptoms on impatiens and New Guinea impatiens include stunting, leaf distortion and blackened spots or rings on foliage and stems. In some cases, terminals will die and the entire plant may collapse. As with other virus diseases infected plants may remain symptomless for a period of time.
Symptoms on Vegetable Crops
In Massachusetts the virus has only been noted on pepper and tomato. On pepper the foliage becomes distorted, the fruit develop circular rings that are slightly indented into the skin. On tomato, foliage becomes spotted and blackened particularly on the inner leaflets toward the top of the plant. The growing point may also become necrotic and the whole plant may wilt. Fruit symptoms are variable. Small necrotic spots to large circular blotches may develop.
Life History
Both viruses are transmitted by vegetative propagation of infected plant material and by the feeding activity of certain species of thrips. In greenhouses, the most important vector is considered to be the western flower thrips (WFT), Frankliniella occidentalis. The virus is associated with thrips in a persistent manner. Larvae become viruliferous after feeding about 30 minutes on an infected plant. After a latent period lasting 3 to 18 days, the thrips can infect new plants after feeding for 5 to 10 minutes. The insects are able to transmit the virus for the rest of their lives. In greenhouses the viruses can be perpetuated through successive crops and weeds resulting in losses year-round. The disease has not been observed to spread significantly in field-planted vegetable crops in Massachusetts.
Management
Elimination of infected plant material and WFTs are the most important management practices. Newly-acquired plant shipments should be inspected for evidence of thrips and symptoms of virus before being introduced into the greenhouse. Symptomatic plants should be discarded or isolated and the supplier should be notified after laboratory confirmation of the disease. Since various weed species can harbor the viruses, thorough weed control is important. If possible, keep vegetatively propagated plants isolated from those produced from seed. Do not grow vegetable transplants in the same greenhouse with susceptible ornamentals. Thrips populations should be monitored with sticky traps. Since only a small number of viruliferous thrips are necessary to start an epidemic, total eradication of WFT should be the goal.
For more information on the life cycle and management of thrips, a 31-page publication Floral Facts: Western Flower Thrips is available from the University of Massachusetts Extension Bookstore. It contains information and color pictures on WFT and INSV (with emphasis on biological control) and is available for $3.00. Click here for a link to the Extension Bookstore and to download a printable order form.
Partial list of plants susceptible to impatiens necrotic spot virus & tomato spotted wilt virus
|
Ornamentals |
cosmos |
phlox |
pepper |
Prepared by Dr. Robert L. Wick, Department of Micrbiology, Fernald Hall, Room 111, University of Massachusetts, Amherst MA 01003. Tel. (413) 545-1045 Fax (413) 545-2532
Resources
See the web site of the American Phytopathological Society for an excellent
article, with photos and diagrams, on Tomato Spotted Wilt Virus
and Related Tospoviruses.
