Impatiens Necrotic Spot Virus - Part I:
Dr. Larry W. Barnes, Professor
Department of Plant Pathology
Texas A&M University System
College Station, TX

Impatiens necrotic spot virus (INSV) can be one of the most serious viral pathogens of greenhouse crops. INSV is sometimes confused with tomato spotted wilt virus (TSWV), a closely related virus that shares many similarities and characteristics with INSV. Although similarities exist between the two, research has determined that INSV is different from TSWV, and is usually the most commonly encountered of the two viruses in greenhouse crops. Because of the ability of INSV to devastate production and cause serious losses and because INSV can cause a confusing range of symptoms, an understanding of this virus can help recognize and manage this serious viral disease to reduce production losses.

Host Range:
The INSV/TSWV host range is extensive and includes many plants common to greenhouse pot flower production, bedding plant production, and foliage production. Table 1 is not all-inclusive but lists some of the common greenhouse crop plants and exemplifies the breadth of the host range.

Table 1. Common greenhouse crops susceptible to INSV.

African Violets
Zebra Plant

Numerous weed hosts of INSV/TSWV are also known resulting in a conservative estimate of susceptible hosts at over 480 species in 50 different plant families.

Part of the frustration of dealing with INSV is the variability of symptoms that can develop following plant infection. Specific plant host infected, stage of plant development at time of infection, and greenhouse environmental conditions (temperature, light intensity, nutrition, etc) can contribute to variable symptoms. Common symptoms of INSV infection can include brown leaf spots, concentric ringspots (either yellow or brown), brown stem lesions, stunting, wilting, vein browning and necrosis, and mosaic line patterns. Leaf mottling and leaf distortion are also suspicious symptoms. Although many of these symptoms can be diagnostic in certain host plants, some of the symptoms might be misinterpreted as caused by viral pathogens or other factors. Pattern of development can also be helpful in interpreting possible INSV symptoms.

INSV is only known to be transmitted by the western flower thrips. The larval stage of the thrips acquires the virus by feeding on an INSV-infected plant and transmits the virus as an adult. Thrips migrating into a greenhouse can be a INSV source, especially if they have fed and cycled on weed hosts outside. Infected plant material introduced into the greenhouse can also serve as an initial source of INSV. Once INSV is inside the greenhouse, thrips can move it throughout the greenhouse and into other susceptible hosts. Various host plants in the same greenhouse may respond differently to infection, preventing an accurate, timely identification of infection. Continued thrips activity intensifies INSV development, plant infection, and plant loss.

Management Strategies:
Reduce the chances of INSV problems by careful vigilance and an integrated management approach. Because of the high loss potential if INSV occurs, growers should have an ongoing program to monitor crops for INSV symptoms in susceptible crops (see Table 1) and a management plan ready for implementation if INSV is observed.

Scouting. Knowledgeable scouting is the foundation for INSV management. Growers must become familiar with the range of symptoms that are possible on the susceptible crops that they growˇtime spent on this is an investment in the truest sense! Routine scouting for thrips activity and use of yellow or blue "sticky cards" to monitor thrips population is important. The petunia cultivars "Super Blue Magic" and "Calypso" can be used as indicator plants, both for the presence of thrips and of INSV. The blue flower color is attractive to the thrips and brown rings develop rapidly on the leaves if INSV infection occurs indicating INSV is present in the immediate area.

Thrips Control. Because thrips are the only known vector, aggressive thrips management is mandatory for good INSV control. Choose insecticides wisely, schedule applications carefully, assure thorough coverage, and make sure that the insecticide mode of action is reasonable. Systemic insecticides may be of limited value; larvae must feed to acquire a lethal dose of the insecticide and feeding can allow transmission of the virus.
Aggressive weed control is an important strategy for INSV control. Remember that weeds inside and outside the greenhouse may serve as both a habitat for the thrips vector as well as a reservior for the INSV pathogen.

Removal Of Infected Plants. An effective scouting program will help identify symptomatic or suspicious plants. Any plants showing symptoms should be removed from the growing area and destroyed using a "take no prisoners" approach. One infected plant remaining on the bench can serve as a source for continued problems. Do not keep INSV-infected plants in a pile outside the greenhouse because thrips can continue to feed on infected plants and migrate into the greenhouse.

Inspect Incoming Plant Material. Be sure to carefully inspect all plant material that is coming into the greenhouse from outside sources. Check plugs, seedlings, cuttings, stock plant material, and any other type of plant material to make sure it is free of INSV symptoms and thrips. Infected cuttings and plugs contributed to the widespread dissemination of INSV in the mid-1980's, and although INSV indexing is routinely practiced, continued vigilance is needed. If at all possible, purchase plant material that has been indexed. Do not hesitate to submit suspicious plants to The Texas Plant Disease Diagnostic Laboratory at College Station for INSV testing.

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