INSV is a viral disease that has become far too common on ornamental plants in the United States. It is in a viral group called the tospoviruses. Another virus that causes similar symptoms is tomato spotted wilt virus. In fact, the two were considered strains of the same virus until the early 90s. INSV is more often found in ornamental plants, while TSWV is more common on vegetable and field crops. Both diseases cause leaf and stem ring spots, as well as mottling and mosaic patterns. There may be a wavy-line pattern through the foliage with parallel lines. The leaf spots may be yellow, white, gray, or brown–black. If you have not seen these viral diseases before, you might mistake them for fungal leaf-spotting diseases. Eventually, plants will be stunted, with small leaves and poor plant vitality. I see INSV most frequently in impatiens. Both viruses are spread by thrips, usually the western flower thrips.
INSV is a big problem in the greenhouse industry because thrips are difficult to control, it takes only one thrips to spread the virus, and many ornamental plants and weeds can host this disease. Annuals and perennials that are propagated in greenhouses and sold to the public may also have the virus. If you have purchased a virus-infected plant or flat of plants, you will deal with the problem all season, or until the plant dies. Can the virus overwinter in Illinois? It is generally believed that INSV will not overwinter in Illinois without a live host plant.
A few hosts of INSV include impatiens, begonia, vinca, cineraria, exacum, cyclamen, chrysanthemum, and alstroemeria; but many more flowering plants, vegetables, and weeds can host this virus.
Virus diseases remain with a plant until its death. For this reason, we need to remove infected plants from the garden and try to prevent further spread. A nice publication available from Colorado State University Extension can be found at http://www.ext.colostate.edu/pubs/garden/02947.pdf. This report on greenhouse plant viruses describes INSV and TSWV in more detail and shows pictures of symptoms with which you should become familiar.