Plant Viruses and Why They Matter (continued from

page 8) spread through virus infected pollen. When TRSV or ToRSV are involved, these are transmitted by nematodes. Since each of these viruses can infect several types of plant hosts, an infection in a nursery production area could lead to widespread infection. The Indiana Department of Natural Resources, Division of Entomology and Plant Pathology (DEPP) requires that roses coming into the State of Indiana be free of symptoms of viruses as outlined in Indiana Administrative Code 312 IAC 18-3-9, viruses of roses. Symptoms expressed by infected roses will vary depending upon the specific viruses composing the infection and the rose cultivar. Symptoms may include chlorotic line patterns, chlorotic ring spot patterns, distorted leaves, and mottled leaves. See Photo 1.

Viruses transmitted by insects and mites create a challenge given their vector mobility and opportunity to spread viruses among large numbers of plants while feeding. Wheat streak mosaic virus (WSMV) is vectored by the wheat curl mite (Aceria tosichella). WSMV can infect wheat, corn, and several grasses. It may be more difficult to control this virus when wheat and corn fields are surrounded by susceptible grasses or areas with wheat debris that can serve as additional sources of infection. The wheat curl mite also serves as a vector for High plains virus which can infect wheat and corn. Is it not uncommon for corn plants to be infected with more than one virus which often increases the severity of decline in the corn host. Maize lethal necrosis (MLN) is a disease of corn caused by coinfection of corn with more than one virus. Maize chlorotic mottle virus (MCMV) and one of several viruses from the family Potyviridae, such as Sugarcane mosaic virus, Maize dwarf mosaic virus, Johnsongrass mosaic virus or Wheat streak mosaic virus. The coinfection of these viruses can frequently cause plant death or severely reduced yield. It was first found in the United States in the late 70’s. Over the past 10 years, MLN has emerged in areas of East Africa, Southeast Asia, and South America as a significant concern.

WSMV is a concern for shippers of corn seed to certain countries like Argentina and South Africa. Freedom from this virus is a requirement for shipments entering these countries. In corn, infected leaves will show a yellowish mosaic pattern of parallel short streaks that become mottled yellow. Leaves may develop chains of small chlorotic ringspots. Plants may be stunted, and leaves may roll toward the mid-rib. See Photo 2.

Impatiens necrotic spot virus (INSV) is a virus of concern for greenhouse growers because it has an extremely large host list (hundreds of spe cies) and is spread by western flower thrips (Frankliniella occidentalis). Depending on plant susceptibility, insect population, and disease se verity, infected plants can eventually die. Just a few infected plants in a greenhouse along with uncontrolled thrips populations can result in widespread decline or make plants unmarketable. Symptoms may include wilting, chlorotic and necrotic ring spots on leaves, sunken spots on leaves, plant yellowing, stunting or poor flowering. It is important to remember that INSV, and all viruses, may not show the same type of symptoms in different types of plants. Western flower thrips also vector Tomato spotted wilt virus (TSWV) which has an extremely large host list. Control of the thrips population and elimination of weeds growing in and near greenhouses is important to controlling these viruses. See Photo 3.

There are a lot of viruses that can infect hosta plants, but Hosta virus X (HVX) is a virus that only infects hosta. It is spread through use of gardening and propagation tools that are contaminated with virus infected sap, or by division of infected plants. It was first detected in the United States in 2006. DEPP inspectors and inspectors in other states found this virus frequently in the following years. There was a lot of ef fort by plant health officials, universities, and hosta societies to educate growers on how to recognize hosta virus symptoms. State and national plant health officials also worked effectively to track international sources of this virus and communicate the

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