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Plant Viruses and Why They Matter

(continued from page 10) increasing problem of HVX incidence in the U.S. to suppliers. Finds of HVX have greatly decreased in Indiana, but it still occurs occasionally. Symptoms can include chlorotic or discolored ring spots, discolored areas along leaf veins, leaf puckering, green and yellow mottled areas, and death of leaf tissue. Symptoms vary between cultivars, and other viruses that infect hosta may show similar types of symptoms. See photos 4 & 5.

Japanese holly fern mottle virus (JHFMoV) is another example of a virus that only infects one type of plant. This virus is only known to occur on Japanese Holly Fern. This virus was first described in 2008 in the southeastern U.S. This virus spreads through grafting with infected plant material and through spores from infected plants. Symptoms can include yellow mottling, mosaic patterns, ringspots, and stunted growth. See Photos 6 & 7.

Tobacco ringspot virus (TRSV) is spread by dagger nematodes and some groups of insects. TRSV has a wide host range including many field crops and weed species. It is another virus of regulatory concern and countries such as Argentina require soybean seed imports be free of this pathogen. In soybeans symptoms may include stunting, shepherd’s crook of the terminal bud, poor pod development, spots on pods, distorted leaves, and discolored stem pith. See Photo 8.

Like the rose mosaic virus complex, the blackberry yellow vein disease (BYVD) complex refers to a grouping of one or more viruses that can be found individually or as mixed infections on the same plant. At least eleven viruses are known to be part of this disease complex, which was first observed in the southern U.S. around 2000. BYVD-associated viruses can be transmitted by pollen, nematodes, whiteflies, thrips, mealybugs, eriophyid mites, aphids, and, potentially, beetles. This makes controlling transmission of the disease very difficult. Symptoms often include chlorosis, ringspots, mottling, vein yellowing and oak leaf line patterns. Repeated years of infection result in a decline in yield and productivity after 5 to 7 years. Prior to the presence of this virus complex, blackberry production areas in the southeast remained productive for 20 years or more. More than 30 viruses have been identified in Rubus spp. See Photo 9.

Identifying a specific virus on a plant requires specialized testing. The ability of viruses to change and generate new molecular variation creates a complex testing challenge. There may be very little information available as to known viruses for particular plant hosts. Tests are usually available for common viruses with large host ranges. Sometimes the diagnosis is straightforward because the virus is a common one, for which tests are readily available. In other cases, nailing down the identity of a virus may involve the equivalent of a research project by a plant virologist and a team of technicians. In many cases diagnosticians agree that the plant is likely infected with a virus but attempts at identification become a dead end.

On occasion we get lucky…

An example of this is with an elderberry grower who had possible virus symptoms in a field. Testing for common viruses with a large host range all came back negative. The Purdue Plant and Pest Diagnostic Laboratory was able to find a plant virologist who was interested in the problem and suspected the plants were infected. The virologist was later able to identify virus particles and determined that a virus in the genus Carlavirus was present in symptomatic leaves. It is known that carlaviruses are mainly spread by aphids and through infected vegetative parts. They can also sometimes be spread by virus contaminated equipment. See Photo 10. How can we stop the introduction of viruses into our industry? Since viruses depend on the cellular structure of their host for reproduction, control is dependent on prevention of infection of the plant. Control of insect or mite populations is essential for management of viruses transmitted by them, as well as weed populations that may harbor them in and around growing areas. Preventative measures may include use of certified virus-free seed or vegetative stock. Genetically engineered virus resistant crops may provide opportunities for effective management of difficult to control diseases that could otherwise result in significant yield loss of food crops. For example, Papaya ringspot virus was devastating in Hawaii prior to the development of genetically resistant plants which allowed for successful management of the disease.

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Plant Viruses and Why They Matter

(continued from page 12)

The world of plant viruses is complex and quite large. Viruses present a difficulty for plant health inspectors and plant disease diagnosticians. Resources to identify the specific virus infecting a plant may not be readily available. However, many common virus problems in greenhouses, nurseries and the private landscape can be identified. Identification of the specific virus or virus group is key to management of the disease because it provides information as to how the virus is transmitted.

If you suspect that you have virus infected plants in your nursery, greenhouse, or landscape, it’s recommended that you try to verify if the problem is a virus and get a possible identification. Samples can be submitted to the Purdue Plant and Pest Diagnostic Laboratory. Their website has instructions on how to send in plant samples at https://ag.purdue.edu/department/btny/ppdl/submitsamples/submit-sample.html

Nursery growers may also contact their local DEPP inspector for assistance in identification of possible virus problems. A map showing the DEPP inspector territories can be found at https://www.in.gov/dnr/entomology/files/ep-Inspector_Terrirories.pdf

About the Author

Angela Rust has been with the Indiana DNR, Division of Entomology and Plant Pathology as a Natural Resources Inspector since 1995. She worked in northeast Indiana based out of Allen County until 2000 and then has spent the last 22 years based out of Perry County in the southwest part of the state. Responsibilities within the division include nursery and dealer certifications, phytosanitary certifications, invasive species survey and regulation, and completion of environmental assessments for the gypsy moth program.