Plant Health: Viral Diseases

Viral ringspot symptoms on epimedium.

Imagine this nightmare scenario: A type of infectious plant disease caused by a pathogen that is too small to detect even with a light microscope. There are no chemical controls. Many of these pathogens have wide host ranges and cause symptoms that are highly variable on different host plants. The pathogen is an obligate parasite, living only inside plant cells and cannot be cultured on artificial media like many fungal and bacterial plant pathogens. This pathogen does not produce spores, but its infectious particles take over the machinery of the host cells, damaging them and causing them to produce more particles of the pathogen. This pathogen develops systemically in the plant, staying with it for the entire life of its host. This is the world of viral plant diseases.

Viral plant diseases were conceptualized more than 100 years ago by Martinus Beijerinck with his description of tobacco mosaic virus (TMV), and many early understandings of all viral diseases comes from work on TMV. Viral plant diseases drew the attention of horticulturists much earlier than the discovery of TMV, though the cause was not understood at the time.

Tulipmania in Holland in the 1500s-1600s was caused by what was determined centuries later to be the tulip breaking virus, resulting in elaborate streaks and slashes in tulip blooms. Though it turns out that this virus damages the plant and its reproductive capability, the flower streaks resulted in a stock market-like boom in tulip bulb prices. In Essential Plant Pathology, there is a tale of one horticultural entrepreneur who parlayed the frenzy for these unique tulips by selling a bulb for “4 tons of wheat, 8 tons of rye, a ton of butter, a 1,000 pounds of cheese, 4 tons of beer, 2 barrels of wine, 4 oxen, 8 swine, 12 sheep, a suit of clothes, a bed and a silver drinking cup.” Apparently the buyer even lost the clothes off his back.
 

Symptoms of viral infection in hosta.

Biology
Viruses are considered non-living because of their inability to reproduce on their own and their need for host cell machinery to replicate and function. Viruses inside plant cells need host plant enzymes, energy, membranes and other structures to replicate and to do their damage of affecting plant cell metabolism, interfering with photosynthesis in the cells, causing yellowing and various discolorations, and with plant cell growth regulators thus resulting in symptoms such as leaf enations (bumps) and distortions.

Virus spread
Viruses typically need wounds to get into plants, but this can be subtle, such as simple mechanical transmission by people handling plants with tobacco mosaic virus. Or it can be spread through wounds from insects, such as western flower thrips feeding on herbaceous plants and transmitting impatiens necrotic spot virus. Aphids and whiteflies and other sucking insects in the Order Homoptera are common vectors of certain viruses. But other organisms including thrips insects, nematodes and fungi can vector viruses from one plant to another. Viruses can also be spread by seed and, less commonly, by pollen.

Symptoms of viral diseases
Plant pathogenic viruses cause a wide range of symptoms, including: stunting, dwarfing, yellowing or chlorosis, ring spots, vein banding, flower breaks, browning or necrosis, rugosity or swelling on leaves, leaf rolling, stem pitting, and bumps or enations on leaves.

Sometimes a single virus such as impatiens necrotic spot virus (INSV) causes a wide range of symptoms on different host plants. When great effort was made to limit virus infections in floral geraniums, it was discovered that almost all the previously virus-infected plants were infected with stunt viruses. If almost everything is stunted you might never know it until plants without those viruses are produced and grow more vigorously.
 

Western flowers thrips are vectors of INSV.

Common viruses of ornamentals
Some of the common diseases of herbaceous annuals and perennials include; INSV, cucumber mosaic virus (CMV), TMV, hosta virus X (HVX) and tomato spotted wilt virus (TSWV).

All of these have a different range of symptoms and most, with the exception of HVX, have broad host range on a number of plant genera. There are, however, many more viruses that we know damage plants, and as diagnostics and identification progress, we are sure to find a number of viruses that are currently not on our radar. This is true as well for many woody ornamentals, for which little is known relative to virus identification and control in many cases, with notable exceptions for rose mosaic and viral problems of many fruit trees and their ornamental cousins.

TMV alone occurs on more than 150 genera of plants, resulting in a mosaic pattern of dark and light green caused by the viral effects on the photosynthetic process in leaf cells. INSV occurs on everything from Aquilegia to Vinca and from Monarda to Malva. All of these viral diseases have their own special details that relate to viral biology, modes of dissemination and host range. TMV is mechanically transmitted (smoking must be regulated around growth of susceptible plants due to transmission from tobacco products), INSV is transmitted by western flower thrips and other thrips species, and the mode of transmission of HVX is unknown.

Management and control
Virus control is largely preventive and requires a program of integrated plant health management. Here are some keys:

• Host resistance. Where available, plant cultivars, species and genera with resistance to problem viral diseases. Rotate to non-host crops when necessary.
   
• Sanitation. Buy only from trusted sources, and if available, buy tissue culture-derived, virus-indexed plants. Plant handlers and propagators should use good tool/equipment and hand sanitation. Follow up detection programs by roguing out plants suspected and confirmed of virus problems. For example, HVX infected plants should be destroyed to minimize infection of other susceptible plants. In a landscape bed, residual root tissue remains a source of virus inoculum that could infect a susceptible hosta planted in the same location until it is completely degraded by microorganisms.
   
• Control of insect vectors. If INSV is an issue, maintain good thrips control, including monitoring with sticky traps and use of insecticides. If producing virus-indexed stock plants, insect control and timing of planting is essential.
   
• Detection. When viral diseases are suspected, consult university and commercial plant diagnostic labs. Train personnel in symptom identification. Use commercially available ELISA (Enzyme-Linked Immunosorbent Assay) and PCR (Polymerase Chain Reaction) test kits that test for characteristic components of the viral proteins and genes for confirmation of viral diseases. (Go to www.agdia.com for examples of virus detection products.)

Viral diseases are pervasive and common problems for the green industry. Advance your learning curve on which viral diseases matter for your business.

Jim Chatfield is associate professor, Ohio State University Extension, Department of Plant Pathology, Department of Horticulture and Crop Science; chatfield.1@osu.edu.