Plant Health: Viral Diseases

Infectious plant diseases are frustrating, and viral diseases of ornamental plants are especially frustrating -- difficult to diagnose, manage and control. One of the reasons infectious diseases are so difficult is that plant pathogens, which include fungi, bacteria, nematodes, phytoplasmas and viruses, are invisible to the naked eye when they enter plants and develop infections inside plant cells. You cannot see the microscopic-sized spores from the rose black spot fungal pathogen (Diplocarpon rosae) arrive on the rose leaf, germinate, penetrate the epidermis and then grow into and digest contents of plant cells. Only days later do you see the black spots and leaf yellowing symptoms of the disease.

Virus particles are much smaller than D. rosae fungal spores and germ tubes. At least we can see fungi and bacteria with light microscopes with their 100× and 430× magnification. For viruses, we need an electron microscope, not an everyday tool for even the most sophisticated growers and most diagnostic labs. Match this with the variability of the symptoms of many viral diseases and the fact that there are no virusides, and you see why viral disease problems can pose a perfect storm for horticulturists.

Hosta aureomarginata with hosta virus XViruses were not discovered until 1898 when tobacco mosaic virus (TMV) was first identified, though not well understood for some time. Hosta virus X was not described until 1996. We still are unaware of the existence and importance of many viral diseases. Plant pathologists often say that fungal diseases are the most common plant diseases, but it may be true that viral diseases are much more prevalent, though we just do not know it yet. Decades ago, when virus-indexing of geraniums was undertaken, we learned of many new viruses in the process. Ringspot and mosaic diseases may have easy-to-spot symptoms, but until you eliminate stunt viruses that are widespread, how do you know the disease and stunting was even present? This is especially true if you need extensive electron microscopy to view the virus particles in the plant tissue.

What are viruses?
Plant viruses are pathogenic particles that cause damage inside plant cells by interfering with the allocation of resources that the plant has produced through photosynthesis. Once viruses get into plants through wounds, from natural wounds such as the branching of roots to insect feeding or propagation cuts, viruses shed their protein coats and then begin using their RNA or DNA to induce the plant cellular machinery to produce more and more virus particles. The viruses then move systemically throughout the plant.

How do viruses get transmitted?
Some viruses, such as TMV are mechanically transmitted, including our handling of infested plants and use of propagation tools. Other viruses are vectored by insects and other pests, especially insects with piercing-sucking mouthparts in the Homoptera order, including aphids, leafhoppers, whiteflies. Impatiens necrotic spot virus (INSV) is a serious disease of greenhouse crops in which the virus is vectored by the western flower thrips insect. The forms of transmission of particular viruses are critical to management strategies: sanitation during handling and propagation for TMV, thrips control for INSV.

What are some symptoms of viral diseases?
This is truly a diagnostic challenge. Symptoms of viral diseases can often mimic those caused by other pathogens and abiotic disorders, including herbicide injury. For example, Julie Beale at the University of Kentucky notes that there is often diagnostic confusion about rose rosette disease vs. glyphosate injury. Both result in distortion of rose shoots, but with rose rosette, look for the overall combination of bunchy distorted shoots, reddish discoloration of the shoots, and excessive thorn production. There is not yet a definitive test for rose rosette, so careful examination of the plants, review of horticultural practices, possible testing for chemical residues are all part of the diagnostic process.

As noted in the Ohio State University Plant Disease Fact Sheet Series titled “Viral Diseases of Greenhouse Floral Crops” (HYG 3065-08, www.ohioline.osu.edu): “Overall symptoms for viral diseases can be quite irregular—often more severe on one leaf or one plant than another. Viruses often cause irregular dark and light green patches (called mosaic or mottle) on the leaves. Viruses can cause chlorosis (yellowing), necrosis (tissue death) or vein clearing. Chlorosis (yellowing) can take the form of circular ring spots or mosaic. Virus symptoms can also include stunting and leaf deformation (e.g., shoestring, puckering, irregular leaf edges), which are often asso­ciated with a mosaic. Many viruses cause flower break symptoms (changes in normal pigmentation) on flowers. However, despite causing severe symptoms on some hosts, viruses can infect other hosts or cultivars without producing symptoms. These asymptomatic plants can be a hidden source of virus infection in the greenhouse.”

What are some approaches to viral disease management?
This is a challenge, especially considering that there are no direct chemical controls for viruses, though there may be chemical controls for insect vectors that transmit viruses, such as with INSV and thrips control.  Here are some key approaches:

Sanitation: This is critical when viral diseases are determined to be a major factor in your operation. Some viruses can survive for extremely long periods of time outside of the host, as well as in or on dead tissue, benches, tools, or plant debris. TMV can easily be moved from one plant to another by the touch of a hand or the carryover of sap on a cutting knife. For this reason it is extremely important to wash your hands in warm, soapy water with crops where virus problems are common. Tools used to take cuttings or clean up plants should be disinfected every few minutes or between contact with stock plants or cultivars. Follow the label recommendations on one of the commercially available disinfectants (e.g., containing the quaternary ammonium salts, hydrogen dioxide) or use a 1:10 dilu­tion of household bleach. Benches and other surfaces that come into contact with infected plant material should also be disinfected. Because viruses can be present in roots, pots and plug trays containing infected plants should not be reused without being disinfected. Weeds serve as alternate or secondary hosts for plant viruses as well as their insect vectors. Keeping weed popula­tions in and around the greenhouse to a minimum is a good sanitary practice that will aid greatly in control­ling viruses. Infected plants should be destroyed, not composted near the operation.

Education: Learn the symptoms and management strategies of the viral diseases of the crops you grow and maintain. Attend seminars, resource materials on the web, universities and from green industry partners.

Vector control: Control vectors when they are key virus transmission agents.

Exclusion: Buy clean virus-indexed plant materials when available.

Roguing: Remove suspect plants before they become sources of infestation for other plants.

Scouting: Train workers to spot atypical symptoms on plants. Use sticky cards to monitor for vectors in greenhouse environments.

Rapid diagnostic kits: For certain key viral diseases, rapid diagnostic kits such as those available from Agdia can help confirm presence of viruses, leading to improved management decisions.

Use resistant varieties: As knowledge improves more information will become available about plant taxa resistant to particular diseases. For example, check ohioline.osu.edu the “Hosta Virus X” fact sheet (HYG 3069-08) for information on which hostas tested positive for this disease.

Dennis Lewandowski, Ohio State University, Department of Plant Pathology contributed to this story.