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Recognizing Plant Viruses

November 07, 2011

There are numerous plant viruses known to attack various ornamental and greenhouse crops.  Viruses are often discussed in conjunction with plant pathogens; however, they are very different from fungal and bacterial diseases. Some of the most common viruses growers observe are cucumber mosaic virus (CMV), impatiens necrotic spot virus (INSV), tomato spotted wilt virus (TSWV), and tobacco mosaic virus (TMV); however, there are numerous others.  

Plant viruses are caused by very tiny subcellular particles that can only be seen using an electron microscope.  These sphere- or rod-shaped particles are composed of small pieces of DNA or RNA encapsulated in a protein coat.  Unlike most plant pathogens, viruses do not divide or produce any type of reproductive structures, such as spores. Instead, they are reproduced inside living cells using ribosomes and other components of its host cell for multiplication.  

Virus particles basically take over plant cells, forcing them to replicate more viruses identical to themselves.  This causes plant cells to function improperly and normal plant operations, such as the production of chlorophyll or cell division, either does not occur or are seriously affected.  Plant viruses affect the growth of plant cells and causes plants to grow abnormally. Viruses cause systemic infections and currently there is no treatment for removing them from infected plants in production.

Viruses enter host plants through some type of wound or injury to a plant cell.  Each type of virus has a specific ‘vector’ or carrier of the virus which transmits it to uninfected plants.   Insect feeding and vegetative propagation are examples of how viruses can be spread or ‘vectored’ from one plant to another.

Plant viruses can be challenging to identify as the symptoms often resemble other cultural problems such as fungal diseases, herbicide injury, and nutritional disorders.  Visible symptoms may include mosaic, mottling, ring patterns, necrotic spots, bumps, and distortion of the foliage, discolored or abnormal flower formation, or abnormal growth or stunting.  Symptoms vary with the virus present and in many cases, plants may exhibit multiple symptoms from one or more viruses. The symptoms observed vary with the type of virus(es) present.  

The age of the plant, how long it has been infected, the environment, and the amount of stress on the plant are additional factors that affect which symptoms are present and the magnitude they are expressed.

Although, several viruses are easy to identify using visual observations, growers cannot always rely solely on visual symptoms to properly diagnose the presence of plant viruses.  Symptomatic plant tissues should be sent to virus testing laboratories for more accurate testing and identification procedures.  These laboratories conduct virus testing using bioassay, serology, nucleic acid analysis, DNA probing, and electron microscopy identification procedures.  
Agdia, Inc. (www.agdia.com) has developed practical and cost effective ImmunoStrip® (serological) test kits for identifying the presence of INSV and other plant viruses. Growers can use ImmunoStrip® at their greenhouses and nurseries to provide quick and reliable results.

Since there are currently no effective methods to cure plants infected with viruses, any symptomatic plants should be removed from the production facility and discarded after the plant materials have been confirmed to have one or more plant viruses.  Failure to remove symptomatic plants from the crop area could result in significantly more infected plants down the road especially if the virus is vectored by insects.

The introduction of plant viruses into production facilities can be reduced by purchasing plant materials from reputable companies which offer clean, virus free materials to the industry.  Inspect all incoming plants for virus symptoms.

The best management strategies involve early detection and controlling the vectors of plant viruses.  Scouting programs can help detect vectors of viruses and help identify plants expressing virus symptoms early.  Controlling insect vectors and weed host plants are currently the most effective means of reducing viruses from infecting crops in greenhouses and nurseries.  

Remember, once a plant has been diagnosed with one or more plant viruses; plants showing virus symptoms should be removed from the production area and discarded immediately.

Paul Pilon
Perennial Solutions Consulting
paul@perennialsolutions.com

 

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