2012 Crop Disease of the Year at the U of I Plant Clinic was a Virus!

What was the main crop disease seen at the U of I Plant Clinic in 2012?  Well, we saw a general increase in soybean virus diseases.  More specifically, we saw a significant increase in a soybean viral disease called, soybean vein necrosis virus on many soybean samples submitted to the U of I Plant Clinic.  

Soybean Vein Necrosis Virus (SVNV)

What is a virus?  a submicroscopic, intracellular, obligate parasite consisting of a core of infectious nucleic acid (either RNA or DNA) usually surrounded by a protein coat (EM of tobacco mosaic virus) APSnet

• Non-living pathogen
• Unable to replicate self 
•  Unable to transmit itself – require a vector to penetrate a cell wall (insect, seed, pollen, human, fungi, nematode, etc.)
• We can’t see it under a microscope, but can see it under an pricey electron microscope (if available)

For more information on plant viruses:  http://www.apsnet.org/edcenter/intropp/PathogenGroups/Pages/PlantViruses.aspx

Some of the common plant virus vectors in Illinois are aphids, thrips, whiteflies, beetles, leafhoppers, plasmodiophorids, nematodes, and mites.   

There can be two different types of insect vector transmission: 

• Non-persistant- feeding behavior is a stylet-born virus transmission, frequent feeding probes of short duration (seconds), ~230 viruses (this type of insect vector looses the virus rapidly if feeding on a non-infected plant

• Persistant- feeding behavior is a circulative transmission, infrequent feeding probes of long duration (minutes to hours), ~80 viruses, (the virus multiplies within this type of insect vector)

Non-persistant insect vector                                                              Persistant insect vector

Courtesy of Oregon State University presentation

 Courtesy of Oregon State University presentation

Plant viral symptoms occur when virus RNA is incorporated in the cell and causes an "interference with the plant physiology.  Typical viral symptomology on plants can be described as mosaic, mottled, ring spots, stunt, leaf roll, curled, streaked, breaks, yellowing, reddening, or crinkled.  In some cases, the plant may be infected with a virus, and not show any symptoms at all.

If a field show symptoms of a virus, the field pattern of symptoms can sometimes mimic a typical insect (vector) distribution in a field.  For example, if you observing what you think are viral symptoms on field edges, this could be due to insect vectors that survive in fence rows.  However, viral symptoms that show up as "hot spots" within the field may be due to a wind, fungal, nematode, or infected seed pattern virus vector.

 The diagnosis of plant viruses in the field is difficult if you are relying only on plant symptomology because most viruses can exhibit similar symptoms.  In addition, other plant problems can sometimes exhibit simliar symptoms to that of viruses, such as herbicide injury.  Definite virus diagnosis is not possible without specialized testing such as Bioassays using indicator hosts, vector transmission assays, electron microscopy, detection based on coat protein or ELISA assay (enzyme-linked immunosorbent assay), or Characterization of viral nucleic acid:  PCR (Polymerase chain reaction amplication) and gels.

Picture of a hosta that has been found to be positive (2 red lines) based on an ELISA quick strip test

If you are testing a plant for a virus, a good sample is needed for virus diagnosis.  Take a representative sample of the symptomatic tissue, which may include: entire leaf, entire tuber, or entire plant.  The plant sample may need to be shipped overnight to the testing facility. If you have a question about testing facilities, you can contact the University of Illinois Plant Clinic:  http://web.extension.illinois.edu/plantclinic/

Plant viruses can survive in nature within vectors, infected debris, seed, and usually only in association with living tissue.  

When scouting for crop virus diseases, you may want to watch for moderate to high insect activity, lower than expected yields, mottled seed (soybean), no yield response to insecticide application, and presence of virus symptoms typical of virus infection.

Plant Virus Management:

Strategy	Effectiveness
Resistance	highly effective when available
Sanitation	Not very effective in field,  greenhouse yes
Rotation	Not very effective
Insecticides / vector control	Timing very difficult, with persistent viruses and very mobile vectors with many generations less effective
Tissue culture	Highly effective,  ie. potato certification programs
Coat protein technology antisense	Highly specific and effective when available, tomato

(Stephanie Porter and Suzanne Bissonnette)