Zika And Zucchini Yellow Mosaic: Analogies Between Human- And Plant-Infecting Viruses
October 31, 2016 
Even with powerful electron microscopes, viruses cannot be identified based on particle shape.
Pictured here are particles of tomato apex
necrosis virus. Photo credit: B. W. Falk
ZIKA, dengue, West Nile, ebola: These exotic-sounding words grab our attention and represent mysterious, but threatening entities or biological phenomena that we might be fascinated with but don’t want to experience. Of course, these are all viruses that infect people and which have occurred lately in outbreaks that caused debilitating diseases in various parts of the human community.
Attracting significant attention lately is the Zika virus, previously a lesser known virus but one that causes dreadful birth defects as well as serious neurological disease in some adults. While we certainly do not equate the seriousness of Zika with agricultural problems, the Zika virus can serve as a reminder about aspects of the many plant-infecting viruses, from alfalfa mosaic to zucchini yellow mosaic, that affect vegetables.
Specialized Tools To ID Zika
Even with powerful electron microscopes, the Zika virus cannot be identified by looking at its particles. In addition, because other pathogens cause similar symptoms, Zika can’t be identified based on symptoms either.
Zika can only be confirmed with specialized tests that detect the virus’ RNA or specific antibodies in the patient’s blood. Zucchini yellow mosaic and all the other plant viruses from “A to Z” likewise defy visual identification. A look at plant virus particles is not enough.
Regarding symptoms, plant viruses cause problems that look fairly generic. For any infected crop, the leaf spots, yellowing, foliage deformity, necrosis, or stunted growth caused by one virus can overlap with similar looks caused by another. A lettuce plant exhibiting stunting, twisted leaves, and brown necrotic spotting could be infected with lettuce mosaic, turnip mosaic, cucumber mosaic, tomato spotted wilt, or impatiens necrotic spot viruses.
To identify a virus in lettuce, one uses diagnostic tools that search for a molecular or serological match with the responsible virus. Fortunately for field professionals, advances have been made in diagnostics and portable, easy-to-use test strips (lateral flow devices) that can be used in the field to test ground-up slurries of leaf tissue for any number of viruses.
Virus identification is no minor issue; the best approach to disease management depends on accurate identification of the pathogen responsible for the symptoms.
Zika In A Family Of Related Viruses
Scientists find common genetic, morphological, and biological commonalities among all organisms. Based on these shared characteristics, researchers classify and organize the organisms into groups. Viruses are no exception to this classification effort. Zika belongs to a virus family called the Flaviviridae. Viruses in this family have similar particle shapes, genetic material, and protein structures. Zika is therefore closely related to family members such as dengue, yellow fever, Japanese encephalitis, and West Nile viruses.
The numerous “A to Z” plant viruses also are arranged into families. For our lettuce example, lettuce mosaic and turnip mosaic viruses both belong to the Potyviridae group, while the tomato spotted wilt and impatiens necrotic spot pathogens are placed in the Bunyaviridae family. Viruses affecting other crops are likewise categorized into families.
To growers battling viruses in their crops, this aspect of virus families might appear to be strictly an academic matter. However, there are practical aspects to virus classification because the viruses within a group tend to have common features, thereby providing agriculturists with some idea of how the pathogens operate in their fields.
For example, the potyviruses are mostly spread between plants by aphids. However, the plant viruses in the Bunyaviridae are vectored by thrips and not by other insects. Another group of plant-infecting viruses, in the Nepovirus genus, are not spread by insects at all but by soilborne nematodes. Knowing the plant virus family, therefore provides a clue to how the pathogen is spread in fields.
Key To Zika Is The Vector
The rapid outbreaks of Zika in South and Central Americas and the Caribbean are primarily due to its mosquito vector.
For our “A to Z” plant viruses, the vast majority also are carried around by vectors. Plant viruses can be spread by insects, mites, and nematodes. A great majority of these vectors are in the Hemiptera order of insects, which includes aphids, whiteflies, mealybugs, and leafhoppers. A key to plant virus outbreaks is often the buildup of vector populations, which results in large numbers of insects spreading the viruses.
For the past few years, virus problems have been heightened in California vegetable crops due, in part, to the long drought. With severely limited rainfall, the natural vegetation, weeds, and pastures surrounding vegetable fields rapidly dry up in the early spring, forcing insect vectors to flee this dead vegetation and flock to nearby vegetable crops.
Preventive Measures
Presently, escaping Zika is only possible by avoiding the bite of a virus-carrying vector, which is a preventative measure. There is no treatment for Zika once the virus is inside the body. Hopefully, Zika vaccines will be available in the near future, again a preventative control.
Plant virus control for vegetables also is strictly dependent on preventative strategies; once a vector injects the virus, there are no treatments for crops. Growers therefore need to manage viruses with a combination of the following preventive and integrated strategies:
• Plant cultivars resistant to the virus;
• Time planting to coincide with reduced vector activity;
• Avoid planting in areas with a history of virus problems (which indicates a reservoir of the virus is in that area);
• Control weeds (which serve as virus sources); and
• Manage vectors with pesticides and other IPM measures.
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