Rodrigo A. Valverde
Like humans and animals, plants also suffer from diseases caused by fungi, bacteria, viruses and other pathogens. Viruses that infect plants, including many crop species, are similar to their relatives that infect humans, such as SARS-CoV-2, which causes COVID-19. However, COVID-19 can lead to severe illness and death for humans. As of February 2021, more than 2.2 million deaths worldwide have been attributed to COVID-19, according to the Johns Hopkins Coronavirus Resource Center. Fortunately, plant viruses do not infect humans and are easily eliminated by cooking. When we consume them in uncooked plant products infected by viruses, the high acidity and enzymes of our digestive system degrade most of them. There are many similarities between plant and human viruses concerning detection, spread and control methods.
Plant viruses spread by mechanical contact from farm equipment or by vectors, or agents that spread the pathogen, such as insects. A classic example of a plant virus is tobacco mosaic virus (TMV), a virus that infects tobacco and many other crops. Although this virus is stable in the environment, it can be eliminated by common disinfectants, including soap, alcohol or bleach. Therefore, to control the spread of TMV, it is a common procedure for field and greenhouse workers to disinfect their hands, tools and equipment. This approach to controlling a plant virus is similar to the recommendations by the Centers for Disease Control and Prevention to eliminate SARS-Co-2 from the surface of our hands or items we touch.
“Changing the environment” to control the plant exposure to insect vectors is a plant virus control approach similar to the use of masks to help control of COVID-19. For example, many crops are protected against the virus vectors by using reflexive mulches, covering plants with fabric, spraying plants with oils or growing them in greenhouses. Most plants affected by viral diseases do not die. After all, keeping the host alive is the only way for the virus to multiply and survive. Nevertheless, as in the case of COVID-19 in humans with certain preexisting conditions, plants debilitated by other viruses or pathogens suffer more severe symptoms when infected by a specific virus and sometimes die.
Like humans and animals, plants can be “vaccinated.” An example of this is the control of a disease of citrus called Citrus tristeza virus (CTV), a disease with worldwide distribution. When infected with severe strains of this virus, most citrus varieties die. At one time, the citrus industry in Brazil, a major citrus-producing country, was being devastated by the virus. However, researchers noticed that trees infected with mild strains of CTV were not infected by the severe or killer strains, and fruit yield was not significantly affected. Therefore, a massive inoculation (“vaccination”) of healthy trees with mild strains of the virus was conducted and saved Brazil’s citrus industry. Today, the massive efforts of private and public institutions to develop a vaccine to protect us against infection by SARS-Co-2 should provide us with a method to alleviate the COVID-19 pandemic.
Rodrigo A. Valverde is a professor in the Department of Plant Pathology and Crop Physiology.
(This article appears in the winter 2021 issue of Louisiana Agriculture.)