Karen-Beth Scholthof, "Environment and Disease: Lessons from Plant Pathology in the Time of COVID-19”

In an effort to engage more explicitly with the world around us, the Agricultural History Society is developing a series of short, web-based essays that will apply the stories and methodologies of our allied fields to the issues that currently affect our daily lives. Anyone with an interest in contributing, please contact Bert Way at away5@kennesaw.edu. Our first contribution is Karen-Beth Scholthof’s essay, “Environment and Disease: Lessons from Plant Pathology in the Time of COVID-19.”

It should be cited as: Karen-Beth G. Scholthof, “Environment and Disease: Lessons from Plant Pathology in the Time of COVID-19,” The Short Rows, published June 17, 2020. https://www.aghistorysociety.org/ahs-blog/karen-beth-scholthof-environment-and-disease-lessons-from-plant-pathology-in-the-time-of-covid-19

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Environment and Disease: Lessons from Plant Pathology in the Time of COVID-19

 Karen-Beth G. Scholthof

As we learn how to live in the time of COVID-19, experts such as Anthony Fauci have advised that social distancing should continue until there are “essentially no new cases, no deaths.” Why must we remain vigilant within our communities even after the apex of the outbreak, through the extended plateau, and a possibly long-term cycle of seasonally circulating virus? Perhaps surprisingly, my area of expertise, plant pathology—the study of plant diseases—offers a understandable explanation. 

Plant pathologists use what’s known as the “disease triangle” to explain how to control outbreaks and seasonal reappearances of a disease. The disease triangle was first conceived more than sixty years ago by George McNew, a plant pathologist at the Boyce Thompson Institute for Plant Research. McNew diagrammed the fact that an epidemic arises from the interaction of three factors: a susceptible host, a virulent pathogen, and a hospitable environment. Here, a simple form of McNew’s disease triangle is helpful to explain the key role of environment in the success of pathogens such as SARS-CoV-2.

The disease triangle. The events needed for disease to occur at a particular time includes a permissive environment, a susceptible host, and a virulent pathogen, as shown on the left. By altering the environmental conditions favorable to the pathogen, disease incidence can be reduced, as shown by the drawing on the right. Image: K.-B.G. Scholthof

 For an example, consider a plant disease where the three key factors consist of a virulent pathogen (Phytophthora infestans, an oomycete or water mold), a field of potato plants, and cool, humid weather. When disease-resistant plants are not available (i.e., we can’t change the host) and we lack good chemical control measures (i.e., we can’t attack the pathogen directly), the cycle of infection can only be disrupted by “breaking” the environmental side of the disease triangle. Similarly, to reduce the incidence of COVID-19, caused by SARS-CoV-2 infection, in our communities, we use social distancing, face coverings, and handwashing—changing our environment reduces the likelihood of exposure to the virus and of exposing others if we are infected. 

As a case study, let’s look at a devastating plant disease outbreak in the mid-nineteenth century. The Great Irish Famine was a humanitarian crisis precipitated by the failure of the potato crop due to late blight disease, caused by the water mold Phytophthora infestans. First-hand reports from Ireland described this blight as seeming to spring up overnight with fields of lush green plants suddenly melting away into a stinking mess, resulting in the near total collapse of the potato crop. Although disease outbreaks also occurred in Europe and North America, the dependence of the Irish poor on the potato for most of their food calories was devastating. The tragic consequences of this plant disease outbreak led to the emigration of a million people from Ireland and another million deaths—a loss of 25 percent of the country’s population. A single pathogen provided a horrifying lesson for the need to scientifically manage, reduce, and control a crop disease. At the peak of the Irish Famine in 1847, the end was not in sight. Even today, late blight disease remains a clear and present danger for farmers and gardeners growing potatoes. 

Plant diseases such as potato late blight provide a lesson in the close link between pathogens, the environment, and society—including how an epidemic drives policy and politics while further unmasking social and economic injustices—issues that strike close to home as COVID-19 has evolved from a distant fear to our daily reality. 

Driving through farmlands, you may see hundreds or even thousands of acres of a single crop. Genetically identical plants are a “monoculture” and are especially vulnerable to emerging pathogens and disease. Today, plant pathologists break these cycles of disease by modifying or controlling the host, the pathogen, or the environment. We may breed crops that are resistant to the disease, plant them in a different way or at different times, or use chemical treatments to protect the plants from harmful fungi, viruses, bacteria, and insects. By changing the host (resistance genes), stopping the pathogen (chemicals), or altering the environment (planting earlier or later) we can control an outbreak of a new disease or seasonal recurrence of a known pathogen. 

For our current pandemic situation, the global human population is the host species, and we are, in essence, a monoculture. SARS-CoV-2, the novel coronavirus, having jumped from an animal host, has become extraordinarily successful at infecting humans. We do not have population-wide immunity to this virus. Again, from Dr. Fauci, we “don't make the timeline, the virus makes the timeline.” Until drugs or vaccines are shown to control the virus and the disease, we can do our part to disrupt an environment that is favorable to the novel coronavirus. And, there may be a single peak of infection, waves of infection, or seasonal recurrence of COVID-19 in our communities. Ongoing infections would suggest that we have not been sufficiently vigilant in disrupting the virus’s favored environment. Today, and for at least the near future, disrupting a favorable environment for the virus is the main element of controlling the spread of the disease, and the disruption must continue until either proven vaccines (to strengthen the host) or medications (to combat the virus) are created.

Social distancing, a face mask, and good hygienic practices are the best community options currently available to break the disease triangle. Simple efforts such as handwashing with soap disrupts the virus’s membrane envelope, preventing it from initiating an infection. With time, we, the host, may acquire ‘herd immunity’ as a form of protection from the virus. Or, the amazing ongoing scientific work may identify safe and effective treatments or a vaccine. 

In the meantime, we must remain vigilant in continuing with the recommended practices that create an unfavorable environment for the virus. We must not become complacent, because when we reach the peak we are at best only halfway toward coming down from this global mountain of disease. 

Karen Scholthof is a professor and virologist in the department of Plant Pathology and Microbiology at Texas A&M University. Her areas of specialization are molecular plant virology and the historiography of tobacco mosaic virus.

For further reading:

Scholthof, K.-B. G. 2007. The disease triangle: pathogens, the environment, and society. Nature Reviews Microbiology 5: 152-156. https://www.nature.com/articles/nrmicro1596