Pathogen vs. Disease: Why Terms Matter - Jim Chatfield


Rose black spot disease, caused by the pathogen Diplocarpon rosae

What’s in a term or a name - clarity of thought and communication. When discussing plant diseases, I always make a point,ad infinitumorad nauseum(your choice), that the pathogen is not the disease. This is a key precept of the Plant Disease Triangle, that for an infectious disease to occur, three things must coincide: a virulent pathogen, a susceptible host plant, and an environment conducive to disease.

Disease is a process. Rose black spot will not occur unless there is the virulent pathogen (Diplocarpon rosae),a susceptible host (certain taxa in the genusRosa), and a certain number of hours of wet foliage at a given temperature. There are roses resistant toDiplocarpon rosae, thus they do not get rose black spot disease.

Planting those resistant roses is a great disease management strategy because it means that not all three components of the disease triangle are present. Of course, the pathogen, working 24/7 like everything in nature, tends to mutate, overcoming resistance to the pathogen in certain roses, so the eternal “Nature red in tooth and claw” always applies.

A case for the idea that names and terms matter occurred this past summer in Ohio and other states when lilacs and rhododendrons and other nursery plants susceptible to thePhytophthora ramorum water mold pathogen escaped plant regulatory quarantine in the Pacific Northwest, were shipped to Oklahoma and then shipped eastward to Walmart and Rural King outlets. A terminology problem occurred with the use of the term Sudden Oak Death for infested plants.

A better term for this disease is PRAM disease. SincePhytophthora ramorumoccurs on many hosts, it is best not to use the term Sudden Oak Death when this pathogen is causing diseases on other host plants. The statement “Sudden Oak Death confirmed in Ohio” muddied the waters, since no oaks were shipped, no oaks have ever been found to have Sudden Oak Death in the East, and the pathogen has not been shown to have become established in the East, even on these other hosts, except in this shipped nursery stock.

Of course the possibility exists that the lilacs and rhododendrons that escaped interception here and were then planted in Ohio can be a source of inoculum leading to the establishment of this pathogen here. However, we have not yet detected that yet in the past two decades when this pathogen was identified in the Pacific Northwest, and therein lies the connection to the Disease Triangle. It is suspected that the Environment Conducive to Disease component of the Disease Triangle will not be satisfied in Ohio, compared to the high-humidity coastal forests of California and Oregon where hundreds of thousands of oaks died from SOD, starting in the 1990s.

Nothing is guaranteed, of course, and vigilance and the highly professional regulatory efforts of the Ohio Department of Agriculture are essential, but remember that the presence of the pathogen is not the same as the presence and establishment of the disease. Terms matter!

Which brings us to the current focus of all our lives: what about COVID-19? Recently, it occurred to me that I did not know what it stood for: the CO (corona) seemed obvious, and the VI (for virus) seemed obvious, but what is the D? Duh, it is for D (disease), and the 19 is for its discovery in 2019. As per our discussion, COVID-19 is the name for the disease. The pathogen’s name, the term for the virus, is SARS-CoV-2. 

So to close, a little bit more of our continuing education: according to the CDC there are currently seven coronavirus pathogens (crownlike spikes on the surface of the virus particle) known to infect humans.

four of these viruses cause common cold symptoms

SARS-CoV causes SARS: Severe Acute Respiratory Syndrome

MERS CoV causes MERS: Middle East Respiratory Syndrome

And now SARS-CoV-2, causing COVID-19. Be safe. 

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