I’ve noticed that graduate school is a systematic unpacking of the nicely packaged boxes of facts and truths that I accumulated over the years. Definitions were hit particularly hard. For instance, in context of horticulture, define plant disease and pathogen. Before I continue I feel I should reveal that experts disagree strongly on subtleties within these definitions. It’s a matter of picking a definition and fighting for it.
Here is my current concise definition of plant disease: an adverse change in the form or function of a plant in response to the affects of a pathogen or environmental factor. We’ll get back to this, but right now the natural follow-up question is: what is a “pathogen?”
This is where things get really interesting. If I pull my Random House Webster’s College Dictionary off the shelf it will tell me: n. any disease-producing agent, esp. a virus, bacterium, or other microorganism. Okay, what about the book next to it on the shelf, my Oxford Concise English Dictionary: n. a bacterium, virus, or other micro-organism that can cause disease. I’m seeing a big problem with those two definitions: they are not the same. How about adding one more definition, that of Agrios’ Plant Pathology (a respected text in the realm of those who study sick plants) an entity that can incite disease. That did not help. Everyone agrees on what a pathogen does (causes disease), yet what is it? How does it affect the plant? Does it need to be small? More importantly, does it need to be alive?
Let’s look at the etymology of the word pathogen. It’s of Greek origin. Two Greek words combine, first is pathos, which means suffering/grief, and second is gennan, which means to produce. So a plant pathogen is something that can produce suffering in a plant. Based upon the the parts of the word itself, there is no indication whether it refers to something that is alive or not, big or small.
This is difficult. A human can cause suffering in a plant. A rock can cause suffering in a plant. A cold day can cause suffering in a plant. Are they plant pathogens?
My immediate response is to say that a pathogen must be a biotic, microscopic agent. It needs to be alive. But what about a virus? And why do I establish the constraints of biotic and microscopic anyway? It’s really just to fit my preconceived notion of what a pathogen is.
Not only am I having trouble deciding if a pathogen needs to be alive or not, I don’t even know if it needs to be present or not to cause the disease. For instance, if a plant lacks sufficient nitrogen it will become diseased. Thus the cause of the disease is the absence of nitrogen. Does this make nitrogen a pathogen? Can something be a pathogen in absentia?
Right now I’m still wrestling with what my definition of a plant pathogen will be, and how I will defend it. It makes me feel rather stupid (and then I remember that experts all over the world wrestle with this too and then I feel depressed instead).
Going back to my definition of plant disease, you’ll notice that it contains the word pathogen. It has to. But I still don’t know what a pathogen is. So therefore I do not know what plant disease is. Arrrrggghhhhhhhh!