Thinking deeper on the disease triangle

Elementary discussions of plant pathology describe the disease triangle almost without fail.

It has become a standard inclusion in many presentations, and a quick image search will find hundreds of graphical designs that have been developed to describe the idea.

The foundational idea appears quite simple at first glance. For a ‘dise-ease’ to express itself requires a combination of three elements:

  1. a susceptible host,
  2. a potentially pathogenic organism,
  3. the proper environment,

This seems like an obvious and simple explanation. We accept it without much question and move on to the next part of the discussion, which usually revolves around controlling ‘pathogens’ when the combination of these three elements is met.

Rather than moving immediately to the control conversation, and assuming an infection has occurred that we have no influence over, we might dig a bit more deeply into each of the three elements. When we understand these three elements fully, they will give us all the information we need to prevent any ‘dis-ease’ from expressing in the first place. Prevention is often easier and more effective than a cure.

When we consider the definition of a ‘susceptible host’, what are the parameters that define a susceptibility? What are the differences between susceptible and resistant cultivars? Does the organism require a certain amino acid and carbohydrate profile that some cultivars do not provide? What are the resistance mechanisms that are present in some cultivars, and in some growing conditions, but not in others?

What defines a ‘potential pathogen’? We know from the research being conducted on the microbiome, as well as the work described by James White and Don Huber in our podcast discussions, there is no correlation between the presence of a potential pathogen and an actual infection. Soil organisms that might become pathogenic (such as fusarium and verticillium) actually develop symbiotic relationships with plants when the plant has a healthy, disease suppressive microbiome. For an organism to infect a plant and produce disease requires compromised soil biology, and a compromised microbiome.

What defines a ‘proper environment’? Our first thoughts generally go to the external climate, humidity and temperature. What about the plants internal environment, and the soil environment? From Olivier Husson’s breakthough work on the biophysical environment required by different organisms, we know that each organism requires the plant and soil to be in a specific redox state. The plant pH and redox, and soil pH, redox, and paramagnetism each need to be within a defined zone before an organism can infect a plant and produce disease.

The solution to effective disease prevention is actually very straightforward. We need to understand exactly what defines a susceptible host, a disease conducive microbiome, and the internal plant ‘environment’ for each pathogen. When we understand these elements, it becomes very easy to manage the crop in a way that prevents these organisms from producing disease, even when the organism is abundantly present, and the climatic environment is considered ideal for disease expression.

For which ‘disease’ organisms would you like to understand the elements of the disease triangle more deeply?