The disease triangle in root diseases

When growing greenhouse crops, it is simply a matter of time before problems occur. These problems can be caused by diseases, insects, fertility problems, pesticide phytotoxicities, plant growth regulator over-application, watering problems, air pollution from heaters, etc. To correct a problem, it is necessary to know what it is and what is causing it.

Similar looking symptoms
Unfortunately, many types of problems have similar looking symptoms, so it is important to test a crop to determine the cause. However, what type of test do you do? For example, if you send in a plant to be tested for disease when the problem is coming from a malfunctioning furnace, the source of the problem will not be identified. Therefore, it is best to start investigating the problem and take note of all the variables that could be causing a problem to narrow down the cause.

Disease Triangle
In the case of plant disease, there are certain conditions that must be in place in order for a disease to get started. This is often simplified in a picture called the disease triangle (Figure 1). The disease triangle points out that three favorable conditions must coexist to cause a disease problem. These three are the pathogen, a susceptible host (plant) and proper environmental conditions.

Figure 1. Disease Triangle. The three factors that have to coexist are presence of a pathogen, proper greenhouse environment for disease development and a susceptible host plant.

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