Use the Disease Triangle to Help Your Plants

I have written several articles about different plant diseases and pests that the horticulture team has dealt with at Minnetrista. I like sharing what choices we make to combat various plant issues for those of you that might be dealing with the same issue in your own garden—or at least give you a heads up on what might be coming your way.

However, I don’t think I’ve ever shared the basic thinking that helps us and other professionals make decisions on strategies for combating problems. We often use a concept called the disease triangle.

The basic premise of the triangle is that three factors must be present in order for disease to occur: a disease or pathogen, a susceptible host, and a favorable environment. Therefore, to combat disease, we try to minimize any of these components that are within our control. It can be helpful to view this as a diagram where each component is represented on its own line. The smaller the triangle, the lower the likelihood of disease.

Each component can be further defined as follows:

Pathogen/Pest–This can include fungus, insects, bacteria, viruses and other microbes. Reduce the presence of the pathogen, and of course, you reduce the possibility of disease. Total abundance, the ability to infect, and the proximity to a susceptible host are factors to consider.

Environment–Conditions that favor disease. For example, a fungus like apple scab needs moisture and warm temps to become a problem, so warm wet springs mean it’s time to think about apple scab. Drought is another example of an environmental factor.

Host–The degree in which the host (your plant) is susceptible to a disease. For example, some apple tree varieties are very susceptible to apple scab, while others are close to completely resistant.

If you want to reduce or eliminate a pest or disease, you have to make the triangle smaller. In some cases that might be selecting disease resistant varieties of plants, which would limit the available hosts. In other cases it might be preventive fungicide when environment factors are right for rapid spread.

In any case, the disease triangle can help you make a decision on which factors to minimize in order to reach the desired outcome of helping your plant.