Understanding the Disease Triangle, a Perfect Storm That Can Destroy Your Plants

By Ashley Barkow, Horticulturist

For those of you who may have never heard of the disease triangle, it’s a pretty simple concept that shows how the potential relationship between a host (plant), a pathogen (disease) and an environment (your garden) can come together to negatively affect the wellbeing of your plants and wildlife.

There are three components to the disease triangle. You must have all three components in order to have a disease. Limiting one, two or three of the factors of the disease triangle will greatly impact the severity of the disease.

disease triangle concept model

When gardening, be sure to keep the principles of the disease triangle in your back pocket. The first component of the disease triangle is the host, the specific plant that can be affected by a pathogen. The second component of the disease triangle is the pathogen, the living organism that causes the disease. The last component of the disease triangle is the environment, which refers to the ideal place where the disease, pathogen and host can survive.

lilac shrub

Lilac shrub

One example of the disease triangle in action would be a lilac shrub growing in a damp, crowded garden that’s infected with powdery mildew. The host is the lilac shrub, the pathogen is the powdery mildew (a fungus), and the environment is the damp, crowded garden. All three of these factors come together to create the perfect atmosphere for a disease to thrive.

If a plant is a susceptible host, and already has a disease, the environment can be altered to help lessen the severity of the disease. Let’s go back to our lilac shrub (host) infected with powdery mildew (pathogen, growing in a damp, crowded garden (environment). By pruning away infected lilac branches and moving away any surrounding plants, airflow is improved, the moist, congested environment is eliminated, and the chances of getting powdery mildew are lessened.

disease triangle concept model - environment

Now, let’s think about the host. Some plants are more susceptible to certain diseases than others. For example, crabapple trees (host) are extremely vulnerable to apple-scab (pathogen), which is a fungus. This pathogen is accelerated in cool, crowded gardens (environment) in the fall or spring.

apple scab on crabapple trees

Apple scab on crabapple trees.

Apple scab produces unsightly black blotches on the leaves, resulting in premature leaf drop. To disrupt the disease triangle and decrease the likelihood of getting apple scab, plant a disease-resistant host, such as an apple scab resistant cultivar, or better yet, an oak tree.

disease triangle concept model - host

So how do we alter the pathogen to decrease the likelihood of disease? Take, for example, a maple tree (host) with tar spot (pathogen). This is a fungal disease that is more of an aesthetic concern than a lethal one.

black tar spot on sugar maple

Black tar spot on sugar maple.

To disrupt the disease triangle, simply rake up and destroy any fallen leaves. Doing so will remove the pathogen from the environment, so it won’t be able to affect the maple tree the following season.

disease triangle concept model - pathogen

To employ the disease triangle in your own backyard, plant disease-resistant plants, remove pathogen-containing materials and minimize environmental conditions that promote the spread of disease.

Want to get a refresh on best pruning practices so you’re ready for spring? Sign up for our next two pruning classes at the Garden! Vijai Pandian, a Brown County Extension Brown Horticulture Educator, will be on hand to answer all your gardening questions for the upcoming growing season.