By Rob Hausmann

As a horticulturist, I commonly get asked the question “What’s wrong with my plant” accompanied with a photo of someone’s beloved ‘pet’ that is riding the struggle bus. If the cause of the problem isn’t related to light or water, the damage or disease can usually be applied to the disease triangle. Disease or damage can be caused by microorganisms like bacteria, fungi, or viruses, or insects.

In horticulture, more specifically Plant Pathology, damage or disease can simply be explained by an illustration called the disease triangle. The Disease Triangle accounts for three factors that must be required/present AT THE SAME TIME for the pathogen to occur. These factors are: your beloved plant bebe - aka The Host, The Environment, and the pest or The Pathogen itself. The Host can refer to the plant itself but also the resistance or susceptibility to a certain pathogen. The Environment is referring to the relative humidity, the soil type, the moisture, the sunlight, all attributing to whether a space is dry or wet, usually. The Pathogen is the factor that can be more mysterious, where did it come from?

My favorite example when explaining the disease triangle is spider mite infestation on a parlor palm ‘Chamaedorea elegans’. Usually this is easily caused by the plant’s environment becoming too dry. Since spider mites thrive in a dry environment (1 factor) and the host plant is present (2 factors)-all that’s needed for the disease aka damage to occur is presence of the pest (3rd factor). These three factors coming together at the same time allow for the perfect storm for damage or disease to occur. Since, the living room is sunny and therefore dry (Environment), the spider mites (Pathogen) could thrive on the plant (Host). This can be remedied by removing one of the three factors, and its typically easiest for us to manipulate the plant’s environment.

“Where do the spider mites come from though?”

Well there are usually two scenarios that answer this question.

Scenario 1: The plant has them present on it when acquired. It’s common for many plant pests to show up at a nursery or garden center just because of the enormous amount of material coming in and out the door. (Sidebar: one should always quarantine new house plants or even landscape plants away from healthy plants to ensure they aren’t infected before installing in their forever home. I usually go one step further and treat them as if they are infected, spraying and monitoring them closely).

Scenario 2: The other source of our little arachnid foes is the wind. That’s right- these tiny sap suckers can easily catch a free ride on a light breeze, fit right through a common window screen and onto a precious plant. For this reason, I hardly ever open my windows at my house. I have enough plants to create enough fresh air anyways! If a spider mite infestation occurs, a good start to the battle is to increase the humidity in the plant’s environment. Systemic insecticides for indoor (non-pollinator) plants are my preventative first line of defense.

The Disease Triangle, can be applied or used for horticulturist and plant parents alike to critically think through a plant disease or damage situation and figure out a solution. Sometimes it can be an easy change of environment if one is lucky!