Written by Elaine Harlin, Portage County Master Gardener Volunteers
What is wrong with my plant? I have asked myself this question so many times, I have lost count. My challenge became how to figure out a solution without killing the plant through what I thought was the right treatment. Behold, the disease triangle!
The plant disease triangle was developed in 1960 by botanist Russell B. Stevens. It is a simple tool used to understand how plants get sick. The idea is that plant sickness needs three equal parts to happen:
Host: Susceptible plant
Pathogen: Fungi, phytoplasma, bacteria, virus
Environment: Air and soil temperatures, rainfall, humidity, soil moisture
Think of it like a fire triangle. A fire needs oxygen, heat, and fuel to burn. If you take one part away, the fire goes out. In the same way, if you change one part of the plant disease triangle, the plant will stay healthy!
I have used the disease triangle to help me understand what is happening with a sick plant to apply the right treatment so one of the three conditions of the disease triangle is eliminated. This means either kill the pathogen, correct the environment, or replace the host plant with a disease resistant plant variety.
I had a New England aster that was growing beautifully in my garden. It had grown to bushel size by August with blooms appearing in late July. We had a very humid and hot July, and I was watering the plant every night when it was cooler. By the second week of September the aster was half brown, almost dead! (See before and after pictures)
Using the disease triangle, I found my New England aster is susceptible to various fungi such as powdery mildew, and rust. They are also susceptible to various insects such as thrips and aphids which carry viruses that infect plants. I cut a sample stem from the aster and looked at it closely. I found rust, powdery mildew, thrips at various stages of development (from egg, larva, pupa and adult), and aphids. My poor aster was a hot mess!
My environment contributed to the fungi due to watering at night and having limited air circulation due to the aster being crowded by other plants. Out of my control but part of the environment, was the extreme heat and high humidity.
The next step was to determine how to manage the pathogen, change the environment, and/or find disease resistant plant varieties. Once you have determined what is affecting your plant, search the internet for management solutions such as, “rust fungus treatment on flowers at Ohio.edu.org” (See link below). Be sure to look for natural solutions for managing plant diseases whenever possible, as this will help to protect beneficial insects in your gardens.
In my case, the aster was too overwhelmed by thrips, aphids, and fungi to manage effectively. I decided to gently remove the entire plant wrapped in newspaper, then to a trash bag for trash removal day. Please note, you should NOT compost diseased plant material as the fungi, bacteria or virus can infect your compost.
Final tips: Know the botanical name of your plant, learn what your plant is susceptible to, and scout your garden often to find potential threats early on. Happy gardening!
More information:
Rust in the flower garden
Plants Get Sick Too!
Disease Triangle: Fundamental Concept for Disease Management
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