Disease Triangle

Do you remember the fire safety lesson from elementary school where they told you that three things had to be in place for a fire to occur? [The answer is fuel, heat, and oxygen.] Take any one of those components out of the equation, and voila! No fire. Well, plant diseases work much the same way.

The three sides of plant disease

For disease to occur, the environment, host plant, and pathogen must all be present. Alter any of those three and make infections less severe and less frequent. Remove one side of that triangle to eliminate the disease. Since prevention is far easier than treatment, the disease triangle is a handy tool for you to use in the garden.

​Environment

You can control common environmental factors with these good cultural practices:

• Ensure good soil structure and fertility with compost.
• Select plants suited to your microclimate and soil pH.
• Install plants in locations conducive to their health, space them properly, and at a time when harmful diseases are less likely.
• Prune plants for good airflow.
• Water early in the morning to give plants time to dry out.
• Use drip emitters, furrow irrigation, and soaker hoses instead of overhead watering.
• Allow the soil to dry out between waterings.
• Control pest populations with beneficial insects, weed removal, and good sanitation.
• Minimize temperature extremes with mulch, proper irrigation, and sunburn protection.
• Remove tree supports as soon as they are not needed.
• Remove diseased plants.

Host

The host is the plant that gets sick. It can also be a plant that shelters a pathogen or disease vector. Some diseases affect only a single host, while others can infect many different types of plants. [Did you know that lilacs grown near apple trees are more likely to get bacterial blight?] Protect host plants from disease with these tips:

• Install disease-resistant plants.
• Select and install plants less likely to attract, feed, or shelter common pests and diseases in your area.
• Reduce disease susceptibility by keeping plants healthy enough to protect themselves.

Pathogens

Pathogens are disease-carrying bacteria, viruses, or other microorganisms. You may be surprised to learn that many pathogens are already in your garden. New arrivals are unusual and notable, which is why quarantines are so effective at protecting your plants. 

If they are not present, pathogens may catch a ride into a landscape on insects, tools, shoes, and newly acquired plants. Chewing insects may create a point of entry for disease or may be infected, transferring viruses or bacteria to the host plant as they feed. You can interrupt the pathogen side of the triangle with these tips:

• Plant disease-free seeds and plants.
• Sanitize tools regularly.
• Eliminate insects that carry the pathogen (vectors).
• Interrupt pathogen life cycles with crop rotations and cover crops.

Time

Many botanists have added time as a factor in disease prevention, converting the disease triangle into a pyramid. They do this because time can be a critical factor in disease development. Diseases take time to infect a plant and reproduce. Water sitting on a leaf for a few minutes may do nothing, while several hours of surface water on the same leaf may be deadly. Regularly monitoring plants for signs of disease can put time in your favor.

How healthy plants defend themselves

If all the disease-carrying pathogens are already present, why don't all plants die a horrible death as soon as they sprout? Like us, plants have evolved ways to protect themselves. They do not have immune systems. Instead, they use these steps in a type of chemical warfare:

1. Detection – Invaders trigger a chemical messaging system that tells nearby cells (and sometimes neighboring plants) to thicken their cell walls or secrete harmful chemicals.
2. Hypersensitive response (HR) – Next, the cells surrounding the invaded area will self-destruct, creating a barrier to further infection. Some dead spots on leaves are the successful defeat of a pathogen, and sometimes not.
3. Systemic acquired resistance (SAR) – Plant cells enter a heightened awareness status; commercial growers artificially stimulate this response with chemicals (plant activators), which are less toxic than fungicides and antibiotics, and their effects last longer.
4. RNA silencing - Host plants can deconstruct an attacker's genetic information into less harmful versions; symptoms may still occur but are mild, and the plants can recover. [And we thought GMOs were a human-only construct!]

All this time, you probably thought plants were relatively passive, right?

Symptoms & disease identification

Plant disease symptoms include wilting, stunting, deformed leaves or other growths, cankers, chlorosis, and more. If you notice disease symptoms, use that information to determine the cause. Contact your local Master Gardeners or the Department of Agriculture for advice. Once you know what your plants are fighting, you can use the disease triangle to break the cycle.

Each microclimate has its own set of problems. Learning about your plants puts you in control.