Editor’s note: This week we have a guest article by Dr. Tony Keinath, a researcher and avid gardener. Amy Dabbs will return in two weeks.

By Anthony Keinath

Special to The Post and Courier

Charleston gardeners certainly are familiar with plant diseases such as powdery mildew, gray mold and root rot.

However, where plant diseases come from often appears to be a mystery. The beginning of a disease is difficult to see with the naked eye since it happens inside the plant or underground.

This is why some diseases seem to appear “out of nowhere.”

A deeper understanding of the causes of diseases and the weather conditions that favor them helps to explain their origins.

Plant pathologists (like me) use a handy diagram, the disease triangle (a three-legged stool), to illustrate that three factors work together to promote disease.

The three corners of the disease triangle are the plant, the pathogen (or disease-causing microorganism) and the environment. The environment is the weather and soil conditions around the plant.

When these three factors are found in the same place at the same time, a plant disease occurs.

For example, root rot develops when a susceptible plant is planted in soil where a root pathogen lives, and there is enough water for the pathogen to be active.

A disease also can be thought of as a play. The plant is the stage, the pathogen is the lead (and often only) actor, and the environment is the props or setting. The disease happens when all three parts come together. Each part by itself is not the play, just like a pathogen growing in a culture dish in the laboratory is not a disease because the plant is not present.

The plant serves as the “stage” for disease and the host for the pathogen. As parts of the plant are invaded by the pathogen, they usually turn yellow and then brown. The distinctive colors and patterns that result are the disease symptoms.

Symptoms that affect the whole plant, such as wilting and stunting, suggest that the roots are where disease started.

The age of the plant can hasten or hinder disease. For example, leaf diseases often start on the older leaves and spread to younger ones. In contrast, root diseases are more likely to develop on young seedlings than on older plants that have “hardened off.”

The most unfamiliar part of the disease triangle is the pathogen.

Plant pathogens are microorganisms that can live in or on living plants. Just as many human diseases are caused by viruses or bacteria, many plant diseases are caused by fungi or plant viruses. To pathogens, the plant is a rich source of food that allows them to reproduce.

It is important to realize that a pathogen can “have a life of its own” apart from the plant when it is not causing a disease. Most pathogens can survive in soil in a dormant state for months or years while the host plant is absent.

As a root grows through soil, it encounters pathogens that respond to the root and attack it.

Other pathogens, such as downy and powdery mildews, survive on living plants. These mildew diseases appear on healthy plants when the pathogen is blown from a diseased plant to a new plant.

The environment around the plant is probably the most important factor that determines if or when disease appears.

Rainfall, dew, fog and air and soil temperatures are the most important weather conditions to watch. Gray mold on pansies or strawberry fruit appears only during cool spring weather.

Other diseases, such as bacterial wilt of tomato, Southern blight of pepper and root knot on squash, appear when soil warms to 85 degrees in late spring.

How long leaves stay wet after a rain, or how late dew or fog lasts in the morning, can be used to predict when disease will start. The longer the leaves stay wet, the more likely they will become diseased.

Downy mildew is sure to show up on collards after a foggy fall morning. Likewise, the longer that soil stays too wet after rain or irrigation, the more likely that seedlings will damp off or roots will rot.

The next time you see a disease on a favorite plant, remember that the environment allowed a pathogen in the air or the soil to attack the plant.

Anthony Keinath is professor of plant pathology at the Clemson Coastal Research & Education Center in Charleston. His expertise is in diseases of vegetables. Contact him at tknth@clemson.edu