It Takes Three to Make a Plant Sick

Infected Lilac leaves
Lilac leaves infected with powdery mildew, showing the white, dusty coating typical of this disease.

By Christine Engelbrecht
Plant Pathology
Iowa State University

It may take two to tango, but three things are required to make a plant sick.  An understanding of this fundamental “Plant Disease Triangle” forms a basis for managing plant health.

The first “side” of the triangle is so obvious it may be overlooked. In order to have a plant disease, you must have a plant. More specifically, you need a susceptible plant, one that is able to get a particular disease. Each plant species is prone to a unique set of maladies. Crabapples and oaks get different diseases. Within a species, plant varieties differ in their susceptibility to various diseases. For example, some crabapple cultivars are decimated by apple scab while others are unaffected. The overall health and vigor of an individual plant also affects its susceptibility to disease.

The second “side” of the Plant Disease Triangle is also simple. Besides a susceptible plant, there must be an organism that can cause disease, or a pathogen.  Most plant pathogens are fungi, which can cause leaf spots, root rots, mildews, wilts,and a variety of other symptoms.  Besides fungi, bacteria, nematodes (microscopic worms) and viruses are other examples of common plant pathogens.

The third “side” of the triangle is perhaps the least obvious, but it is crucial.  The susceptible plant and the pathogen must interact together in a favorable environment in order to result in plant disease.  For many fungal diseases, “favorable environment” means warm and wet. But some diseases are favored by cool weather, dry conditions, or a certain soil pH. Each disease is favored by a slightly different combination of humidity, temperature and other environmental factors. Even when a large population of a pathogen is present on a susceptible plant, there will be no disease unless the conditions are just right.

For example, powdery mildew is a common disease of lilac, causing a white, dusty coating on the leaves in mid to late summer.  In order to have this disease, three things are required—the plant (a susceptible lilac bush), the pathogen (the powdery mildew fungus), and a favorable environment (in this case, humid but not wet conditions and moderate temperatures).

How can we use this knowledge to manage plant problems? Because three things are necessary for plant disease, we can prevent disease in our gardens by altering any one of the three factors.

For example, we can reduce the impact of the host plant by choosing disease-resistant varieties or species that are relatively disease free. Several varieties of lilac are available that are resistant to powdery mildew. Maintaining good plant vigor through proper watering and fertilizing will also make plants less prone to disease.

Alternately, we can attack the pathogen side of the triangle. The pathogen can be reduced by removing debris and weeds where it may survive, rotating crops so that pathogens do not survive year to year on the same crop, or controlling insects that may carry the pathogen. In our lilac example, we could remove diseased debris at the end of the growing season, or use fungicides to reduce the amount of the pathogen on the leaves. 

The environment can be managed in many ways to reduce disease. Humidity and leaf wetness are often conducive to disease, and these can be minimized by spacing, staking and pruning plants to promote airflow. We can remove weeds that impede airflow. We can change watering practices, avoiding overhead watering that increases leaf wetness, and watering in the morning rather than the evening so leaves have time to dry out before night. In our lilac example, spacing the bushes to maximize airflow between them and planting them in full sun can help to minimize humidity in the canopy, thereby reducing powdery mildew problems.

By understanding that three things are required for plant disease--host plant, pathogen, and favorable environment--we can use a three-pronged approach to manage plant diseases. 

Contacts :
Christine Engelbrecht, Plant Pathology, (515) 294-0581,
Jean McGuire, Extension Communications and Marketing, (515) 294-7033,

There is one high resolution photo available for use with this week's column:
lilac11907.jpg Caption: Lilac leaves infected with powdery mildew, showing the white, dusty coating typical of this disease.  Powdery mildew results when a susceptible plant and the mildew fungus interact in a warm, humid environment.