As we gear up for the 2023 planting season, I wanted to revisit some basic plant pathology concepts in order to bring you soybean disease management insights. I will discuss some of these ideas throughout the season in this Soybean Pathology 101 Series.

What is the Disease Triangle?

The disease triangle (or pyramid) is a classical concept in plant pathology and helps us to understand some elements must exist before a plant disease can occur. A soybean disease will only occur if a susceptible PLANT HOST, a PATHOGEN, and a favorable ENVIRONMENT are present in that field. Some plant pathologists will also add TIME as a fourth factor and then call it a pyramid.

Figure 1: The disease triangle (Image credit: Leo Rocha).


A susceptible PLANT HOST, in our case, is a soybean plant that can be infected and sustain a pathogen. Different soybean varieties can vary in their susceptibility to a specific pathogen, so check seed catalogs and talk to your seed dealer if you are trying to manage a particular disease in your area. Some pathogens may even be able to infect a resistant variety, but this will result in little to no yield loss when compared to an infection of a susceptible line. Crop rotation can also be important in this context. For example, corn is a non-host of the soybean cyst nematode (SCN) and frogeye leaf spot; therefore corn can a good non-host crop to rotate with soybeans.


A PATHOGEN is a disease-causing microorganism (bacteria, fungi, or nematode). A soybean disease can only occur if that specific pathogen is present. Some management practices can reduce pathogen populations in a field, while other pathogens can move through the wind; therefore management practices may be less effective.


The ENVIRONMENT refers to weather and local climate variables. Soybean diseases may be influenced by temperature, humidity, soil compaction, moisture level, texture, PH, and so on. The response of some diseases to weather helps to explain why we observe more yield losses caused by certain diseases in some years. For example, the disease pressure of soybean foliar pathogens (ex.: frogeye leaf spot) was particularly low during the 2022 season due to the low moisture in the air. The drought also made the symptoms of nematodes and other soil-borne diseases more evident later in the season since a weakened root system can not withstand dry conditions very well.

How can you use the disease triangle with soybean disease management?

The disease triangle may sound too conceptual, but it can help us to understand basic disease management practices. Let’s use the disease triangle concepts to develop a soybean cyst nematode management program.

  • SCN, the PATHOGEN, needs to be present in the field. This pathogen is widespread across Illinois, so it is important to sample your fields to assess the presence and population levels you are dealing with. If SCN is not in your field yet, implement practices to prevent entry and keep populations on the lower spectrum. Once SCN is identified in an area, we can not eliminate it, but certain management practices, such as crop rotation and seed-applied nematicides, can help lower SCN populations.
  • A susceptible soybean variety (HOST) must be present in the field. Since corn and wheat are non-hosts of SCN, crop rotation can effectively reduce SCN populations. Planting SCN-resistant varieties is also very beneficial. Remember that we need to protect our SCN-resistant varieties, so make sure to rotate sources of resistance (PI 88788 vs. Peking) if possible.
  • An ideal ENVIRONMENT for soybean growth is also favorable for SCN development, so management practices targeting the environment may be limited. Sandy soils may be more favorable for SCN development and can intensify symptoms. SCN also prefers temperatures from 75F to 85F and has limited development below 59F or above 95F.

I hope this refresher on the disease triangle helps you better understand soybean management practices this season. Be sure to check out the links to the valuable resources below if you want to learn more.


Mueller D., Wise K., Sisson A., Smith D., Sikora E., Bradley C., and Robertson A. 2016. A farmer’s guide to soybean diseases –  Chapter 2: Understanding soybean diseases and disorders. APS Press: St. Paul, MN. Pages 5-21.

Chen S., Kurle J., Malvick D., Potter B., Orf J. 2021. Soybean cyst nematode management guide. University of Minnesota Extension. Available online:

The SCN Coalition

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About the Author: Leo Rocha

Leo Rocha grew up in Southeastern Brazil on a small coffee farm and has been involved in agriculture for as long as he can remember. Leo received a Bachelor of Science in Agronomy from Universidade Federal do Espirito Santo. He joined Southern Illinois University as a graduate student in 2017 and later received a Ph.D. in Agricultural Sciences with an emphasis on Plant Pathology and Nematology. His research projects focus on soybean pathology and cyst nematode management practices. Leo is a post-doctoral researcher at SIU Carbondale and looks forward to sharing soybean pathology and disease management information with Illinois farmers.

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