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Introduction to Root Diseases – Disease Triangle

When growing greenhouse crops, it is simply a matter of time before problems occur. These problems can be caused by diseases, insects, fertility problems, pesticide phytotoxicities, plant growth regulator over-application, watering problems, air pollution from heaters, etc. To correct a problem, it is necessary to know what it is and what is causing it.

Similar looking symptoms

Unfortunately, many types of problems have similar looking symptoms, so it is important to test a crop to determine the cause. However, what type of test do you do? For example, if you send in a plant to be tested for disease when the problem is coming from a malfunctioning furnace, the source of the problem will not be identified. Therefore, it is best to start investigating the problem and take note of all the variables that could be causing a problem to narrow down the cause.

Disease Triangle

In the case of plant disease, there are certain conditions that must be in place in order for a disease to get started. This is often simplified in a picture called the disease triangle (Figure 1). The disease triangle points out that three favorable conditions must coexist to cause a disease problem. These three are the pathogen, a susceptible host (plant) and proper environmental conditions.

Root Disease Triangle PRO-MIX

“Figure 1. Disease Triangle. The three factors that have to coexist are presence of a pathogen,
proper greenhouse environment for disease development and a susceptible host plant.”


Pathogen: There are several types of organisms that cause plant diseases. These include fungi, bacteria, viruses, nematodes, mycoplasmas and spiroplasmas. These pathogens need to be present in the greenhouse in order to set the stage for a disease problem. Shifting the focus to root disease pathogens, they can be present in the greenhouse in plant debris, weeds growing under the bench, dust coming in from a field or gravel driveway, contaminated tools, stock plants, left over plants, insects, etc.

Host: A host is a plant that can become infected by a pathogen. Not all plants are susceptible to attack from the same pathogen as some pathogens have preference for certain plants. For example, Thielaviopsis typically attacks pansies, petunias, pansies, snapdragon, verbena, etc., but does not typically infect marigolds. Within the same plant species there can be differences in susceptibility to a specific pathogen. For example, calibrachoa that are compact and slow growing often have more issues with root disease than faster growing cultivars. Also, consider that within the same species of plants some may have been breed to have resistance to certain pathogens.

If a grower has more disease problems with a type or series of plant, they are often removed from the production schedule as they often cost more to produce due to higher fungicide application costs and plant loss. Also when these plants have disease, they introduce more inoculum in the greenhouse, putting other crops at risk.

Environment: This is the most complex of the three, but can also be manipulated to reduce root disease issues. Any environment that causes plant stress can make a plant more susceptible to a plant disease. The main factors to consider are as follows:

  • Watering – watering is often the major concern when it comes to root disease problems. Pythium and Phytophthora are water molds and prefer a wet growing medium. Overwatering provides the best environment for these pathogens and also stresses plant roots, making them susceptible to attack. Fusarium, Rhizoctonia and Thielaviopsis also require moist media, but do not need wet growing media to thrive. Often they attack plants that are under stress, which can be triggered from overwatering.
  • Temperature – some root disease pathogens, such as Pythium ultimum, prefer cooler growing medium temperatures, while Pythium aphanidermatum prefer warm growing media. The crop itself has preferred temperature range, so if the crop is grown hotter or colder it becomes stressed and more susceptible to pathogen attack. For example, vinca prefers hot growing conditions, but if growing medium temperatures are below 75°F or 24°C, they are more susceptible to root disease pathogens.
  • Air Movement – As with foliar diseases, poor air movement through the crop increases potential for root disease. Humid air stays trapped in the plant canopy, thereby reducing plant water use from the growing medium. The growing medium dries out more slowly, making it easier to overwater and therefore plants are under stress and susceptible to root disease pathogens.
  • Humidity – As with air movement, high humidity slows the dry down rate of the growing medium via limited water usage by the plant and slow evaporation form the growing medium surface. If the growing medium dries out slowly, it takes more time to dry the growing medium which increase plant stress and susceptibility to root disease pathogens.


Plant diseases can be problems for growers from time to time. As seen above there are a number of factors that must occur at the same time in order for root disease to start.

If you have further questions about PRO-MIX BIOFUNGICIDE* or PRO-MIX BIOFUNGICIDE* + MYCORRHIZAE products and how they can minimize root disease, please contact your Premier Tech Horticulture Grower Services Representative or your Regional Sales Representative.

*All BIOFUNGICIDE products are available under BIOSTIMULANT in Canada and in Latin America.

PRO-MIX® is a registered trademark of Premier Horticulture Ltd.

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