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Fungicides: Know when to spray and what factors to consider

Read Time: 8 minutes

By Tracy Kinch April 13, 2022

Proper fungicide application and timing is crucial so it’s important to be familiar with what diseases can occur on your crops and how to best manage them


Knowing when, or even if, to spray fungicide is tough to nail down. I find that keeping a diligent eye on your crops and considering some key factors of disease can help in correctly timing fungicide application. One of the most helpful tools I’ve found when considering use of fungicides is the disease triangle. The disease triangle is composed of three pillars: pathogen, host, and environmental conditions. When you have two of the three pillars, you’re prone to disease presence.

disease triangle - in page

The disease triangle 


Your plants are the host. When considering disease, knowing the condition of the plant and the factors affecting it can help decide if fungicides are needed. Host tends to be the most manageable pillar in the disease triangle as there are lots of resources available to help you learn what diseases can occur on specific crops. Researching what disease your varieties are susceptible to will help you better prepare for disease prevention as some newer varieties tend to have higher resistance. While resistant cultivars are important in disease management, they are only part of the solution and shouldn’t be relied on alone.

Now that we've recognized the host, let's dive into the next tip of the triangle - the pathogen.


Pathogens are usually in the soil, where it can be difficult to tell what is present, or they blow in from other fields. An example of a pathogen and host together would be if you planted canola on stubble with clubroot. When considering pathogen and host together, I find that it’s best to think about what steps you can take to control or limit the risk of spread from field to field. 

Here are 5 simple tips to eleiminate the spread of pathogens:

  • Rinse your boots. When visiting a field with pathogens present, you’re less likely to spread it to other fields if you clean your boots between fields. 
  • Use bleach on your vehicle. Driving in a field with disease present can make it just as easy to spread to other fields as wearing the same unwashed boots. If you want to prevent the spread, sanitize your vehicles before visiting other fields. 
  • Visit the diseased field last. I’ve found that the easiest way of preventing the spread of pathogens from field to field, is to visit the infected field last and sanitize your equipment when you’re finished. 
  • Test your seed. Using seed with fusarium can add to the likelihood of there being pathogen present in your field. My recommendation is that you test your seed for fusarium and use pathogen- free seed to prevent pathogen presence in hosts. If the seed you’ve planted does have fusarium, using seed treatments can cut down on the percentage of fusarium present.  
  • Rotate your crops. Certain pathogens have specific hosts. If you avoid planting that same commodity on the infected field, you can cut down on risk of reoccurrence.  
    • Ie. Aphanomyces only affects pulses, so if you keep pulses off a field for eight years, it should break the cycle of pathogen. 

Host and pathogen are two prongs of the disease triangle that can be assessed fairly early on in the season whereas the last prong, environmental conditions, can be a little more difficult to time. 

Environmental conditions

Environment can affect the host depending on moisture, humidity, and time of year. This part of the triangle is the most out of your control and also the hardest to predict. Different diseases thrive in certain conditions so it’s important to understand the disease that you’re considering spraying for and what environmental conditions are conducive for its development.  

If we have a wet and humid spring, I think it’s likely that fungicide application will need to be seriously considered to cut down on risk of disease. A good way to visually test this is the “wet pant” test. At what time of day can you walk through your field and come out with dry pant legs? The longer in the day it takes for the field to be dry enough for you to walk through and come out dry, the higher the risk of disease. 

There are also some diseases that thrive in cooler, dryer conditions so it’s important to remain vigilant and fully understand what diseases the crops you’re planting can be susceptible to.

When to assess the situation

Fungicide timing is very important to get infection under control before it becomes serious. Know the signs and symptoms as the sooner you catch the disease, the more likely you can treat it and prevent it from getting worse. Fungicides will be most effective at slowing disease development when applied at the very first sign of symptoms. In my experience, I’ve found that the best way to assess disease risk prior to visible presence is to use weather and disease forecasting maps. If there is summer rain, heavy dew in the mornings, and/or high humidity conditions, this is when I would recommend to my customers that they should consider spraying.

Knowing at what stage your crops are most prone to infection is also a helpful tool for preventing disease. For example:

  • Fusarium head blight is a flowering disease (early emerging heads).
  • Sclerotinia infects canola flowers (target first flowers).
  • Molds infect pulses as the canopies thicken and close (target pre-closure).
  • Stripe rust in barley becomes present in temperatures from 10-18 degrees. These temperatures along with six hours of dew are optimal environments for infection and disease development. 


Mid-June to mid-July is when disease activity is most commonly present. The disease triangle can be an excellent tool for helping to make most of your fungicide decision ahead of time and it’s a tool I use very often with farmers. If you do end up needing to spray, by using this tool you’ll be ready to hit the crop at the appropriate, most effective time. If you think there might even be a chance that you’ll need a fungicide, pencil it into your plan. Talk to your rep about whether the present conditions might be an issue for disease. There’s no harm in being proactive. 

Tracy head shot

Tracy Kinch

Tracy grew up in southern Alberta where she graduated from the University of Alberta with a Bachelor of Science in Ecology. In her past experience, she has completed carbon modeling research with Agriculture and Agri-Food Canada. She's also done extensive blackleg research in Australia. Most recently, Tracy has worked as a private agronomist for a large farm in central Alberta before joining the Cargill Saskatchewan team in Fall 2021 as a Market Development Agronomist out of Moose Jaw, SK. She has a passion for plant pathology and loves researching in all aspects of agronomy.