Fungicides: Know when to spray and what factors to consider

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.

The disease triangle 

Host

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.

Sometimes, when wanting to spray for a specific disease, the plants (host) staging is outside of the ideal window but in some cases, an application may still be a good idea. Missing the ideal window can be the result of numerous reasons including weather, equipment breakdowns, or other fields needing to be tended to first. When spraying for something like sclerotinia, for example, the ideal window for canola is generally 20-40% bloom. If conditions are ideal, infection can still occur past 40% bloom. Some products may have a curative affect as well, so spraying at 50% bloom can still be very effective.

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

Pathogen

Pathogens are usually introduced to a host through the soil or wind, where it can be difficult to tell what is present. An example of a pathogen and host together would be if you planted canola on pulse stubble that is a carrier of canola disease. White mold is a disease that can impact both lentils and canola so the risk of pathogen carry-over is higher between host plants. When considering pathogen and host together, I find that it’s best to think about the whole picture and not get too focused on just the current conditions.

It's hard to measure disease spore levels and a lot of pathogens can last for years in the soil, waiting for the perfect environmental conditions. F. graminearum and sclerotinia can survive on residues buried 20 to 25cm deep for more than four years with a large host species range. Even if disease pressure is low, this doesn't mean that risk is non-existent.

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, such as powdery mildew, 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.

There are three factors you should consider when assessing the crop in-season for a fungicide application:

• What were the conditions of that field last season? What diseases may have been present the last time that crop species was grown there?
  • High disease pressure can carry over from one growing season to the next in the form of soil or residues pathogens.
• How does that crop look this season? 
  • Thick plant stands are more prone to locking disease in the canopies. Fungicides help the plant survive disease pressure and can help lock in the yield potential, protecting it against yield robbers.
• What is the economic value of the crop?
  • Disease result in billions of dollars in economic losses and management inputs each year.

Conclusion

Mid-June to mid-July is the ideal window for assessing disease and spraying a fungicide to protect your crop. 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.