The Growing Point | June 2020

Flag leaf vs. head timing: when should I apply fungicide?

The question of head vs flag leaf timing fungicide applications is a common one. If an extra pass over the field is required for disease protection, it would be a good measure to ensure the pass you choose is the one that provides the greatest payback.

Like most agronomy questions, the answer is not simple. Depending on your location, the answer may change from year to year.

A few factors to assess risk can be evaluated prior to and in-season to help answer this question.

  1. Variety disease resistance – If your variety is weak on leaf disease or Fusarium head blight (FHB) resistance there is a greater risk of disease and therefore a greater potential for return on investment for a fungicide application.
  2. Rotation and disease history – Short rotations carry with them a greater risk of disease proliferation. If you commonly encounter FHB, any host crop within 1-2 years will increase your risk.
  3. Monitor your rainfall –Research by Dr. Sheri Strydhorst on AC Foremost (an older cultivar with an MS rating to leaf spots) looked at different rainfall amounts and fungicide timings. Locations which received greater than 4.5” of rainfall from seeding to flag leaf emergence displayed a yield response to flag leaf fungicide applications. Keep in mind that this number should only be used as a rough guideline. Response to flag leaf fungicide also depends on year, variety, rotation, and other agronomic management factors.
  4. Scout during herbicide and early elongation timing – scouting for early symptoms of disease can give you an indication that leaf disease on the flag and penultimate leaf may be a risk later on if the crop continues to see rainfall and moisture.
  5. Continued monitoring – Continue monitoring your field after herbicide timing as you head into flag leaf emergence. This is especially important in years with increased rainfall.
  6. Fusarium risk map familiarity – Become familiar with your province’s FHB risk map. Alberta’s can be found here. Start monitoring the map around flag leaf timing. Think of this tool the way you think of cart tank monitors; you don’t wait until the tank is empty to plan your fill, you monitor the tanks continuously to ensure you are prepared when a decision needs to be made. Bookmark the page and revisit every few days.

Using these tools pre-season and in crop will arm you with the ability to make a confident decision regarding fungicide application timing.

Much of Dr. Strydhorst’s recent research from 2014 to 2016 indicates that a head timing application results in the same, or greater, yield responses as compared to flag leaf application (Figure 1). That being said, it would be foolish to skip monitoring for flag leaf timing if you’re planning to time fungicide application for head leaf timing.

Consider the following scenario: You’ve have already decided that FHB timing application of fungicide provides a better return than flag leaf fungicide. Because of this, you see no concern to scout for foliar leaf disease and simply wait to apply a FHB fungicide. As you apply your FHB fungicide you notice that the field colour seems dull and slightly brown. Upon closer inspection, the flag leaves have been infected with septoria leaf complex and 30% of your leaf area is dead. Your FHB fungicide will protect what is left of the leaves and the head from disease but no amount of head timing fungicide application can remedy the loss of yield due to do the previously lost photosynthetic leaf area.

 



The above example may be a bit extreme, but it gets the point across. Each fungicide application, whether it be flag leaf or FHB timing, should be made based on the risk level at and approaching that crop stage. If while approaching flag leaf, you see visual signs of leaf disease in the lower canopy and environment is conducive for the leaf disease to spread, a flag leaf fungicide application is likely warranted. If flag leaf disease risk is low during that time, begin assessing FHB risk after flag leaf emergence. Even if you do apply a flag leaf fungicide, it would be wise to still monitor FHB risk to determine if a second application is warranted at head timing. However, most small plot research has indicated that dual application does not provide an economical beneficial.

This article is adapted from the article “When is an in-crop fungicide worth it?” originally published in Better Farming Prairie Agriculture May/June 2020 Edition