Fungicide application for protection against scab – what do I do when I can’t hit the “perfect” timing?

Carl A. Bradley, Extension Plant Pathologist, University of Kentucky

Just like the porridge in the “Goldilocks and the Three Bears” story, there is a “just right” timing for when to apply a fungicide for protection against Fusarium head blight (a.k.a. scab) of wheat. That “just right” timing is the Feekes 10.5.1 growth stage (beginning flowering), when anthers are just beginning to extrude from the middle part of the wheat head. Unfortunately, not all main stems and tillers will be at this stage at the exact same time, but when 50% of the wheat heads are at this Feekes 10.5.1 growth stage, that is considered the “just right” timing for applying a fungicide for protection against Fusarium head blight.

Oftentimes, when the risk of Fusarium head blight is high, it will be raining during this critical time when a fungicide should be applied for protection. Decisions sometimes have to be made to either spray a field a little early or a little late, based on the weather. From a multi-year, multi-state research trial that was funded by the USDA-Agricultural Research Service - U.S. Wheat & Barley Scab Initiative, the timing of Caramba and Prosaro fungicides were evaluated. Treatments were applied at Feekes 10.5 (when heads had emerged but were not yet flowering), at Feekes 10.5.1 (early flowering), or at 5 days after Feekes 10.5.1. In general, the results of this research showed that when these products were applied prior to flowering (Feekes 10.5), the control of Fusarium head blight and deoxynivalenol (DON) in the harvested grain was statistically less than the early flowering (Feekes 10.5.1) application time. The results also showed that the application 5 days after Feekes 10.5.1 had slightly less control of Fusarium head blight symptoms compared to the Feekes 10.5.1 application, but it did have statistically-similar control of DON in the harvested grain. In addition, another multi-year study conducted at the Ohio State University and the University of Illinois showed that Caramba or Prosaro fungicide applications up to 6 days after Feekes 10.5.1 often were statistically similar to applications at Feekes 10.5.1 (see published article here: http://apsjournals.apsnet.org/doi/pdf/10.1094/PDIS-01-14-0091-RE). So, according to these research studies, it is best to have the “just right” timing of Feekes 10.5.1, but if you have to be slightly off, it is best to be slightly later (up to about 6 days) rather than slightly earlier than Feekes 10.5.1.

Other things to consider:
 • Currently, the Fusarium head blight risk map is showing a low risk for Kentucky (www.wheatscab.psu.edu).
• Rainfall received the past couple of days and rainfall that is forecasted for the next few days may increase the risk of Fusarium head blight.
• Stripe rust is being observed in several areas in the state on susceptible wheat varieties. Fungicides recommended for protection against Fusarium head blight (i.e. Caramba and Prosaro), also will protect leaves from stripe rust infection.