By Pierce Paul, Cereal Pathologist
After more than 12 years of running my own wheat foliar fungicide efficacy trials here in Ohio, and analyzing data collected by my colleagues across the country, I have come to the conclusion that there is rarely ever a benefit to making more than one foliar fungicide application to wheat in Ohio. In fact, even a single application may not be beneficial if the cultivar is resistant to the prevalent disease in your area or conditions are not favorable for disease development.
So if you have to make a single application, go with the one that is most likely to provide the greatest benefit in terms of disease control and return on your investment. Here are a few tips to help you make that choice, but you’ll have to scout fields to see what’s there, especially if you do not know the susceptibility of your cultivar, and pay careful attention to product labels:
- For early-season diseases such as Septoria and powdery mildew, if your cultivar is susceptible, you are better off applying a fungicide at Feekes 8 (flag leaf emergence) or at Feekes 10 (boot) than at jointing (Feekes 6), for this is when we see the greatest benefit in terms of protection of the flag leaf and yield return. For mildew and Septoria, the residual effect of an early application is often not sufficient to protect the flag leaf, the most important source of sugars for grain development.
- For mid- and late-season diseases such as rusts and Stagonospora leaf and glume blotch, the target growth stages should be between boot and heading (Feekes 10.5), unless you see Stagonospora lesions or rust pustules very early in the seasons (at or before jointing). For rust in particular, if the fungus survived in Ohio due to a mild winter, a flag leaf application may be needed to keep the disease in check, as this disease can spread rapidly and damage the upper leaves before grain fill is complete, if not controlled early.
- Splitting half-rate fungicide applications (half at jointing and the other half at Feekes 8 or Feekes 10) are no better than a single application at Feekes 8 or Feekes 10. Plus, this type of program only increases the risk of fungicide resistance and may damage the crop by adding multiple sets of wheel tracks.
- Make sure you still have the option of using your best fungicides later in the season, just in case head scab and vomitoxin become a problem at flowering time. You should avoid applying the same active ingredient multiple times during the growing season to the same wheat field. Since Prosaro (prothioconazole + tebuconazole) and Caramba (metconazole) are your best fungicides for managing scab and vomitoxin, make sure you have the option of using one or the other at flowering. There are lots of other effective (and sometimes cheaper) fungicides to choose from for earlier applications.
- If the risk for head scab is high (wet, humid conditions during the days leading up to heading and flowering), do not apply a strobilurin fungicide after Feekes 10, as this may result in higher levels of vomitoxin contamination of the grain. You would be better off using your strobilurin or strobilurin+triazole combination products early in the season (between Feekes 8 and 10) to minimize potential problems with vomitoxin and free-up Prosaro or Caramba for application at flowering.