You probably won’t be surprised to hear that this week’s weather is going to put you in a bind, in more ways than one. From a wheat disease perspective, many of us are looking at wheat that is approaching heading or flag leaf.
You may be seeing some leaf diseases in the lower canopy. We also know that it’s now time to prepare for managing Fusarium head blight (a.k.a. head scab). As many of you know, this is a disease of wheat and barley that can lead to the production of toxins in grains. The fungus that causes this disease enters through the flowers and infects the developing grain. Warm and humid environmental conditions during wheat flowering or barley heading favor the growth of the fungus.
So the question becomes: do you spray for leaf diseases now and come back for a second pass with a head scab product? Or do you hold out and just spray once at flowering?
The answer depends on how close you are to heading, what diseases are present in your field now, and how willing you are to run through your field twice. The goal is flag leaf protection here, but lesions on lower leaves can give rise to spores that can infect higher leaves. If you’ve got a bunch of blotch in your canopy now, and you’re at Feekes 9 or earlier, it may be worth your time to make a trip through the field with a fungicide now, as most of these products are systemic and will give you a few weeks of control. If your wheat is already heading when you discover leaf diseases, it might be more economical to wait until flowering begins so that you can apply Prosaro or Caramba for head scab, which will also give you great control of flag leaf diseases.
To check the efficacy of labeled fungicides on various wheat diseases, please see this chart.
Most areas of PA are now at a “Low” risk for the development of scab in wheat and barley. While the humidity levels are high, the temperatures have not been high enough for the fungus to make spores. This can change quickly, so keeping an eye on the FHB Risk Assessment Tool will become critical for those farmers who have wheat beginning to flower in the next few weeks. This forecasting site, is an online model that helps us predict infection risk levels everywhere in the state. It has been improved over the last year to include some new features and better accuracy. Visit it at your convenience, or sign up to have updates e-mailed or texted directly to you.
Use your judgement based on your experience and your local conditions. Be prepared to spray a fungicide on fields that are at medium to high risk at flowering. Remember, sprays applied prior to flowering will not provide significant suppression of scab or toxin production, however, a spray up to a week after the beginning of flowering can offer good disease and toxin reduction. Caramba or Prosaro are effective on scab and give control of most leaf diseases and glume blotch. They do not need to be tank mixed with another product to control these diseases. If either these products is unavailable, Proline and Folicur (which together provide the same chemicals as Prosaro) may be tank mixed at a rate of 3 + 3 fl oz/A. Spray nozzles should be angled at 30° down from horizontal, toward the grain heads, using forward- and backward mounted nozzles or nozzles with a two directional spray, such as Twinjet nozzles.
Once wheat begins flowering, there is about a 5-6 day window to apply a fungicide. The labels state the last stage of application is mid-flower and there is a 30-day to harvest restriction. Do not use any of the strobilurins (Quadris, Headline), or strobilurin/triazole (Twinline, Quilt, Stratego) combination products at flowering or later. There is evidence that they may cause an increase in mycotoxin production.
At this point in the season, the only way to reduce the scab problem is to spray. But in general, do not rely solely on fungicides, as they will provide at most a 50–60% reduction in scab severity and vomitoxin. Choose resistant wheat varieties, and time sprays properly to achieve greater control.