Most of the wheat in the northern half of the state is still between Feekes growth stage 7 (second node formed on the stem) to 8 (early flag leaf emergence), but in the southern half of the state, wheat is much further along. Understandably, wet weather and rain fall near wheat heading time, folks are asking questions about head scab and vomitoxin. Based on some of the questions, here are a few Questions & Answers from Dr. Pierce Paul, OSU Extension specialist, of things to remember and consider as you make your head scab management decision.
Q: What should I apply for head scab and vomitoxin control?
A: Prosaro, Caramba, or Miravis Ace. In OSU research, they are just as effective when applied at the correct growth stage.
Q: What is the correct growth stage for applying a fungicide to control vomitoxin and head scab?
A: The best results are seen when the application is made at or shortly after early anthesis (Feekes 10.5.1). Anthesis – fresh anthers are seen sticking out of the central portion of the head.
Q: How do I make an application decision when some of the heads are at the right growth stage while other heads are not?
A: Unfortunately, wheat growth is never uniform, that is why we recommend 50% anthesis as a guide for apply a fungicide for scab control.
Q: Does 50% mean half of the head is out or at anthesis, or half of all the heads/tillers I examine?
A: It means half of all the heads you examine. If you select 20 heads, and 10 of them have anthers sticking out of the central part of the head, you are at 50% anthesis. However, you will need to examine way more than 20 heads/tillers to get a good estimate. In fact, you should examine multiple sets of 20 heads at multiple locations (50 to 60) across the field – the larger the field and the more variable the field, the more heads/tillers you should example. And focus your attention on the primary tillers.
Q: What if I miss the 50% growth stage, is the fungicide still going to work?
A: YES, once it is applied within the first 4-6 days after 50% anthesis. A “late” application often do just as well as the 50% anthesis application. However, the later you wait, the greater the risk of missing the application window, if it rains and you cannot get into the field.
Q: What if I apply early (to beat the rain), say, before 50% early anthesis or heading?
A: You are still going to get some scab and vomitoxin control, but not as good as if you apply at or shortly after 50% early anthesis.
Q: I follow all the guidelines but still get scab. Why is that?
A: Unfortunately, none of the fungicides are 100% effective. That is why we often say scab suppression — the word control can be misleading. You will see the best results if the fungicide is applied to a moderately resistant variety than to a susceptible variety.
Q: I follow all the guidelines, get good scab control, but still get vomitoxin. Why is that?
A: Quite often, good scab control comes with good vomitoxin reduction. But in some years, particularly when cool, wet, rainy conditions occur between head and harvest, vomitoxin levels may increase, even if scab levels remain low and unchanged.
Q: Would I see better results in terms of scab and vomitoxin control if I make two applications?
A: Based on a limited number of experiments with wheat, better scab and vomitoxin control was seen when Miravis Ace was applied at 50% anthesis followed by Caramba, Prosaro or tebuconazole 4-6 days after than when a single application was made at 50% anthesis.
Q: If it is cold, should I even be concerned about scab and vomitoxin?
A: Scab develops best under warm, humid, or rainy conditions. Cold or cool weather close to heading definitely reduces the risk of scab. For those fields at Feekes 8-9 (9-full flag leaf emergence), continue to monitor the progress of the crop — if there is a lot of moisture in the air and warm weather is in the forecast.