By Pierce Paul, Ohio State University Extension
Most of the wheat in the northern half of the state is still between Feekes growth stage 8 (early flag leaf emergence) and 9 (full flag leaf emergence), but in the southern half of the state, wheat is much further along. Malting barley is even further along than wheat, and will soon be approaching the heading growth stage. Understandably, given the wet weather we have had so far this season, folks are asking questions about head scab and vomitoxin. Based on some of the questions I have been asked over the years, here are a few 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 my experience, 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 in wheat are seen when the application is made at or shortly after early anthesis (Feekes 10.5.1). For barley, the best results are seen when the treatment is applied at or shortly after heading (Feekes 10.5).
Q: How do I know that the crop is at the right growth stage for scab control with a fungicide?
A: For wheat, anthesis – fresh anthers are seen sticking out of the central portion of the head. For barley, heading – heads are fully out of the boot, i.e., out of the leaf sheath of the flag leaf.
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. The growth of barley tends to be even more uneven than wheat. That is why we usually recommend 50% heading as a guide for apply a fungicide.
Q: Does 50% mean half of the head is out or at anthsis, or half of all the heads/tillers I examine?
A: In wheat 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.In barley, it means half of all the tillers you examine. For barley, if you examine 20 tillers, and 10 of them have the head fully emerged, you are at 50% heading. 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 in wheat. A “late” application often does 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. YES, once it is applied within the first 4-6 days after 50% heading in barley. Quite often, waiting until 75+% of the heads are fully out do just as well. 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 in wheat, but not as good as if you apply at or shortly after 50% early anthesis. You are still going to see some scab and vomitoxin control in barley, but not as good as if you apply at or shortly after 50% heading.
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 the term 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. Less information is available on two-treatment programs on barley.
Q: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, continue to monitor the progress of the crop – there is a lot of moisture in the air and warm weather is in the forecast. Also, keep your eyes on the scab forecasting system at http://www.wheatscab.psu.edu/. Contact your county, field, or state (me at firstname.lastname@example.org) extension specialists for guidance.
Read more about head scab at: https://ohioline.osu.edu/factsheet/plpath-cer-06