By Pierce Paul, Ohio State University
Is it still worth putting on a fungicide? If so, what would be the best time to put it on? Via phone, email, or in person, these have been the most frequently asked questions over the last 5 days. The short and simple answer is “it depends”. Some fields have been hit by stripe rust while others have been hit by freezing temperatures, all within the last 7 day. Understandably, most people were planning to hold off on making a fungicide application until flowering (anthesis: Feekes 10.5.1) to get the best control of head scab and vomitoxin. Several of the fields affected by stripe rust are still between Feekes 10 (boot) and 10.5 (heading), at least a week before flowering. Since it is never really profitable to make multiple fungicide applications to wheat, the question now becomes, should I apply a fungicide now (at Feekes 10.5) to control stripe rust or should I just wait until Feekes 10.5.1 to target head scab.
You should base your decision on how much rust is in your field, your current growth stage, the scab forecasting system, and the scab resistance of your variety. If stripe rust is on the flag leaf in multiple hot spots across the field, you should consider applying a fungicide since this is a very explosive diseases and the weather is favorable for its spread. You already have rust but do not know if you are going to get scab. If you are close enough to flowering (at 10.4-10.5), you will get some scab suppression if you apply now, but remember that you usually get better scab control with applications at 10.5.1 (early flowering) or 2-6 days after 10.5.1, than with applications at 10.5 (3-5 days before 10.5.1). Keep in mind that high levels of rust before or during flowering usually have the greatest impact on yield. Rust causes losses by reducing the number of kernels per head and the size of the kernels, and by lowering test weight (Factsheet PLPANT-CER-12). Scab, on the other hand, causes losses by reducing the size of the kernels, lowering test weight and contaminating grain with vomitoxin (Factsheet PLPANT-CER-06).
Before making your fungicide decision, take a look at the scab forecasting system (www.wheat scab.psu.edu) to see what the risk is. For wheat flowering today (May 16), the forecasting system says that the risk for scab is moderate on susceptible varieties across most of the state, and will likely decrease over the next few days because of the cold snap. On moderately susceptible and moderately resistant varieties, the risk is low today and over the next three days (Factsheet PLPANT-CER-03). The forecasting system is 80% accurate, meaning that it is correct more than 3 out of 4 times.
Things to consider:
1- Find out if the variety in your field is moderately resistant or even moderately susceptible to scab, if it is, then the risk for scab is predicted to be low over the next few days. Consider applying a fungicide now to control rust and keep your eyes on the scab forecasting system to see if the risk increases. Moderate scab resistance (or moderate susceptibility) + the Feekes 10.5 application of Prosaro or Caramba for rust control should help to suppress scab, but keep your eyes on the forecasting system.
2- If your variety is susceptible to scab, but you are still at Feekes 10.5 the forecasting system indicates that the risk for scab will likely decrease (due to the cold weather) by the time the crop start flowering. Consider applying the fungicide now to control rust and keep your eyes on the scab forecasting system to see if the risk increases.
3- If your variety is susceptible to scab and you are already at Feekes 10.5.1, an application made at this time will help to control both scab and stripe rust.
4. If your variety is susceptible to scab, you are still at Feekes 10.5, and you want to consider making two application (one for rust and second for scab), consider using a cheap but effective rust fungicide (propiconazole for example) and leave your best scab fungicide (Prosaro or Caramba) for the second application at Feekes 10.5.1.
5. At this point, it is hard to tell whether the recent cold snap (May 15) will severely damage the wheat crop to the point of making a fungicide investment worthless. The level of damage will depend on how cold it got, for how long, the specific variety (some are more cold-temperature-tolerant than others), and how much of the field is affected (see Assessing the Wheat Crop for Freeze Damage and Twisted Wheat: Cold temperatures or herbicide damage?). It may take up to 7-10 days before the level of freeze injury can be determined. Whether or not you choose to wait to assess the damaged before applying a fungicide, you can still use suggestions 1 to 4 above as a guide for making the fungicide decision.